Biomes of western North America at 18,000, 6000, and 0 14C yr B.P. reconstructed from pollen and
packrat midden data
Robert S. Thompson1 and Katherine H. Anderson2
1
U.S. Geological Survey, Earth Surface Processes Team, Box 25046, MS980, Denver, CO 80225, USA.
2
Katherine H. Anderson, Institute of Arctic and Alpine Research (INSTAAR), University of Colorado,
Boulder, CO 80303, USA.
Address for correspondence: Dr. R.S. Thompson, U.S. Geological Survey, Earth Surface Processes Team,
Box 25046, MS980, Denver, CO 80225, USA (fax +1 303 2365349, e-mail:
[email protected])
Ms. for Journal of Biogeography, BIOME 6000 special issue.
1 March, 2000
Biome reconstructions for western North America
(A)
2
ABSTRACT
1 A new compilation of pollen and packrat midden data from western North America provides a refined
reconstruction of the composition and distribution of biomes in western North America for today and for
6000 and 18,000 radiocarbon years before present (14C yr B.P.).
2 Modern biomes in western North America are adequately portrayed by pollen assemblages from lakes and
bogs. Forest biomes in western North America share many taxa in their pollen spectra and it can be difficult
to discriminate among these biomes. Plant macrofossils from packrat middens provide reliable identification
of modern biomes from arid and semiarid regions, and this may also be true in similar environments in other
parts of the world. However, a weighting factor for trees and shrubs must be used to reliably reconstruct
modern biomes from plant macrofossils.
3 A new biome, open conifer woodland, which includes eurythermic conifers and steppe plants, was defined
to categorize much of the current and past vegetation of the semiarid interior of western North America.
4 At 6000
14
C yr B.P., the forest biomes of the coastal Pacific Northwest and the desert biomes of the
Southwest were in near-modern positions. Biomes in the interior Pacific Northwest differed from those of
today in that taiga prevailed in modern cool/cold mixed forests. Steppe was present in areas occupied today
by open conifer woodland in the northern Great Basin, while in the central and southern Rocky Mountains
forests grew where steppe grows today. During the mid-Holocene, cool conifer forests were expanded in the
Rocky Mountains (relative to today) but contracted in the Sierra Nevada. These differences from the forests
of today imply different climatic histories in these two regions between 6000 14C yr B.P. and today.
5 At 18,000 14C yr B.P., deserts were absent from the Southwest and the coverage of open conifer woodland
was greatly expanded relative to today. Steppe and tundra were present in much of the region now covered
by forests in the Pacific Northwest.
Key words: packrat middens, pollen data, plant macrofossil data, plant functional types, biomes, vegetation
changes, western North America, last glacial maximum, mid-Holocene
Biome reconstructions for western North America
(A)
3
INTRODUCTION
Biomes represent broad physiognomic vegetation types that are based on the co-occurrence of plant species
that respond individualistically to climatic gradients and climatic change. Biomes depict vegetation
formations on a global basis based on the structure, physiognomy and climatic adaptations of the plant
functional types of which they are composed (Prentice et al., 1992). Plant functional types (PFTs) are
collections of plant taxa grouped by stature, leaf form, phenology and climatic adaptations. The
characterization of vegetation in terms of biomes provides the basis for modelling past and future vegetation
changes (e.g. Harrison et al., 1995; Kutzbach et al., 1998; Neilson et al., 1998) and using the same terms
for vegetation from different regions facilitates comparisons between the results of numerical climate
modelling and evidence of past vegetation (e.g. Jolly et al., 1998a; Joussaume et al.,1998; Williams et al.,
1998).
Prentice et al. (1992) provided the original definitions of PFTs and biomes used in this paper. Subsequent
regional syntheses for Europe (Prentice et al., 1996), the former Soviet Union and Mongolia (Tarasov et al.,
1998), China (Yu et al.,1998; Yu et al., this issue), Africa and Arabia (Jolly et al., 1998b; Elenga et al., this
issue), Beringia (Edwards et al., this issue), and Canada and the eastern United States (Williams et al.,1998,
this issue) refined these definitions and methodology, and extended them into a wide range of climatic and
vegetational circumstances. In this paper, we use pollen and plant macrofossil data to portray biomes in
western North America at 18,000 radiocarbon years before present (18,000
maximum or LGM), 6000
14
C yr B.P. (the mid-Holocene), and 0
14
C yr B.P.: the last glacial
14
C yr B.P. (today). We discuss the
changes in biomes between these time periods in terms of broad-scale changes in climate.
Western North America is treated separately from Canada and the eastern United States for the following
reasons: (1) it has an endemic flora that differs from those of Canada and the eastern United States; (2) it
has a unique data source (packrat middens) and palynological settings in desert and mountain environments
that are different from the other two regions; (3) its climate has sharp gradients, is generally more arid than
that of the eastern United States, and its winter temperatures are generally milder than those of Canada or
the northeastern and midwestern United States; and (4) its mountainous terrain requires special data analysis
and can limit interpolation among sites.
(B)
Modern climate and vegetation
The modern climate of western North America has strong geographic and elevational gradients in both
temperature and precipitation (Fig. 1). Winter precipitation from the North Pacific supports wet
environments in the Pacific Northwest, along the coast of California and in the northern Rocky Mountains.
Rain shadows associated with the Sierra Nevada, Transverse and Cascade Ranges cause aridity in the
Biome reconstructions for western North America
4
interior western United States. This aridity is offset in some areas near the Mexico-United States border and
in the southern Rocky Mountains by summer monsoonal rainfall from subtropical sources. Seasonal and
diurnal temperature ranges are low in coastal regions, but are high in many parts of the arid and semiarid
interior. These geographic gradients of climate interact with the mountainous physiography of western
North America to create a mosaic of forest, woodland, steppe/grassland, and desert vegetation. These
patterns are depicted in Fig. 2, where the categories representing potential natural vegetation from Küchler
(1964) have been grouped (Table 1) to approximate the biomes defined by Prentice et al. (1992). These
categories are based on observations of the current and historic vegetation and are purposefully simplified
to facilitate comparison between the fossil and modern data (for example, grassland and steppe are merged
here as it is difficult to distinguish these in the fossil record).
(A)
DATA AND METHODS
(B)
Pollen data
Pollen records provide quasi-continuous evidence of vegetation change through time. These data are
quantified as percentages of each taxon for each spectrum in the record; however, modern pollen
percentages frequently do not have linear relationships with the abundances of species within the terrestrial
vegetation. Pollen data are generally of low taxonomic resolution, and can usually be identified only at the
family or genus level. Fossil pollen data have been the primary source of information on biomes in other
regions, and their inclusion here allows some degree of comparability with previous studies. The pollen data
included here are from original counts wherever possible, or were digitized from published diagrams when
counts were unavailable. Raw pollen counts appear to provide a better discrimination between non-arboreal
biomes (Jolly et al., 1998b; Yu et al., this issue), but digitized data have been used to reconstruct biomes
successfully in other regions (e.g. Prentice et al., 1996; Tarasov et al., 1998; other papers in this issue).
(B)
Packrat midden data
Packrats (Neotoma spp.) are small rodents that collect leaves, sticks, fruits and other materials from the area
within tens-of-meters of their nests and bring these items into their homes for food, nesting material, etc.
Through time their nests can become cemented by their desiccated urine, and in dry caves and rockshelters
in western North America these urine-cemented "packrat middens" can be preserved for tens-of-thousands
of years (see papers in Betancourt et al., 1990 for further discussion). Packrat middens are unique to
western North America, although similar deposits left by other animals have been found in arid regions in
the Middle East, South America, South Africa and Australia (Betancourt et al., 1990). Plant remains from
packrat middens: (1) can generally be identified to the species level, and a given midden plant assemblage
appears to provide a detailed inventory of the species growing within 50m (or less) of the packrat’s nesting
Biome reconstructions for western North America
5
site; (2) are extremely well-preserved and provide excellent material for radiocarbon dating; and (3) occur
in assemblages that appear to represent a short interval of time, perhaps as short as the few years of an
individual packrat’s lifespan. In sum, packrat middens provide a detailed inventory of plant species for short
intervals of time in the past.
Although vegetation assemblages preserved in packrat middens have been studied since the 1960s, no
uniform method of quantification has been developed, and indeed there have been more quantification
schemes than investigators in the field. In any case, there is little evidence that relative abundance within the
assemblages has much meaning, except to indicate the past presence or absence of a given plant species at a
given midden site. In this paper we used data from the original publications (USGS/NOAA NGDC Packrat
Midden database) and converted the various quantification schemes into a simple four-digit scale where: 0 =
absent, 1 = rare (and possibly a contaminant); 2 = present; and -9 = cannot determine presence or absence
(this usually pertains when the original investigator published an incomplete list of plant species and it is
thus not possible to determine if something is really absent). Packrat middens with very few reported taxa
were omitted from the analysis.
(B)
Data sets
Modern pollen assemblages were obtained for 66 sites (Table 2). Raw pollen counts were obtained for 16
sites, and digitized data were used for the rest. Modern (and near-modern) packrat midden assemblages (35
samples) were obtained from 18 sites (Table 3). To facilitate comparisons with the older time slices, we
used only the core tops of pollen sediment cores for the 0
14
C yr B.P. time horizon. We did not use the
diverse array of surface samples available for western North America. For packrat middens, we used all
samples with radiocarbon ages within the last 1000 radiocarbon years.
The data set for 6000 14C yr B.P. consists of 76 pollen sites (Table 2), of which 14 were obtained as raw
pollen counts and 62 were digitized, and 34 packrat midden assemblages (23 sites) (Table 3). The data set
for 18,000 14C yr B.P. consists of 21 pollen sites (Table 2), of which 7 were obtained as raw pollen counts
and 14 were digitized, and 17 packrat midden assemblages (13 sites) (Table 3). With original pollen data
for the 6000 14C yr B.P. time slice we used the pollen spectrum closest to 6000 14C yr B.P., whereas for the
digitized pollen data we interpolated to determine the approximate 6000
14
C yr B.P. level. With packrat
midden data for 6000 14C yr B.P., we included all samples dated within 1000 radiocarbon years of 6000 14C
yr B.P. (in other words, those samples that dated between 7000 and 5000 14C yr B.P.). The same protocols
were followed for the 18,000 14C yr B.P. time slice.
Biome reconstructions for western North America
(B)
6
Method of assigning biomes to pollen and packrat midden assemblages (biomization)
The biomization procedure has been fully described by Prentice et al. (1996) and Prentice & Webb (1998).
There are four steps in the biomization procedure: (1) assignment of individual plant taxa to plant functional
types (PFTs); (2) specification of the set of PFTs that can occur in each biome; (3) calculation of the affinity
score of a given vegetation assemblage to every biome; and (4) assignment of the vegetation assemblage to
the biome for which it has the largest affinity score. In cases where the affinity score for two or more biomes
is equal, a tie-breaking rule is applied to determine the biome attributed to the sample, following Prentice et
al. (1996).
We prepared separate plant taxon-PFT matrices for pollen (Table 4) and for packrat midden assemblages
(Table 5) based on our knowledge of the ecology and biology of the individual plants, and on the
descriptions of the flora and vegetation given in Benson (1982), Benson & Darrow (1981), Kearney &
Peebles (1960), Little (1971, 1976, 1977), Thompson et al. (1999a, 1999b) and Turner et al. (1995). The
pollen matrix includes 74 individual pollen taxa and the packrat midden assemblage matrix includes 418
individual species.
We began with the PFT classification based on pollen data used for Europe (Prentice et al., 1996) and
subsequently tested and modified for other northern hemisphere regions (e.g. Jolly et al., 1998b; Tarasov et
al., 1998; Yu et al., 1998; Williams et al., 1998), and modified the pollen to PFT assignments to fit the
North American situation. The PFT definitions, the assignment of individual pollen taxa to PFTs, and the
assignment of PFTs to biomes are broadly consistent with the definitions used in adjacent regions of north
America by Williams et al. (this issue) and Edwards et al. (this issue).
There are difficulties with the pollen-to-PFT relation in western North America for conifers, as many pollen
taxa include both boreal and temperate species (most notably Picea, Abies and Larix). In Eurasia, Larix and
Picea are confined to the boreal zone (Prentice et al., 1996; Tarasov et al., 1998). However, both taxa also
occur in more temperate settings in North America (Little, 1971; Thompson et al., 1999a, 1999b). For
example, Larix lyallii and L. occidentalis both live in temperature environments in the interior regions of
the Pacific Northwest, while Picea sitchensis occurs in temperate coastal forests from southern Alaska to
northern California. We therefore allowed Larix to occur as both a boreal summergreen conifer (as in
Eurasia) and a cool-temperate conifer, while Picea was classified as both a boreal evergreen conifer (as in
Eurasia) and a cool-temperate conifer.
The species-level identifications of the midden plant macrofossils led us to define five new PFTs: woodland
conifer (wc), woodland shrub (ws), frost-sensitive desert shrub or succulent (ds2), desert shrub or succulent
(ds) and steppe shrub (ss). Woodland conifers include pinyon pine and woodland juniper, whereas the
Biome reconstructions for western North America
7
woodland shrub PFT includes species in the genera Amelanchier, Berberis, Cercocarpus, Cowania,
Fraxinus, Garrya, Prunus and Ribes, among others. Frost-sensitive desert shrubs or succulents include
plants characteristic of the present-day Sonoran Desert, including species in the genera Agave, Bursera,
Cereus, Cercidium and Ferocactus, among others. The desert shrub PFT includes less-frost-sensitive
species including species in the genera Acacia, Celtis, Coleogyne, Dalea and Ephedra. Steppe shrubs
include species in the genera Artemisia, Atriplex, Chrysothamnus, Ephedra, Gutierrezia and Grayia, among
others.
We created a PFT to biome matrix (Table 6) to assign the PFTs in Tables 4 and 5 to the biomes defined in
Prentice et al. (1992) and Prentice et al. (1996). In the case of tie-breaks, biomes are assigned in the order
they appear in Table 6.
We identified a new biome (open conifer woodland: OC) to represent the pinyon-juniper woodlands and
subalpine conifer woodlands of the American Southwest, Great Basin and Colorado Plateau. This biome is
characterized by the co-occurrence of eurythermic conifer, woodland conifer, woodland shrub, steppe shrub
and steppe forb PFTs. The biome occurs in dry conditions near the lower moisture requirements of conifers,
and is associated with colder conditions than the xerophytic woods/scrub biome in Europe (Prentice et al.,
1996), Africa (Jolly et al., 1998b) and southern California (see discussion above). The open conifer
woodland biome can be identified from both pollen and midden data.
The biomization procedure required some modifications when applied to the packrat midden data. In
particular, the highly localized sampling area represented by packrat midden assemblages, coupled with the
depauperate arboreal flora of much of the region, means that they characteristically contain relatively few
fossil remains from trees and shrubs but an abundance of material from forbs, many of which may have
been represented by only a few individual plants. If the standard method of calculating affinity scores was
applied in this situation, it would be extremely difficult to generate assignments to arboreal biomes. We
therefore applied a weighting scheme to the packrat midden assemblages, whereby tree macrofossils were
given a weighting of 3, shrub macrofossils were given a weighting of 2 and forbs were given a weighting of
1. These weighting factors were determined through experimentation: we tried several schemes and selected
the weightings that provided the best biomization of present-day midden assemblages when compared with
the mapped modern extents of biomes interpreted from the Küchler (1964) map of potential natural
vegetation in North America. Except for these weighting factors, the calculation of affinity scores and the
assignment of packrat midden samples to biomes was done in the standard way.
Biome reconstructions for western North America
(A)
RESULTS
(B)
Predicted vs observed modern biomes
8
Pollen data provide the evidence for biome reconstructions in the more northerly and higher elevation
biomes (Fig. 3), whereas packrat middens are largely restricted to the Southwestern deserts, Great Basin and
Colorado Plateau. The pollen- and macrofossil-based reconstruction of modern biome distributions (Fig. 3)
corresponds reasonably well with the observed patterns in vegetation distribution (Küchler, 1964; Fig. 2).
The desert, steppe, conifer forest and taiga vegetation are correctly placed geographically. The predicted
and observed geographic patterns for selected biomes are as follows:
Tundra. Only one pollen site (Little Lake, OR; Worona & Whitlock, 1995) is reconstructed as representing
this biome and it occurs within a present-day region of cool conifer forest. This misassignment is due to the
high percentages of Alnus, Cyperaceae and Poaceae that occur at this site, presumably due to historic land
use.
Taiga. It is difficult to use pollen data to discriminate among the taiga, cool conifer forest, cold mixed forest
and cool mixed forest biomes in western North America. However, biomization of the modern pollen data
do result in the correct placement of taiga along the Idaho/Montana border and in south-central Utah.
Cold deciduous forest. The core-top pollen spectrum from Waits Lake, WA (Mack et al., 1978), in a region
of present-day cool conifer forest, is incorrectly assigned to the cold deciduous forest biome. This biome
has an identical taxon list with the taiga, cool conifer, cold mixed and cool mixed forest biomes, except that
the cold deciduous forest biome lacks Picea and Abies. The Waits Lake surface spectrum is dominated by
Pinus pollen with significant representations of Alnus and Larix. The allocation to cold deciduous forest
occurs because Abies is present in a very small amount and Picea is absent.
Cool conifer forest. Cool conifer forest is correctly reconstructed from pollen spectra in the Pacific
Northwest and Sierra Nevada, as well as in the northern panhandle of Idaho and in adjacent Montana.
However, in the Yellowstone (Wyoming) region, Colorado and Utah, modern core-top pollen spectra from
this biome (as mapped in Fig. 2) are often misassigned to the open conifer woodland biome. Pinus, the most
prolific pollen producer in western North America, is present in both cool conifer forest and open conifer
woodland as is Cupressaceae, which in these regions is represented by the genus Juniperus (another
abundant pollen producer). Cool conifer forest includes many taxa not present in open conifer woodland,
including: (1) coniferous forest trees (Abies, Pseudotsuga, Picea); (2) riparian hardwoods with restricted
geographic coverages and low representations in pollen spectra in these regions (Alnus, Betula, Cornus,
Corylus, Salix); and (3) taxa that are poorly recorded in pollen spectra (Ericaceae, Populus, Shepherdia
Biome reconstructions for western North America
9
canadensis). Our data set indicates that most sites in these cool conifer forests have less than 1%
Pseudotsuga (although this tree is common in these forest), and at most only a few percent of Abies and
only rare Picea. The under-representation of major taxa in the pollen assemblage makes it difficult to obtain
a correct biome assignment. For example, Edwards et al. (this issue) have shown that it is difficult to
correctly predict the extent of the Larix-dominated cold deciduous forests in eastern Siberia because Larix
is chronically under-represented in the pollen record. The cool conifer forest samples in the Rocky
Mountains have an additional problem in that they contain significant amounts of Artemisia, Poaceae and
other steppe taxa. These taxa occur in open conifer woodland but not in cool conifer forests. Their
occurence at high elevation sites in the Rocky Mountains is presumably due to pollen blown upslope from
lower elevation habitats. Collectively, the under-representation of key species of the cool conifer forest and
the presence of steppe taxa in the pollen spectra results in the misassignment of samples to the open conifer
woodland biome.
Open conifer woodland. The distribution of open conifer woodland (Fig. 2) is reasonably well captured in
the biome reconstruction (Fig. 3), although pollen-based reconstructions overestimate the extent of the
biome in the central and northern Rocky Mountains. Pollen spectra in modern steppe/grassland
environments in eastern Washington, western Montana, central Wyoming, northeastern Colorado and the
Great Basin are also mistakenly categorized as open conifer woodland. This is the reverse of the situation
described in the preceding paragraph, as here the long-distance blow-in of Pinus pollen (e.g. Mack &
Bryant, 1974) combined with the local steppe/grassland taxa in the pollen spectra, results in a wrong
assignment to open conifer woodland. Packrat midden assemblages, with their species-level inventory of
plant remains, do not suffer the same problems as pollen data in representing this biome.
Xerophytic woods/scrub. The single midden-based reconstruction of xerophytic woods/scrub in California is
correctly placed. There are no pollen or midden records yet from the larger area of present-day xerophytic
woods/scrub in southern California (Fig. 2).
Steppe/grassland. There are cases where the biomization of pollen data results in an incorrect categorization
of steppe or grassland as open conifer woodland due to long-distance transport of conifer pollen. In other
cases, such as the Ruby Marshes in northeastern Nevada (Thompson, 1992), the pollen site itself is located
in steppe, but woodland vegetation occurs within a few kilometers. Here the steppe/grassland and open
conifer woodland biomes form a mosaic, and we do not consider this to be a failed biomization from a
regional perspective.
Desert. The modern hot deserts of the American Southwest and northwest Mexico are recorded only in
packrat midden assemblages, as this environment is typically too arid for lakes. The present-day desert
biome is correctly placed by the midden data, even in a situation such as the Grand Canyon where pockets
Biome reconstructions for western North America
10
of modern desert occur (and are correctly reconstructed from the midden data; Table 3, Fig. 3) but are too
small to appear on the present-day vegetation map (Fig. 2).
0
14
C yr B.P. Summary. Although there are minor problems with the assignment of individual sites to
biomes, it is clear that the biomization technique adequately captures the complex patterns in the vegetation
of western North America and thus can be used to reconstruct past changes in vegetation distribution with
some confidence. However, there are problems, including: (1) the differing taxonomic resolution and spatial
coverage of pollen spectra and packrat midden plant assemblages affect the results in that middens provide
an extremely local but taxonomically precise view of biomes, whereas pollen data provide a more
taxonomically generalized and regional perspective (especially in mountainous regions where significant
amounts of pollen from one vegetation association can blow into another); (2) present-day land use can
skew pollen assemblages and lead to misassignments of surface pollen samples to biomes; (3) the
depauperate tree flora can make it difficult to segregate forest biomes, particularly because trees such as
Picea and Abies occur in both boreal and temperate plant assemblages; and (4) certain pollen types that are
key markers for specific biomes (e.g. Pseudotsuga, Larix, and Abies) are under-represented in pollen
assemblages.
The taiga, cool conifer, cold mixed and cool mixed forest biomes in western North America are difficult to
differentiate on the basis of pollen assemblages. All four share the following taxa: Abies, Alnus, Betula,
Cornus, Cupressaceae, Ericaceae, Larix, Larix/Pseudotsuga, Myricaceae, Picea, Pinus, Populus, Salix,
Shepherdia canadensis and the combined taxon Taxodiaceae/Cupressaceae/Taxaceae. In their original
definitions (Prentice et al., 1992, 1996), the cold mixed forest biome was characterized by the presence of
the ctc1 PFT (see Table 5 for key to abbreviations), whereas cool conifer forests were characterized by the
bec PFT. In Europe, obligate ctc1 taxa (e.g. Cedrus and Taxus) are not common in the pollen records and
the distinction between cold mixed and cool conifer forests largely rests on the presence of obligate bec taxa
such as Picea and Pinus (Hyploxylon). Undifferentiated Picea is not confined to ctc1 in western North
America because it is also considered a cool-temperate conifer, and Pinus (Hyploxylon) in this region
covers a range of environments from subalpine to open conifer woodland. Consequently, for western North
American pollen records, the cool conifer and cold mixed forest biomes have identical taxon lists and are
assigned to the cool conifer forest biome.
The pollen-based definition of the taiga biome lacks Corylus, Pseudotsuga, Taxaceae, Taxodiaceae and
Tsuga, taxa that are present in the cool conifer, cold mixed and cool mixed forest biomes. Most
palynologists have not distinguished Pseudotsuga from Larix, nor Taxaceae and Taxodiaceae from each
other or from Cupressaceae. Corylus is rarely recorded, and is never abundant, in western pollen records. In
practice, then, it is the presence, absence, or abundance of Tsuga pollen that allows us to distinguish taiga
from the other three biomes. Tsuga is present only in the northern Rocky Mountains, Cascade Range and
Biome reconstructions for western North America
11
Sierra Nevada, so it is only in those regions that one could potentially differentiate taiga from the other
forest biomes. The cool mixed forest biome includes several taxa that are not present in the other three
forest biomes: Acer, Ceanothus, Fraxinus, Quercus, Rhamnus. These taxa are usually found in minor
amounts in pollen records in western North America, and it is probable that in most cases the cool mixed
forest biome cannot be reliably separated from the other three forest biomes.
The mountainous terrain of western North America may further confound attempts to differentiate taiga,
cool conifer, cold mixed and cool mixed forest biomes in pollen spectra. Wind-blown pollen can be
translocated from one vegetation association to another in a short geographic (but perhaps large elevational)
distance. In addition, many of the potentially distinctive taxa for these biomes are poor pollen producers
(e.g. Acer, Ceanothus, Corylus, Fraxinus and Rhamnus) and may not be recorded in pollen spectra from
their native vegetation.
The taxonomic resolution of packrat midden macrofossil assemblages makes it possible to segregate the
species characteristic of the forest biomes. Unfortunately, packrat middens are not preserved in wet
environments, and although cool conifer and cold mixed forest biomes are recognized in fossil middens, the
vast majority of the midden assemblages reflect open conifer woodland, steppe and desert biomes.
(B)
Western North American biomes at 6000 14C yr B.P.
The predicted distribution of biomes across western North America at 6000 14C yr B.P. (Fig. 3) is similar to
the modern distribution, in that forests were established in the Pacific northwest, open conifer woodland
occurred in the central Great Basin and the southwestern deserts had reached their modern configuration.
However, there are subtle regional differences between the reconstructed biomes at 6000 14C yr B.P. and the
modern vegetation patterns. Three biomes were reconstructed from pollen data at one site each for 0 14C yr
B.P. (cold deciduous forest, xerophytic woods/scrub, and, incorrectly, tundra), and none of these was
reconstructed from the 6000 14C yr B.P. data. The other biomes reconstructed for 0 14C yr B.P. were present
at 6000 14C yr B.P. in western North America and are discussed below.
Taiga. Taiga is rare in the 0
14
C yr B.P. reconstruction (Fig. 3) but is present at several sites in northern
Idaho and adjacent eastern Washington and western Montana at 6000 14C yr B.P. (Fig. 3). Relatively small
changes in pollen percentages occurred at the sites where taiga was replaced by cool conifer forest between
6000 and 0
14
C yr B.P. The key factor appears to be the arrival of, or increase in, Tsuga pollen in these
records. Although this genus is present at relatively low levels (1 to 5%) in the modern spectra, the presence
of this key taxon is decisive in discriminating cool conifer forest from taiga.
Biome reconstructions for western North America
12
At Waits Lake (Mack et al., 1978b), taiga at 6000 14C yr B.P. was replaced by cold deciduous forest at 0
14
C yr B.P. These two biomes share all PFTs except bec (Tables 4 and 6), which is represented only by
Abies and Picea in western North America. At Waits Lake, Abies was present at approximately 1% in the
mid-Holocene and was absent in the modern spectrum, and this minor difference caused the shift from one
biome to another. At the Lost Trail Pass Bog site in western Montana (Mehringer et al., 1977) slightly
higher levels of Larix/Pseudotsuga pollen at 6000
14
C yr B.P. resulted in identification of cool conifer
forest, and its decline to modern levels led the biomization procedure to identify taiga (which lacks ctc)
from the present day spectrum. Taiga was identified in southern Utah at both 0 and 6000 14C yr B.P. (Fig.
3).
Cool conifer forest. Fossil pollen data indicate that this biome attained its modern extent in the Pacific
Northwest prior to 6000
14
C yr B.P. (Fig. 3), and was also present in western Montana. It apparently
covered more area than today in parts of Colorado and northeast Arizona because it was reconstructed for
sites that were identified as having open conifer woodland at 0
14
C yr B.P. (although, as previously
14
discussed, the reconstructions for 0
C yr B.P. are probably in error; the sites today host a depauperate
form of cool conifer forest). For several of the Colorado pollen records (e.g. Cottonwood Lake and
Keystone Iron Bog: Fall, 1985, 1988), Picea and Abies pollen grains were more abundant at 6000 than at 0
14
C yr B.P., and declined as Pinus and Artemisia increased through the late Holocene. This taxonomic shift
from greater ctc pollen taxa to increased ec and ss pollen resulted in the change in biomes reconstructed
between the two time periods. The opposite trend occurred in the Sierra Nevada of California, where open
conifer woodland was replaced by cool conifer forest after 6000 14C yr B.P. at several sites. Here ctc pollen
taxa (Abies, Tsuga) arrived or increased between the mid- and late Holocene, while Pinus pollen declined.
Open conifer woodland. In the Great Basin and Colorado Plateau regions, open conifer woodland occupied
essentially the same area at 6000
14
C yr B.P. that it occupies today. In southeastern Idaho and adjacent
Wyoming, steppe was present at a few sites occupied today by woodland. In the 0 14C yr B.P. reconstruction
(Fig. 3), depauperate cool conifer forests in the Rocky Mountains were incorrectly reconstructed as open
conifer woodland. This is probably also the case in the same region for 6000 14C yr B.P.
Steppe/grassland. Pollen data indicate that this biome had the same geographic range at 6000 14C yr B.P. as
it has today in the Great Basin, eastern Washington, Wyoming and on the Great Plains.
Desert. Biomization of packrat midden plant assemblages indicate that the Southwestern deserts had
attained their modern northern limits by 6000 14C yr B.P.
6000
14
C yr B.P. Summary. Pollen and packrat midden data indicate that the mid-Holocene biomes of
western North America were similar to those of the present day. Cool-temperate conifer (ctc) trees were
Biome reconstructions for western North America
13
more abundant than today in pollen records in the Colorado Rockies, but less abundant in records in the
interior of the Pacific Northwest and in the Sierra Nevada. Small shifts in the percentages of these trees
between the two time periods caused apparent changes in biomes through time, although the overall region
covered by conifer and mixed forests remained essentially the same between the two time periods.
Steppe/grassland and desert biomes at 6000 14C yr B.P. occupied largely the same areas as they do today.
(B)
Western North American biomes at 18,000 14C yr B.P.
While the environments of western North America appear to have been quite similar at 0 and 6000 14C yr
B.P., they were radically different at the last glacial maximum (LGM, 18,000 14C yr B.P.). During the LGM,
the Laurentide Ice Sheet covered most of what is now Canada and montane glaciers were present in many of
the western mountains. Large lakes were present across the Great Basin and in parts of the Southwest (e.g.
Thompson et al., 1993). The reconstructed vegetation patterns were also very different (Fig. 3). The taiga
and desert biomes were apparently absent. The remaining biomes are discussed below:
Cool conifer, cold mixed, and cold deciduous forests. In the Pacific Northwest, biome reconstructions from
pollen data indicate that the areas of modern conifer and mixed forests were largely occupied by steppe
(although bec elements were present) and perhaps even tundra at the LGM (Barnosky et al., 1987).
However, cool conifer forests were present in western Oregon. This biome, along with cold mixed and cold
deciduous forest, was identified from plant macrofossils in packrat middens from various elevations in the
eastern Grand Canyon (Fig. 3).
Open conifer woodland. Pollen and packrat midden data both indicate that this biome had a greatly
expanded range (relative to today) at the LGM, suggesting that the vegetation at that time was very open.
The open conifer woodland biome occurred within its current range in the Great Basin and Colorado
Plateau, and was also present in the modern desert regions of the Southwest. Pollen records indicate its
presence in Colorado and Wyoming, and there is little evidence that these are misassigned cool conifer
forests as were the Holocene sites in these states. Cool-temperate conifers (ctc) are largely absent from
macrofossil records in the interior north of the Grand Canyon, so essential elements of the cool conifer
forests appear to have been missing. In the Great Basin and surrounding areas, the LGM open conifer
woodlands were characterized by subalpine pines and by prostrate juniper with steppe plants. These
woodlands were comprised of woodland conifers (pinyon pines and woodland junipers) in association with
steppe and cold-tolerant desert elements in those areas that are desert today.
Xerophytic woods/scrub. Warm-temperate sclerophyll scrubs (wte2) were recovered from late Pleistocene
packrat middens on the western side of the southern Sierra Nevada in California, near sites where these
Biome reconstructions for western North America
14
plants grow today. Warm-temperate sclerophyll scrubs have not been recovered from LGM sites elsewhere
in western North America.
Tundra. This biome was reconstructed for one site in western Washington, in a region where other sites
were reconstructed as steppe (Fig. 3). The presence of lemming and reindeer fossils in southern Idaho and
geomorphic evidence of frost wedges in Wyoming (Thompson et al., 1993 and references therein) suggests
that this biome may have been more widespread in the northern interior of western North America than
indicated by the currently available botanical data.
Steppe/grassland. Steppe elements were present in LGM vegetation across western North America during
LGM time. However, eurythermic conifers were present (at least regionally) in the present domain of steppe
in the Great Basin and surrounding region, so pollen and macrofossil data from this region are classified as
open conifer woodland (Fig. 3). Steppe is reconstructed for the LGM for the presently tree-covered western
Washington.
(B)
Climatic implications of changes in biomes
The correspondence of biomes interpreted from the Küchler (1964) map of potential natural vegetation map
(Fig. 2, Table 1) with climatic parameters (MTCO: mean temperature of the coldest month; GDD5: growing
degree days on a 5°C base; and α: a moisture index ranging from 0=extremely dry to 1=extremely wet
based on Thornthwaite and Mather 1955, 1957) are shown in Figs 4 and 5 and in Table 7. These climatic
data were obtained by using a 25-km equal-area grid of present-day climate values (Bartlein et al., 1994;
Thompson et al., 1999a, 1999b) and assigning each grid point in western North America to a biome based
on the Küchler (1964) map.
Tundra is restricted to mountain tops and occurs where MTCO is less than -2.5° C, GDD5 is less than 1000
and α is generally above 0.75. The GDD5 upper limit for tundra seems high when compared to other regions
(cf Prentice et al., 1992); this could reflect the difficulty in estimating present-day climate at high elevations
from the nearest low-elevation sites. However, preliminary analysis of tundra in Alaska suggests that tundra
in that region commonly occurs with GDD5 values as high as 500 to 750. The cool conifer forest biome is
the most widespread forest vegetation in the western United States and occurs under a variety of climatic
conditions (Table 7). It can occur under somewhat colder winter temperatures, and generally requires more
moisture, than open conifer woodland. Open conifer woodland is comprised of eurythermic conifers and
steppe plants and can survive under colder conditions than xerophytic woods/scrub and drier and warmer
climates than cool conifer forest. Xerophytic woods/scrub vegetation cannot survive freezing temperatures
and, as its name implies, grows under warm and dry conditions.
Biome reconstructions for western North America
15
Grassland and steppe are not distinguished as separate biomes in our reconstructions based on pollen and
macrofossil data. However, they are segregated on the Küchler (1964) map (Fig. 2), and Figs 4 and 5
illustrate their climatic differences. Grassland occurs on the Great Plains, in parts of the Southwest, in the
Central Valley of California and in the interior Pacific Northwest. Steppe is widespread across the semiarid
interior of the western United States from the Great Plains to the lee of the Sierra Nevada and Cascade
Ranges. Grassland survives under a wide variety of winter temperatures (MTCO -15 to 10° C), and is
associated with GDD5 2000 to 4500 and α of 0.4 to 0.7. Steppe overlaps with grassland in its climatic
tolerances, but can live in drier environments (α as low as 0.15), has lower GDD5 requirements (GDD5 as
low as 1000) and does not occur under warm winter conditions (MTCO is below 0°C). Desert occurs in the
Southwest and in southeastern-most California. It has lower moisture requirements than steppe and
grassland (α between 0.05 and 0.4), higher GDD5 requirements (3000 to 6000) and MTCO above freezing.
The climates associated with our 0 14C yr B.P. reconstructed biomes are illustrated in Fig. 6 (see also Table
7b). Where we do not have precise elevational data for a site, we associated the climate of the closest
gridpoint on our 25-km grid with the pollen or packrat midden site. Climatic “envelopes” for the Küchlerbased biomes were visually interpreted from Fig. 4 and are included on Fig. 6 to allow comparisons of the
climates of the Küchler-based biomes and the reconstructed biomes. The left-hand panels of Fig. 6 illustrate
the reconstructed climates for the cool conifer forest localities. Nearly all of the sites identified as belonging
to this biome fall within the climate envelope from the Küchler-based biome, and thus it appears that the
biomization assigns these surface pollen samples correctly.
The central panels of Fig. 6 plot the climates for the 0 14C yr B.P. sites that were reconstructed as having
open conifer woodland vegetation. In these panels the climatic envelopes (from Fig. 4) for this biome are
shown, along with those for cool conifer forest. Many sites in modern cool conifer forest in the northern
Rocky Mountains were incorrectly placed in open conifer woodland by the biomization procedure. The data
in Fig. 6 show these sites occurring within the moisture requirements of the cool conifer forest biome
(which exceed those of open conifer woodland) but at the cold end of the cool conifer forests. Inspection of
these data indicate that the cool conifer forest sites that are misidentified as open conifer woodland occur
near the lower limits of the forest in relatively dry regions of northeastern Nevada, central Utah, and western
Colorado. At the cold end of its tolerance many of the ctc trees are absent from this biome, and pollen
spectra converge on those of the open conifer woodland.
The right-hand panels of Fig. 6 illustrate the apparent modern climates of sites reconstructed as
steppe/grassland or desert biomes at 0
14
C yr B.P. In this plot the steppe/grassland and desert samples
adhere closely to the freezing-line divide apparent in the Küchler-based data. However, sites within both
Biome reconstructions for western North America
16
biomes appear to exceed the permissible values for the moisture index. Here the assignment of nearest gridpoint climates may be causing difficulties, as in this region of high relief we may be incorrectly using
precipitation values from higher elevation sites.
(B)
Vegetation and climate of western North America at 6000 14C yr B.P.
The presence of cool conifer forest in the maritime region of the Pacific Northwest at 6000
14
C yr B.P.
suggests that relatively little climatic change occurred there between the mid-Holocene and the present. In
the interior part of this region, the shift from taiga at 6000 14C yr B.P. to cool conifer and cold deciduous
forests at 0
14
C yr B.P. may reflect a shift to milder climates in the late Holocene, with modern warmer
winters perhaps being the key factor. The reconstruction of cool conifer forests at several sites in Colorado
and northeastern Arizona that are characterized as open conifer woodland or steppe in the modern
reconstruction indicates that conditions were wetter than present at 6000 14C yr B.P. In contrast, steppe was
more extensive in the northern Great Basin at 6000 14C yr B.P. compared to present, and sites in the Sierra
Nevada that are in cool conifer forest today were in open conifer woodland during the mid-Holocene. These
shifts in vegetation imply drier than modern conditions in the northern Great Basin and Sierra Nevada at
6000 14C yr B.P. As noted by Williams et al. (this issue), warmer and drier climates at 6000 14C yr B.P. are
also reflected in the contraction of forests and expansion of grassland in the upper Midwest.
The contrast between drier conditions in these regions and wetter conditions in the southern Rocky
Mountains has been noted in previous reconstructions of the mid-Holocene climates of western North
America (e.g. Thompson et al., 1993), and, as discussed in that paper, appears to reflect an enhanced
summer monsoon circulation in the Southwest coupled with warmer than modern conditions across most of
western North America (as illustrated in Fig. 1, the area of relatively moist mid-Holocene climates lies
within the modern track of the summer monsoon). Despite this proposed augmentation of the summer
monsoon, the biome reconstructions imply little discernible difference between 6000 and 0 14C yr B.P. in
the deserts of the Southwest.
(B)
Vegetation and climate of western North America at 18,000 14C yr B.P.
The occurrence of steppe and tundra at the LGM in now-maritime western Washington indicates conditions
both colder and drier than today (Barnosky et al., 1987). Cold climates associated with steppe are also
inferred for the northern Great Plains near the Laurentide Ice Sheet (Williams et al., this issue). The
presence of cool conifer forest in western Oregon at 18,000
14
C yr B.P. indicates that there was a strong
temperature and moisture gradient along the coast of the Pacific Northwest.
Biome reconstructions for western North America
17
Open conifer woodland was present at most of the LGM sites from Wyoming south to the Mexican border,
indicating open vegetation with MTCO remaining above -10°C (Fig. 4). In the Great Basin and adjacent
regions, this woodland included xerophytic subalpine conifers instead of the current dominance of
woodland conifers, suggesting that the climate was at the cold and dry end of the range of open conifer
woodland. In contrast, this biome in the present-day Southwestern deserts included pinyon pine and
woodland juniper, suggesting that the climate was cool (not cold) and wet relative to today. Open conifer
woodland also occurred in the southeastern United States during the LGM (Williams et al., this issue), and
it is possible that it is favored by the lower levels of atmospheric carbon dioxide that occurred in the late
Pleistocene (as may have occurred with dry-tropical biomes: Jolly & Haxeltine, 1997).
Overall, the distributions of biomes in western North America at 18,000
14
C yr B.P. are consistent with
earlier palaeoclimatic reconstructions (Thompson et al., 1993) and modelling (Kutzbach et al., 1993, 1998;
Thompson et al., 1993). Key factors of the LGM climate that differed significantly from today include: (1) a
southward displacement of the westerlies, and their enhanced persistence through the year; (2) virtual
elimination of the present summer monsoon circulation; and (3) a strong temperature gradient through the
interior region, with very cold temperatures to the north and mild conditions along the Mexican border.
(A)
DISCUSSION AND CONCLUSIONS
(B)
The biomization method
Pollen and packrat midden data appear to provide realistic reconstructions of present-day biomes in western
North America. However, to achieve these results the weighting had to be increased for trees (3x) and
shrubs (2x) to be able to reconstruct the woodland, forest and desert biomes. Our ability to differentiate
among taiga, cool conifer forest, cold deciduous forest and cold mixed forest from pollen data is limited due
to a depauperate flora and taxonomic overlap among the PFTs. In addition, our ability to discriminate
between cool conifer forest (at the cold end of its range) and open conifer woodland appears to be limited
by the very low representations of key ctc trees (especially Pseudotsuga) and by blow-in of pollen from
steppe plants in regions of high relief.
The new biome, open conifer woodland, provides a good description of both the modern and late
Pleistocene vegetation of the semiarid interior of western North America. It likely occurs in other regions of
the world. For example, the vegetation of the colder, drier parts of the circum-Mediterranean region is also
characterised by an intimate mix of conifers and steppic vegetation (e.g. the juniper woodlands of interior
Spain and Greece: Polunin & Walters, 1985). However, vegetation types that are apparently analogous to
our open conifer woodland biome occupy rather limited areas in Europe and the Middle East today. This
may explain why they were not classified as a separate biome in the original biomization of Europe
Biome reconstructions for western North America
18
(Prentice et al., 1996) and have not been recognised in subsequent biomizations (e.g. Jolly et al., 1998b).
Given that juniper woodlands may have been more widely distributed in Eurasia during other periods of the
late Quaternary, it would be useful to re-examine the vegetation records from these other regions in the light
of our recognition of open conifer woodlands as a distinct biome.
Single examples of plant macrofossils from a few sites have been used to supplement pollen data in
biomizations of other regions (e.g. northern Russia: Texier et al., 1997; Africa: Jolly et al., 1998b).
However, the biomization procedure has never been applied to assemblages of plant macrofossils or to an
extensive plant macrofossil data set encompassing a range of different biomes. Our reconstructions show
that identical biome reconstructions are obtained using plant macrofossils and pollen spectra, when both
types of data are available from the same region. Specifically, the changing extent of open conifer woodland
since the LGM is clearly documented from both pollen and plant macrofossil sites. The comparability of
biome reconstructions from the two data sources opens up new possibilities for the systematic mapping of
vegetation changes during the late Quaternary in more arid regions where pollen data are sparse. Although
packrat middens are unique to western North America, similar deposits have been found in e.g. the Middle
East (Fall et al., 1990), South Africa (Scott, 1990) and Australia (Nelson et al., 1990), as well as in South
America. The systematic exploitation of the macrofossil records from these deposits could substantially
enhance our understanding of the Late Quaternary dynamics of arid regions.
Pollen and plant macrofossils record vegetation at rather different spatial scales: pollen data provide
regional reconstructions of vegetation changes at a coarse level of taxonomic precision, while macrofossil
data provide records of more local changes based on species-level identifications. Potentially, these
differences in spatial resolution could be exploited in several ways. For example, macrofossil data could be
used to determine whether specific taxa represented in a pollen record were locally present or were derived
by long-distance transport. Takahara et al. (this issue) have shown that the upslope transport of pollen from
extra-local vegetation leads to incorrect biome reconstructions for nearly half of the pollen records from
Japan. The use of plant macrofossil data to establish local presence, in conjunction with pollen data, could
therefore substantially improve biome reconstructions in mountainous regions. In cases where different
species within a genus are characteristic of different biomes, such as is the case with different species of
Pinus, Abies and Picea in western North America, it may also be possible to exploit the higher taxonomic
resolution of plant macrofossil data to guide biomizations at pollen sites (Jolly et al., 1998b; Takahara et
al., this issue).
(B)
Changes in the distribution of PFTs in western North America since the last glacial maximum
Plant species have responded to changes in climate and atmospheric chemistry in individualistic fashions
over the time since the LGM (Thompson, 1988). We have explored aspects of this history in regard to
Biome reconstructions for western North America
19
changes in biomes. Aspects of the vegetation history of western North America can be also seen in the
changing affinity scores of PFTs. For example, boreal evergreen conifers (bec) were present across most of
the region at 18,000 14C yr B.P. (Fig. 7), except for the present-day low elevation deserts of the Southwest.
By 6000 14C yr B.P. this PFT attained its modern range in the Rocky Mountains and Pacific Northwest, and
lost much of its late Pleistocene habitat in the Southwest. Woodland conifers (wc) also changed their
distributions through time: they lived in the modern Southwestern deserts at 18,000
14
C yr B.P. and
14
dispersed northward and upward in elevation to near their modern limits by 6000 C yr B.P. (as they lost
their late Pleistocene habitats in the Southwestern deserts).
The steppe shrub PFT (ss) was important across its modern range at 18,000
14
C yr B.P., and was also
present in the Pacific Northwest and the Southwestern deserts (Fig. 8). This implies that this complex of
species has been a major constituent of western vegetation under both late Pleistocene and Holocene
climatic regimes. In contrast, taxa from the desert shrub or succulent PFT (ds) were minor components of
western vegetation at the LGM, and have expanded into their modern range during the Holocene.
Collectively the histories of the four PFTs in Figs 7 and 8 illustrate differing patterns of PFT response to
climatic change. Some, such as the steppe shrub PFT, have maintained near-modern geographic (although
perhaps not elevational) ranges since the LGM, whereas others have modified their geographic ranges to
greater or lesser extents.
(B)
Vegetation and climate changes in western North America in the Late Quaternary
The vegetation reconstructed through the objective biomization method in western North America is in
good agreement with more informal reconstructions for 18,000, 6000 and 0 14C yr B.P. (e.g. Thompson et
al., 1993). The biomization procedure provides a uniform method of describing vegetation changes through
time, as well as providing a potential means to identify underlying climatic causes. Geologic and biologic
data indicate that the present-day climatic patterns (Fig. 1) of western North America were greatly changed
at the LGM. For example, the present-day climate of western Washington State in the Pacific Northwest is
characterized by high levels of precipitation and mild temperatures. This was apparently not true at 18,000
14
C yr B.P.; instead glacial, faunal and vegetation data suggest that this region experienced dry climates with
cold winters (Barnosky et al., 1987; Thompson et al., 1993). In contrast, the desert regions of the
Southwestern United States today have hot and dry climates, whereas palaeolacustrine, faunal and
vegetational data indicate that the climate was relatively cool and moist at 18,000 14C yr B.P. (Street-Perrott
et al., 1989; Thompson et al., 1993), apparently due to a southerly displacement (relative to today) of the
westerlies. The biomes reconstructed for both of the regions at the LGM fit well within this framework, with
the cold dry climates of the Northwest being associated with steppe and tundra, and the cool moist
Southwestern climates with open conifer woodlands (Fig. 3). The reconstructed biomes for the midHolocene are also well aligned with previous palaeoclimatic syntheses. Particularly noteworthy are the
Biome reconstructions for western North America
20
contrasting changes in biomes in the southern Rocky Mountains and Sierra Nevada. The former area
apparently fell within a region of greater-than-modern summer monsoonal precipitation at 6000 14C yr B.P.
(Fall, 1988; Thompson et al., 1993), while the latter area experienced greater-than-present drought at this
time (Anderson, 1987; Thompson et al., 1993). The reconstructed biomes reflect this pattern, with the
Rocky Mountains having more sites covered with cool conifer forest at 6000 14C yr B.P. than today, while
several present-day cool conifer forest sites in the Sierra Nevada were open conifer woodland in the midHolocene (Fig. 3).
(A)
ACKNOWLEDGEMENTS
This paper is a contribution to BIOME 6000. Sandy Harrison graciously provided editorial advice and
motivation. Our assignment of pollen taxa to PFTs was made in consultation with Pat Anderson, Pat
Bartlein, Mary Edwards, Eric Grimm, Steve Jackson, Pierre Richard, Tom Webb, Cathy Whitlock and Jack
Williams. We thank Jack Williams for the software used in the biomization procedure, and Tom Webb, Dan
Muhs and Jack Williams for detailed and constructive reviews. We thank Laura Strickland for her help in
data preparation. Silvana Schott provided a detailed technical edit of the manuscript. We are grateful to
Colin Prentice for advice on the application of the biomization procedures to Western North America and
for comments on an earlier draft of the manuscript.
(A)
BIOPIC
Robert S. Thompson is a geologist with the Global Change and Climate History Team of the U.S.
Geological Survey who studies late Cenozoic vegetation and climate change, primarily in western North
America. Katherine H. Anderson is a research associate with the Institute for Arctic and Alpine Research
(INSTAAR) at the University of Colorado. She applies quantitative techniques to reconstruct past climates
from pollen and plant macrofossil data in North America.
Biome reconstructions for western North America
21
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Biome reconstructions for western North America
31
TABLE AND FIGURE CAPTIONS:
Table 1. Present-day biomes in western North America interpreted from Küchler (1964; see Fig. 2).
Table 2. Characteristics of the 0, 6000 and 18,000
14
C yr B.P. pollen sites from western North America.
Site names with asterisks indicate digitized data, those without an asterisk were taken from the North
American Pollen Database from the sample level closest to the target age. Negative radiocarbon ages
indicate dates that are younger than A.D. 1950. Dating control (DC) codes are based on the COHMAP
dating control scheme (Webb, 1985; Yu & Harrison, 1995). For site with continuous sedimentation
(indicated by a C after the numeric code), the dating control is based on bracketing dates where 1 indicates
that both dates are within 2000 years of the selected interval, 2 indicates one date within 2000 years and the
other within 4000 years, 3 indicates both within 4000 years, 4 indicates one date within 4000 years and the
other within 6000 years, 5 indicates both dates within 6000 years, 6 indicates one date within 6000 years
and the other within 8000 years, and 7 indicates bracketing dates more than 8000 years from the selected
interval. For sites with discontinuous sedimentation (indicated by D after the numeric code), 1 indicates a
date within 250 years of the selected interval, 2 a date within 500 years, 3 a date within 750 years, 4 a date
within 1000 years, 5 a date within 1500 years, 6 a date within 2000 years, and 7 a date more than 2000
years from the selected interval. Biome codes (Biome) are given in Table 6. For mapping purposes some
sites (indicated by ‡) which are too close to one another have been displaced slightly.
Table 3. Characteristics of the 0, 6000 and 18,000
14
C yr B.P. packrat midden sites from western North
America. Dating control codes (DC) follow the scheme described in Table 2. Biome codes (Biome) are
given in Table 6. For mapping purposes some sites (indicated by ‡) which are too close to one another have
been displaced slightly.
Table 4. Assignments of pollen taxa from western North America to the PFTs used in the biomization
procedure.
Table 5. Assignments of plant macrofossil (midden) taxa from western North America to the PFTs used in
the biomization procedure.
Table 6. Assignment of PFTs to biomes used in the biomization for western North America.
Biome reconstructions for western North America
32
Table 7. Climatic tolerances of biomes in western North America, based on (a) present-day potential natural
vegetation, and (b) present-day pollen and plant macrofossil assemblages. MTCO temperatures are
approximated to the nearest 2.5° C; GDD5 values to the nearest 500 growing-degree days; and α values to
the nearest 0.05. The values in 7 (a) were obtained through visual inspection and interpretation of Figs 5
and 6; the values in 7 (b) through visual inspection and interpretation of Fig. 5. Cold deciduous forests and
xerophytic woods/scrub are omitted from 7 (b) because they only occur once.
Figure 1. Present-day climate of western North America. These four panels illustrate the biseasonal
precipitation regime and the range of temperature and precipitation conditions experienced over this
mountainous region. Modern mean temperature (right) and mean precipitation (left) in January (top) and
July (bottom) over western North America.
Figure 2. Vegetation of the western United States based on the Küchler (1964) map of potential natural
vegetation. The categories used by Küchler have been grouped to approximate the biomes used by Prentice
et al. (1996) and in this paper (with the exception of the steppe and grassland categories, which can be
differentiated here but are more difficult to discriminate in pollen and macrofossil data).
Figure 3. Biomes for 0, 6000, and 18,000 14C yr B.P. in western North America. The 0 14C yr B.P. biomes
are on: (1) pollen assemblages from modern core tops from fossil pollen sites and lacustrine surface
samples, and (2) packrat middens from the past 1000 radiocarbon years. Pollen sites are represented by a
circle, midden sites by a triangle.
Figure 4. Bivariate plots of the estimated modern climatic ranges of the vegetation categories shown in Fig.
2.
Figure 5. Histograms illustrating the univariate estimated modern climatic ranges of the vegetation
categories shown in Fig. 2.
Figure 6. Bivariate plots of the estimated modern climatic ranges of the western North American biomes
mapped for 0
14
C yr B.P. in Fig. 3. Pollen sites are represented by a circle, midden sites by a triangle.
Figure 7. Affinity scores for selected plant functional types in western North America for 18,000, 6000, and
0
14
C yr B.P. The upper series of panels illustrates the occurrence of boreal evergreen conifers including
species of Picea, Abies, and Larix. The lower series illustrates the past occurrence of species of pinyon
Biome reconstructions for western North America
33
pines and woodland junipers (woodland conifers). Pollen sites are represented by a circle, midden sites by a
triangle.
Figure 8. Affinity scores for selected plant functional types in western North America for 18,000, 6000, and
0
14
C yr B.P. The upper series of panels illustrates the occurrence of steppe plants, such as Artemisia,
whereas the lower series illustrates shrub and succulent species that are now largely confined in hot deserts.
Pollen sites are represented by a circle, midden sites by a triangle.
Biome reconstructions for western North America
34
Table 1 Present-day biomes in western North America interpreted from Küchler (1964; see Fig. 2).
Biome
tundra
cool conifer forest
temperate
deciduous/cool mixed
forest
open conifer
woodland
xerophytic
woods/scrub
grassland
steppe
desert
Küchler categories
alpine meadows and barren
spruce-cedar-hemlock forest, cedar-hemlockDouglas fir forest, silver fir-Douglas fir forest,
fir-hemlock forest, mixed conifer forest,
redwood forest, red fir forest, lodgepole pinesubalpine forest, pine-cypress forest, western
ponderosa forest, Douglas fir forest, cedarhemlock-pine forest, grand fir-Douglas fir forest,
western spruce-fir forest, eastern ponderosa
forest, Black Hills pine forest, pine-Douglas fir
forest, Arizona pine forest, spruce-fir-Douglas fir
forest, Southwestern spruce-fir forest, western
ponderosa forest, Oregon oak/California mixed
evergreen forest
cedar-hemlock-Douglas fir forest, Oregon oak
woodlands, California mixed evergreen forest,
California oaklands, oak-juniper woodlands,
mountain mahogany-oak (in part), northern
floodplain forest
juniper-pinyon woodland, juniper woodland
Key genera
Agrostis, Carex, Festuca, Poa
Abies, Cupressus, Picea, Pinus,
Pseudotsuga, Sequoia, Thuja, Tsuga
chapparal, coastal sagebrush, California oaklands
(in part)
Adenostoma, Arctostaphylos,
Ceanothus, Eriogonum, Quercus,
Salvia
Acacia, Agropyron, Andropogon,
Aristida, Artemisia, Bouteloua,
Buchloe, Danthonia, Distichlis,
Festuca, Hilaria, Juniperus,
Muhlenbergia, Poa, Prosopis,
Quercus, Setaria, Spartina, Stipa
fescue-oatgrass, California steppe, California
tule marshes, fescue-wheatgrass, wheatgrassbluegrass, grama-galleta steppe, grama-tobosa
prairie, wheatgrass-needlegrass shrubsteppe,
galleta-three awn shrubsteppe, mesquite-buffalo
grass, mesquite-acacia-savanna, foothills prairie,
grama-needlegrass-wheatgrass, grama-buffalo
grass, wheatgrass-needlegrass, wheatgrassbluestem-needlegrass, wheatgrass-grama-buffalo
grass, bluestem-grama prairie, sandsagebluestem prairie, shinnery, northern cordgrass
prairie, oak savanna, juniper-oak savanna,
fescue-mountain muhly prairie
sagebrush steppe, Great Basin sagebrush steppe,
saltbush-greasewood, mountain mahogany-oak
(in part)
mesquite bosques; blackbrush, creosote bush,
creosote bush-bur sage, palo verde-cactus shrub,
grama-tobosa shrubsteppe, ceniza shrub, gramatobosa shrubsteppe, Trans-Peco shrub savanna
Arbutus, Cercocarpus, Juniperus,
Populus, Pseudotsuga, Quercus, Salix,
Thuja, Tsuga, Ulmus
Artemisa, Juniperus, Pinus
Agropyron, Artemisia, Atriplex,
Cercocarpus, Quercus, Sarcobatus
Cercidium, Colegyne, Flourensia,
Franseria, Larrea, Leucophyllum,
Opuntia, Prosopis
Biome reconstructions for western North America
35
Table 2 Characteristics of the 0, 6000 and 18,000 14C yr B.P. pollen sites from western North America. Site
names with asterisks indicate digitized data, those without an asterisk were taken from the North American
Pollen Database from the sample level closest to the target age. Negative radiocarbon ages indicate dates that
are younger than A.D. 1950. Dating control (DC) codes are based on the COHMAP dating control scheme
(Webb, 1985; Yu & Harrison, 1995). For site with continuous sedimentation (indicated by a C after the
numeric code), the dating control is based on bracketing dates where 1 indicates that both dates are within
2000 years of the selected interval, 2 indicates one date within 2000 years and the other within 4000 years, 3
indicates both within 4000 years, 4 indicates one date within 4000 years and the other within 6000 years, 5
indicates both dates within 6000 years, 6 indicates one date within 6000 years and the other within 8000
years, and 7 indicates bracketing dates more than 8000 years from the selected interval. For sites with
discontinuous sedimentation (indicated by D after the numeric code), 1 indicates a date within 250 years of
the selected interval, 2 a date within 500 years, 3 a date within 750 years, 4 a date within 1000 years, 5 a date
within 1500 years, 6 a date within 2000 years, and 7 a date more than 2000 years from the selected interval.
Biome codes (Biome) are given in Table 6. For mapping purposes some sites (indicated by ‡) which are too
close to one another have been displaced slightly.
Site
no.
1
3
4
5
6
18
20
21
22
24
26
28
29
30
31
32
37
40
42
43
44
45
46
47
48
49
50
51
53
54
55
57
59
60
61
62
56
65
68
69
70
71
73
74
75
76
77
78
79
83
85
88
89
92
94
Site name
MODERN SAMPLES
Alkali Creek*
Antelope Playa*
Balsam Meadows*
Barrett Lake*
Battle Ground Lake*
Como Lake*
Cottonwood Pass Pond*
Creston Fen*
Cub Creek Pond*
Cygnet Lake Fen
Dead Man Lake*
Diamond Pond*
Divide Lake*
Dome Creek Meadow*
Emerald Lake
Exchequer Meadow*
Fryingpan Lake*
Gold Lake Bog
Gray's Lake*
Great Salt Lake*
Guardipee Lake
Hager Pond*
Hall Lake*
Hay Lake, Arizona
Head Lake*
Hedrick Pond
Hidden Cave*
Hoh Bog*
Hurricane Basin*
Ice Slough*
Indian Prairie Fen
Jacob Lake
Keystone Iron Bog*
Kirk Lake*
La Poudre Pass Bog*
Lake Cleveland*
Lake Isabel Bog*
Lily Lake ‡
Little Lake, Oregon
Long Lake*
Lost Lake, Montana
Lost Trail Pass Bog*
Marion Lake*
Mariposa Lake, WY
Mayberry Well
Mckillop Creek Pond*
Mineral Lake*
Mission Cross Bog*
Molas Lake
Nichols Meadow*
Pangborn Bog*
Posy Lake*
Potato Lake*
Rattlesnake Cave*
Ruby Marshes*
Lat.
(°N)
Long.
(°W)
Elev.
(m)
No. of
14
C
dates
DC
Age of
chosen
depth
Target
age
Biome
38.75
43.50
37.17
37.60
45.67
37.55
38.83
47.58
45.17
44.65
36.24
43.25
43.95
40.02
44.07
37.00
38.62
43.65
43.00
41.00
48.55
48.67
47.82
34.00
37.70
43.75
39.33
47.75
37.97
42.48
44.63
34.33
38.87
48.12
40.48
42.32
40.07
43.77
44.17
40.07
47.63
45.75
49.33
44.15
33.70
48.33
46.73
41.78
37.75
37.43
48.83
37.95
34.08
43.52
41.13
106.83
105.45
119.50
119.02
122.48
105.50
106.41
118.75
110.17
110.60
108.95
118.33
110.23
107.03
110.30
119.08
111.67
122.05
111.58
112.50
112.72
116.92
122.30
109.43
105.50
110.60
118.75
124.25
107.55
107.90
122.58
110.83
107.03
121.50
105.78
113.63
105.62
110.32
123.58
105.60
110.48
113.97
123.00
110.23
108.30
115.45
122.20
115.48
107.68
119.57
122.58
111.70
111.50
112.62
115.48
2800
1450
2005
2816
<300
3523
3700
n/a
2500
2530
2780
1265
2628
3165
2634
2219
2720
1465
1946
1280
1233
860
104
2780
2300
2073
1251
n/a
3650
1950
988
2285
2920
190
3103
2519
3310
2469
217
3210
1019
2152
305
2730
2080
920
436
2424
3200
1509
n/a
2653
2222
1996
1818
3
3
6
6
11
2
3
3
3
6
5
11
3
5
3
6
5
5
15
1
3
12
6
6
6
5
16
8
6
4
5
3
7
8
3
3
3
2
13
5
5
16
7
3
1
5
7
7
2
2
3
4
4
1
25
n/a
n/a
n/a
n/a
n/a
n/a
n/a
n/a
n/a
n/a
n/a
n/a
n/a
n/a
n/a
n/a
n/a
n/a
n/a
n/a
n/a
n/a
n/a
n/a
n/a
n/a
n/a
n/a
n/a
n/a
n/a
n/a
n/a
n/a
n/a
n/a
n/a
n/a
n/a
n/a
n/a
n/a
n/a
n/a
n/a
n/a
n/a
n/a
n/a
n/a
n/a
n/a
n/a
n/a
n/a
n/a
n/a
n/a
n/a
n/a
n/a
n/a
n/a
n/a
0
n/a
n/a
n/a
n/a
458
n/a
n/a
29
n/a
n/a
-34
n/a
n/a
106
n/a
0
n/a
n/a
n/a
n/a
350
25
n/a
n/a
n/a
n/a
n/a
0
243
n/a
-35
n/a
n/a
0
211
n/a
n/a
n/a
81
n/a
n/a
n/a
n/a
n/a
n/a
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
OC
STEP
COCO
OC
COCO
OC
OC
OC
COCO
OC
OC
STEP
TAIG
OC
OC
COCO
TAIG
COCO
OC
OC
OC
COCO
COCO
STEP
OC
OC
OC
COCO
STEP
OC
COCO
STEP
COCO
COCO
STEP
OC
OC
OC
TUND
OC
STEP
TAIG
COCO
OC
OC
COCO
COCO
STEP
OC
COCO
COCO
COCO
OC
OC
OC
Reference
Markgraf & Scott, 1981
Markgraf & Lennon, 1986
Davis et al., 1985
Anderson, 1990
Barnosky, 1985a
Shafer, 1989
Fall, 1988
Mack et al., 1976
Waddington & Wright, 1974
Whitlock, 1993
Wright et al., 1973
Wigand, 1987
Whitlock & Bartlein, 1993
Feiler et al., 1997
Whitlock, 1993
Davis & Moratto, 1988
Shafer, 1989
Sea & Whitlock, 1995
Beiswenger, 1991
Mehringer, 1985
Barnowsky, 1989
Mack et al., 1978b,d
Tsukada et al., 1981
Jacobs, 1985
Shafer, 1989
Whitlock, 1993
Wigand & Mehringer, 1985
Heusser, 1978
Andrews et al., 1975
Beiswenger, 1987
Sea & Whitlock, 1995
Jacobs, 1983
Fall, 1985, 1988
Cwynar, 1987
Short, 1985
Davis, 1981
Short, 1985
Whitlock, 1993
Worona & Whitlock, 1995
Short, 1985
Barnosky, 1989
Mehringer et al., 1977
Mathewes, 1973
Whitlock, 1993
Markgraf, unpub
Mack et al., 1983
Tsukada et al., 1981
Thompson, 1984
Maher, 1961
Koehler & Anderson, 1994
Hansen & Easterbrook, 1974
Shafer, 1989
Anderson, 1993
Davis, 1981
Thompson, 1992
Biome reconstructions for western North America
36
100
102
103
105
106
107
109
113
116
118
119
Slough Creek Pond
Soleduck Bog*
Splains Gulch*
Swamp Lake*
Swan Lake*
Teepee Lake*
Tioga Pass Pond*
Waits Lake*
Wessler Bog*
Williams Fen*
Woski Pond
44.93
47.92
38.83
37.95
42.33
48.33
37.92
48.17
48.17
47.33
37.73
110.35
124.47
107.08
119.82
112.42
115.50
119.27
117.67
124.50
117.58
119.63
1884
73
3160
1554
1452
1270
3018
n/a
25
n/a
1212
4
1
4
5
3
8
4
8
1
6
3
n/a
n/a
n/a
n/a
n/a
n/a
n/a
n/a
n/a
n/a
n/a
0
n/a
n/a
n/a
n/a
n/a
n/a
n/a
n/a
n/a
-36
0
0
0
0
0
0
0
0
0
0
0
OC
COCO
TAIG
COCO
OC
COCO
STEP
CLDE
COCO
STEP
COCO
Whitlock & Bartlein, 1993
Heusser, 1973
Fall, 1988
Smith & Anderson, 1992
Bright, 1966
Mack et al., 1983
Anderson, 1990
Mack et al., 1978c,d
Heusser, 1973
Nickmann, 1979
Anderson & Carpenter, 1991
1
3
4
5
6
9
10
11
12
13
14
18
19
20
21
22
24
25
28
29
30
31
32
37
38
40
42
43
44
45
46
47
48
49
50
53
54
55
59
60
61
62
63
56
64
65
66
68
69
70
71
73
74
76
77
78
79
81
83
85
86
88
91
92
93
94
100
102
103
105
106
6000 14C yr B.P. SAMPLES
Alkali Creek*
Antelope Playa*
Balsam Meadows*
Barrett Lake*
Battle Ground Lake*
Big Meadow*
Blacktail Pond*
Bogachiel River Site*
Bonaparte Meadows*
Buckbean Fen*
Carp Lake*
Como Lake*
Copley Lake*
Cottonwood Pass Pond*
Creston Fen*
Cub Creek Pond*
Cygnet Lake Fen
Davis Lake*
Diamond Pond*
Divide Lake*
Dome Creek Meadow*
Emerald Lake
Exchequer Meadow*
Fryingpan Lake*
Gardiners Hole*
Gold Lake Bog
Gray's Lake*
Great Salt Lake*
Guardipee Lake
Hager Pond*
Hall Lake*
Hay Lake, Arizona
Head Lake*
Hedrick Pond
Hidden Cave*
Hurricane Basin*
Ice Slough*
Indian Prairie Fen
Keystone Iron Bog*
Kirk Lake*
La Poudre Pass Bog*
Lake Cleveland*
Lake Emma*
Lake Isabel Bog*
Lake Washington*
Lily Lake ‡
Lily Lake Fen ‡
Little Lake, Oregon
Long Lake*
Lost Lake, Montana
Lost Trail Pass Bog*
Marion Lake*
Mariposa Lake, WY
Mckillop Creek Pond*
Mineral Lake*
Mission Cross Bog*
Molas Lake
Mud Lake*
Nichols Meadow*
Pangborn Bog*
Pinecrest Lake*
Posy Lake*
Rapid Lake*
Rattlesnake Cave*
Redrock Lake*
Ruby Marshes*
Slough Creek Pond
Soleduck Bog*
Splains Gulch*
Swamp Lake*
Swan Lake*
38.75
43.50
37.17
37.60
45.67
48.92
44.97
47.88
48.75
44.30
45.92
37.55
38.87
38.83
47.58
45.17
44.65
46.58
43.25
43.95
40.02
44.07
37.00
38.62
44.92
43.65
43.00
41.00
48.55
48.67
47.82
34.00
37.70
43.75
39.33
37.97
42.48
44.63
38.87
48.12
40.48
42.32
37.90
40.07
47.67
43.77
43.77
44.17
40.07
47.63
45.75
49.33
44.15
48.33
46.73
41.78
37.75
48.50
37.43
48.83
50.50
37.95
42.62
43.52
40.67
41.13
44.93
47.92
38.83
37.95
42.33
106.83
105.45
119.50
119.02
122.48
117.42
110.60
124.33
119.08
110.25
120.88
105.50
105.08
106.41
118.75
110.17
110.60
122.25
118.33
110.23
107.03
110.30
119.08
111.67
110.73
122.05
111.58
112.50
112.72
116.92
122.30
109.43
105.50
110.60
118.75
107.55
107.90
122.58
107.03
121.50
105.78
113.63
107.63
105.62
122.22
110.32
110.32
123.58
105.60
110.48
113.97
123.00
110.23
115.45
122.20
115.48
107.68
119.75
119.57
122.58
121.50
111.70
109.20
112.62
105.50
115.48
110.35
124.47
107.08
119.82
112.42
2800
1450
2005
2816
<300
1040
2018
533
1021
2363
714
3523
3250
3700
n/a
2500
2530
282
1265
2628
3165
2634
2219
2720
n/a
1465
1946
1280
1233
860
104
2780
2300
2073
1251
3650
1950
988
2920
190
3103
2519
3730
3310
6
2469
2469
217
3210
1019
2152
305
2730
920
436
2424
3200
655
1509
n/a
320
2653
3135
1996
3095
1818
1884
73
3160
1554
1452
3
3
6
6
11
8
3
2
14
4
13
2
7
3
3
3
6
16
11
3
5
3
6
5
2
5
15
1
3
12
6
6
6
5
16
6
4
5
7
8
3
3
5
3
5
2
6
13
5
5
16
7
4
5
7
7
2
5
2
3
1
4
4
1
7
25
4
1
4
5
3
2C
1C
2C
3C
1C
2C
5C
7C
1C
2C
2C
1C
1C
2C
4C
7C
1C
1C
n/a
3C
2C
4C
1C
1C
4D
2C
2C
3D
5C
1C
2C
6C
2C
4C
2C
2C
3C
2C
2C
3D
2C
2C
3D
2C
4C
2C
4D
2C
2C
1C
1C
2C
4C
1D
2C
1C
7C
7D
1D
5D
7D
1C
2C
4D
1C
1C
2C
4C
1C
4C
4C
n/a
n/a
n/a
n/a
n/a
n/a
n/a
n/a
n/a
n/a
n/a
n/a
n/a
n/a
n/a
n/a
6124
n/a
n/a
n/a
n/a
6018
n/a
n/a
n/a
6147
n/a
n/a
6166
n/a
n/a
5639
n/a
6077
n/a
n/a
n/a
5919
n/a
n/a
n/a
n/a
n/a
n/a
n/a
5892
6285
5990
n/a
5978
n/a
n/a
6505
n/a
n/a
n/a
5818
n/a
n/a
n/a
n/a
n/a
n/a
n/a
n/a
6000
6000
6000
6000
6000
6000
6000
6000
6000
6000
6000
6000
6000
6000
6000
6000
6000
6000
6000
6000
6000
6000
6000
6000
6000
6000
6000
6000
6000
6000
6000
6000
6000
6000
6000
6000
6000
6000
6000
6000
6000
6000
6000
6000
6000
6000
6000
6000
6000
6000
6000
6000
6000
6000
6000
6000
6000
6000
6000
6000
6000
6000
6000
6000
6000
6000
6000
6000
6000
6000
6000
OC
STEP
OC
OC
COCO
TAIG
OC
COCO
TAIG
OC
COCO
OC
COCO
COCO
OC
OC
OC
COCO
OC
TAIG
STEP
TAIG
COCO
TAIG
OC
COCO
STEP
OC
STEP
TAIG
COCO
OC
OC
OC
OC
TAIG
STEP
COCO
COCO
COCO
OC
OC
OC
OC
COCO
TAIG
OC
COCO
OC
STEP
COCO
COCO
OC
TAIG
COCO
STEP
OC
OC
OC
COCO
COCO
OC
OC
OC
OC
OC
OC
COCO
COCO
COCO
STEP
Markgraf & Scott, 1981
Markgraf & Lennon, 1986
Davis et al., 1985
Anderson, 1990
Barnosky, 1985a
Mack et al., 1978a
Gennett, 1977
Heusser, 1978
Mack et al., 1979
Baker, 1976
Barnosky, 1985b
Shafer, 1989
Fall, 1988
Fall, 1988
Mack et al., 1976
Waddington & Wright, 1974
Whitlock, 1993
Barnosky, 1981
Wigand, 1987
Whitlock & Bartlein, 1993
Feiler et al., 1997
Whitlock, 1993
Davis & Moratto, 1988
Shafer, 1989
Baker, 1983
Sea & Whitlock, 1995
Beiswenger, 1991
Mehringer, 1985
Barnowsky, 1989
Mack et al., 1978b
Tsukada et al., 1981
Jacobs, 1985
Shafer, 1989
Whitlock, 1993
Wigand & Mehringer, 1985
Andrews et al., 1975
Beiswenger, 1987
Sea & Whitlock, 1995
Fall, 1985, 1988
Cwynar, 1987
Short, 1985
Davis, 1981
Carrara et al., 1984
Short, 1985
Leopold et al., 1982
Whitlock, 1993
Whitlock, 1993
Worona & Whitlock, 1995
Short, 1985
Whitlock, 1989
Mehringer et al., 1977
Mathewes, 1973
Whitlock, 1993
Mack et al., 1983
Tsukada et al., 1981
Thompson, 1984
Maher, 1961
Mack et al., 1979
Koehler & Anderson, 1994
Hansen & Easterbrook, 1974
Mathewes & Rouse, 1975
Shafer, 1989
Fall, 1988
Davis, 1981
Maher, 1972
Thompson, 1992
Whitlock & Bartlein, 1993
Heusser, 1973
Fall, 1988
Smith & Anderson, 1992
Bright, 1966
6128
Biome reconstructions for western North America
107
109
113
116
118
Teepee Lake*
Tioga Pass Pond*
Waits Lake*
Wessler Bog*
Williams Fen*
48.33
37.92
48.17
48.17
47.33
115.50
119.27
117.67
124.50
117.58
1270
3018
n/a
25
n/a
8
4
8
1
6
1C
1C
2C
7C
1C
6
8
11
14
25
26
27
33
42
47
49
51
57
58
68
75
77
79
83
89
94
18,000 14C yr B.P. SAMPLES
Battle Ground Lake*
Bechan Cave
Bogachiel River Site*
Carp Lake*
Davis Lake*
Dead Man Lake*
Devlins Park*
Fargher Lake*
Gray's Lake*
Hay Lake, Arizona
Hedrick Pond
Hoh Bog*
Jacob Lake
Kalaloch*
Little Lake, Oregon
Mayberry Well
Mineral Lake*
Molas Lake
Nichols Meadow*
Potato Lake*
Ruby Marshes*
45.67
38.00
47.88
45.92
46.58
36.24
40.02
45.88
43.00
34.00
43.75
47.75
34.33
47.55
44.17
33.70
46.73
37.75
37.43
34.08
41.13
122.48
111.00
124.33
120.88
122.25
108.95
105.55
122.52
111.58
109.43
110.60
124.25
110.83
124.33
123.58
108.30
122.20
107.68
119.57
111.50
115.48
<300
1370
533
714
282
2780
2953
n/a
1946
2780
2073
n/a
2285
35
217
2080
436
3200
1509
2222
1818
11
1
2
13
16
5
6
3
15
6
5
8
3
3
13
1
7
2
2
4
25
2C
7D
7C
1C
3C
7C
1C
2C
6C
6C
4D
7D
7C
6D
2C
7C
2D
7D
7C
4C
1C
18236
17770
17408
18144
17618
18332
18062
37
6000
6000
6000
6000
6000
TAIG
OC
TAIG
COCO
STEP
Mack et al., 1983
Anderson, 1990
Mack et al., 1978c
Heusser, 1973
Nickmann, 1979
18000
18000
18000
18000
18000
18000
18000
18000
18000
18000
18000
18000
18000
18000
18000
18000
18000
18000
18000
18000
18000
STEP
OC
STEP
STEP
STEP
OC
OC
COCO
OC
OC
OC
TUND
OC
STEP
COCO
OC
STEP
OC
OC
OC
OC
Barnosky, 1985a
Davis et al., 1984, Davis, 1990
Heusser, 1978
Barnosky, 1985b
Barnosky, 1981
Wright et al., 1973
Legg & Baker, 1980
Heusser, 1983
Beiswenger, 1991
Jacobs, 1985
Whitlock, 1993
Heusser, 1978
Jacobs, 1983
Heusser, 1972
Worona & Whitlock, 1995
Markgraf, unpub NAPD
Tsukada et al., 1981
Maher, 1961
Koehler & Anderson, 1994
Anderson, 1993
Thompson, 1992
Biome reconstructions for western North America
Table 3 Characteristics of the 0, 6000 and 18,000
38
14
C yr B.P. packrat midden sites from western North
America. Dating control codes (DC) follow the scheme described in Table 2. Biome codes (Biome) are
given in Table 6. For mapping purposes some sites (indicated by ‡) which are too close to one another have
been displaced slightly.
Site name
Sample
Lat.
Long.
Elev.
Code
(°N)
(°W)
(m)
Sample
type
C14 Target DC
age
age
Biome
References
MODERN SAMPLES
Ajo Loop
4
31.97 112.78
550
midden
980
0
4D
DESE
Van Devender, 1987
Ajo Loop
3A
31.97 112.78
550
midden
990
0
4D
DESE
Van Devender, 1987
Ajo Loop
2C
31.97 112.78
550
midden
130
0
1D
DESE
Van Devender, 1987
Ajo Loop
2B
31.97 112.78
550
midden
30
0
1D
DESE
Van Devender, 1987
Bison Alcove
MOD
38.73 109.50
1317
midden
405
0
2D
OC
Sharpe, 1991
Bison Alcove
MOD
38.73 109.50
1317
midden
355
0
2D
OC
Sharpe, 1991
Dog Canyon
1
32.83 105.92
1615
midden
360
0
2D
DESE
Dolores
4(1)
37.52 108.55
2100
midden
900
0
4D
OC
Van Devender, 1985
Dolores
3
37.52 108.55
2100
midden
580
0
3D
OC
Van Devender, 1985
Dolores
4(2)
37.52 108.55
2100
midden
990
0
4D
OC
Van Devender, 1985
Eureka View ‡
#3
37.33 117.78
1450
midden
535
0
3D
DESE
Gatecliff
#5
39.00 116.78
2319
midden
0
0
1D
OC
Greenwater Valley ‡
G11C
36.17 116.60
1410
midden
740
0
3D
DESE
Cole & Webb, 1985
Greenwater Valley ‡
G11A
36.17 116.60
1410
midden
0
0
1D
DESE
Cole & Webb, 1985
Greenwater Valley ‡
G9D
36.17 116.60
1350
midden
290
0
2D
DESE
Cole & Webb, 1985
Greenwater Valley ‡
G15
36.17 116.60
1380
midden
0
0
1D
DESE
Cole & Webb, 1985
Greenwater Valley ‡
G7
36.17 116.60
1360
midden
0
0
1D
DESE
Cole & Webb, 1985
Greenwater Valley ‡
G8
36.17 116.60
1340
midden
0
0
1D
DESE
Cole & Webb, 1985
Greenwater Valley ‡
G6B
36.17 116.60
1350
midden
270
0
2D
DESE
Cole & Webb, 1985
Greenwater Valley ‡
G11B
36.17 116.60
1410
midden
400
0
2D
DESE
Cole & Webb, 1985
Greenwater Valley ‡
G4A
36.17 116.60
1350
midden
200
0
1D
OC
Cole & Webb, 1985
Hance Canyon ‡
HC2B
36.03 111.97
1200
midden
0
0
1D
DESE
Hidden Forest
#3B
36.57 115.10
2380
midden
820
0
4D
OC
Spaulding, 1981
Horse Thief Hills ‡
#3(1)
37.35 117.80
1575
midden
200
0
1D
DESE
Spaulding, 1980
Horse Thief Hills ‡
#3(2)
37.35 117.80
1575
midden
200
0
1D
DESE
Spaulding, 1980
Horse Thief Hills ‡
#3(2)
37.35 117.80
1575
midden
200
0
1D
DESE
Spaulding, 1980
Horse Thief Hills ‡
#2
37.35 117.80
1635
midden
200
0
1D
OC
Spaulding, 1980
Horseshoe Mesa ‡
HM1B
36.03 111.98
1100
midden
0
0
1D
DESE
Cole, 1981
Kings Canyon
KC7
36.80 118.80
1270
midden
90
0
1D
XERO
Cole, 1983
Kings Canyon
MODINDUR 36.80 118.80
1270
midden
0
0
1D
XERO
Cole, 1983
McCullough Range
1(1)1
35.75 115.17
1045
midden
960
0
4D
DESE
Spaulding, 1991
Navar Ranch
18C
31.90 106.15
midden
789
0
4D
DESE
Van Devender & Toolin, undat.
San Andres
1
32.83 105.92
1555
midden
430
0
2D
DESE
Van Devender et al., 1984
Sierra Bacha
3
29.83 112.47
100
midden
320
0
2D
DESE
Van Devender et al., 1994
Wide Rock Butte
LEVEL1
36.12 109.33
2100
midden
0
0
1D
OC
Schmutz et al., 1976
Van Devender, 1987
Van Devender et al., 1984
Spaulding, 1980
Thompson & Hattori, 1983
Cole, 1981
6000 14C yr B.P. SAMPLES
Ajo Loop
1A
31.97 112.78
550
midden
5240
6000 4D
DESE
Chuar Valley
CH1
36.17 111.92
1430
midden
6830
6000 4D
OC
Council Hall Cave
#1A
39.33 114.10
2040
midden
6120
6000 1D
OC
Thompson, 1984
Eureka View
#4(3)
37.33 117.78
1435
midden
5435
6000 3D
DESE
Spaulding, 1980
Cole, 1981
Biome reconstructions for western North America
Eureka View
#4(2)
37.33 117.78
1435
midden
6795
6000 4D
DESE
39
Spaulding, 1980
Eureka View
#4(1)
37.33 117.78
1435
midden
5595
6000 2D
DESE
Spaulding, 1980
Fishmouth Cave
4
37.42 109.65
1585
midden
6100
6000 1D
OC
Betancourt, 1984
Grandview Point ‡
GP4
36.00 111.98
2200
midden
5510
6000 2D
OC
Hornaday Mountains
1C
31.98 113.60
240
midden
6065
6000 1D
DESE
Cole, 1981
Van Devender et al., 1990
Lava Tube
99999
43.55 122.57
1640
midden
5690
6000 2D
STEP
Bright & Davis , 1982
Lubkin Canyon ‡
1
36.53 118.05
1264
midden
5090
6000 4D
STEP
Koehler & Anderson, 1995
Lubkin Canyon ‡
2
36.53 118.05
1264
midden
5610
6000 2D
OC
Lucerne Peak ‡
#1
34.50 117.00
1097
midden
5800
6000 1D
DESE
King, 1976
Marble Canyon
1A
32.83 105.92
1580
midden
5430
6000 3D
DESE
Van Devender et al., 1984
Koehler & Anderson, 1995
McCullough Range
3(5)
35.75 115.17
1045
midden
5060
6000 4D
DESE
Spaulding, 1991
McCullough Range
3(1)
35.75 115.17
1045
midden
5510
6000 2D
DESE
Spaulding, 1991
McCullough Range
3(3)
35.75 115.17
1045
midden
6480
6000 2D
DESE
Spaulding, 1991
McCullough Range
3(4)
35.75 115.17
1045
midden
6800
6000 4D
DESE
Spaulding, 1991
midden
6360
6000 2D
DESE
Van Devender & Toolin, nd
1700
midden
6330
6000 2D
DESE
Van Devender & Toolin, 1983
Navar Ranch
1C1
31.90 106.15
Rhodes Canyon
4
33.18 106.60
Rhodes Canyon
6B
33.18 106.60
1700
midden
6950
6000 4D
DESE
Van Devender & Toolin, 1983
Sierra Bacha
1J
29.83 112.47
200
midden
5340
6000 3D
DESE
Van Devender et al., 1994
Sunset Cove ‡
#1
34.50 117.00
972
midden
5880
6000 1D
OC
King, 1976
Tse an Bida Cave
BI6B
36.00 112.00
1450
midden
6800
6000 4D
OC
Cole, 1981
Valleyview
#1
39.50 114.72
2350
midden
6250
6000 1D
OC
Thompson, 1984
Valleyview
#1
39.50 114.72
2350
midden
6670
6000 3D
OC
Waterman Mountains
9D
32.35 111.45
760
midden
5540
6000 2D
DESE
Thompson, 1984
Andersen & Van Devender, 1991
Waterman Mountains
9D
32.35 111.45
760
midden
4845
6000 5D
DESE
Andersen & Van Devender, 1991
Waterman Mountains
12A
32.35 111.45
760
midden
6195
6000 1D
DESE
Andersen & Van Devender, 1991
Waterman Mountains
12A
32.35 111.45
760
midden
5920
6000 1D
DESE
Andersen & Van Devender, 1991
Van Devender, 1973
Wellton Hills
#5A
32.60 114.12
175
midden
6600
6000 3D
DESE
Wellton Hills
#5A
32.60 114.12
175
midden
8150
6000
DESE
Van Devender, 1973
Wide Rock Butte
LEVEL4
36.12 109.33
2100
midden
6210
6000 1D
COMX
Schmutz et al., 1976
Wolcott Peak
4
32.45 111.47
862
midden
5350
6000 3D
DESE
Van Devender, 1973
Big Boy
2
32.83 105.92
1555
midden 18300 18000 2D
OC
Chuar Valley ‡
CH18C2
36.17 111.92
1770
midden 18490 18000 2D
COMX
Cole, 1981
Chuar Valley ‡
CH18B
36.17 111.92
1770
midden 18800 18000 4D
OC
Cole, 1981
Eyrie
#3(2)
36.63 115.28
1855
midden 18890 18000 4D
OC
Spaulding, 1981
Flaherty Mesa
#2
36.48 115.25
1770
midden 18790 18000 4D
OC
Spaulding, 1981
Hance Canyon ‡
HC4
36.03 111.97
1100
midden 17400 18000 3D
OC
Cole, 1981
Horseshoe Mesa ‡
HM6
36.03 111.98
1450
midden 18630 18000 3D
XERO
Cole, 1981
Kings Canyon
#5A2
36.80 118.80
1275
midden 19130 18000 5D
XERO
Cole, 1983
Kings Canyon
#5A2
36.80 118.80
1275
midden 17520 18000 2D
XERO
Cole, 1983
Nankoweap ‡
NA9C
36.25 111.95
2020
midden 18130 18000 1D
COCO
Cole, 1981
Nankoweap ‡
NA9B
36.25 111.95
2020
midden 17950 18000 1D
COCO
Cole, 1981
Pontatoc Ridge
#4B
32.35 110.88
1463
midden 17950 18000 1D
OC
VanDevender & Thompson, unpub.
Rampart Cave
RatLayer
36.10 113.93
535
midden 18890 18000 4D
OC
Phillips, 1977
Streamview
#2
39.33 114.10
1860
midden 17350 18000 3D
OC
Thompson, 1984; Thompson & Mead, 1982
Vulture Canyon
#6
36.10 113.93
645
midden 17610 18000 2D
OC
Phillips, 1977
Vulture Canyon
#17
36.10 113.93
645
midden 17100 18000 4D
OC
Mead & Phillips, 1981
Willow Wash
#4D
36.47 115.25
1585
midden 17070 18000 4D
OC
Spaulding, 1981
7
18,000 14C yr B.P. SAMPLES
Van Devender et al., 1984
Biome reconstructions for western North America
40
Table 4 Assignments of pollen taxa from western North America to the PFTs used in the biomization
procedure.
Abbr.
aa
Plant functional type
arctic/alpine dwarf shrub
bec
bs
boreal evergreen conifer
boreal summergreen
ctc
cool-temperate conifer
df
desert forb
ds
ds2
ec
desert shrub or succulent
frost-sensitive desert shrub or
succulent
eurythermic conifer
g
h
s
sf
grass
heath
sedge
steppe forb
ss
ts
steppe shrub
temperate summergreen
ts1
ts2
cool-temperate summergreen
intermediate-temperate summergreen
ts3
wc
ws
wtc
warm-temperate summergreen
woodland conifer
woodland shrub
warm-temperate conifer
wte
warm-temperate broadleaved
evergreen
cool-temperate broadleaved
evergreen
warm-temperate sclerophyll shrub
wte1
wte2
Pollen taxa
Alnus undiff., Betula undiff., Brassicaceae, Caryophyllaceae,
Dryas-type, Fabaceae, Oxyria, Polygonum bistortoides-type,
Potentilla-type, Ranunculaceae, Salix, Saxifragaceae, Thalictrum,
Umbelliferae/Apiaceae
Abies, Picea
Alnus undiff., Betula undiff., Cornus, Larix, Larix/Pseudotsuga,
Myricaceae, Populus, Salix, Shepherdia canadensis
Abies, Larix/Pseudotsuga, Picea, Pseudotsuga, Taxaceae,
Taxodiaceae (Other, in California),
Taxodiaceae/Cupressaceae/Taxaceae, Tsuga
Ambrosia-type, Brassicaceae, Caryophyllaceae,
Chenopodiaceae/Amaranthus, Ephedra, Eriogonum,
Euphorbiaceae, Fabaceae, Nyctaginaceae, Ranunculaceae,
Sphaeralcea-type, Tubuliflorae/Other Asteraceae,
Umbelliferae/Apiaceae
not present
not present
Cupressaceae/Taxaceae, Pinus,
Taxodiaceae/Cupressaceae/Taxaceae
Poaceae
Ericaceae
Cyperaceae
Ambrosia-type, Artemisia, Brassicaceae, Caryophyllaceae,
Cercocarpus-type, Chenopodiaceae/Amaranthus, Ephedra,
Eriogonum, Fabaceae, Potentilla-type, Ranunculaceae, Sarcobatus,
Saxifragaceae, Sphaeralcea-type, Tubuliflorae/Other Asteraceae,
Umbelliferae/Apiaceae
not present
Acer, Alnus undiff., Anacardiaceae, Aquifoliaceae, Carya,
Ceanothus, Clethra, Cornus, Fraxinus, Myricaceae,
Ostrya/Carpinus, Populus, Quercus, Rhamnus, Salix, Ulmus
Betula undiff., Corylus, Fagus, Larix, Larix/Pseudotsuga, Tilia
Castanea, Celtis, Cephalanthus, Fabaceae, Juglans, Magnoliaceae,
Morus, Nyssa, Platanus
Celtis, Cephalanthus, Liquidambar, Nyssa
not present
not present
Taxodiaceae (Other, in California),
Taxodiaceae/Cupressaceae/Taxaceae, Taxodium
Aquifoliaceae, Chrysolepis/Lithocarpus, Magnoliaceae, Quercus
Aquifoliaceae
Ceanothus, Chrysolepis/Lithocarpus, Quercus, Rhamnus
Biome reconstructions for western North America
41
Table 5 Assignments of plant macrofossil (midden) taxa from western North America to the PFTs used in the biomization procedure.
Abbr.
aa
bec
bs
ctc
Plant functional type
arctic/alpine dwarf shrub
boreal evergreen conifer
boreal summergreen
cool-temperate conifer
df
desert forb
ds
desert shrub or succulent
Plant macrofossil taxa
Cirsium sp., Phlox sp.
Juniperus communis, Picea engelmannii, Picea pungens, Picea engelmanii, Pinus flexilis
Ribes cf. montigenum, Shepherdia canadensis, Shepherdia sp.
Abies concolor, Abies magnifica, Calocedrus decurens, Juniperus communis, Juniperus occidentalis, Juniperus
scopulorum, Juniperus sp., Pinus flexilis, Pinus lambertiana, Pinus longaeva, Pinus ponderosa, Pseudotsuga menziesii
Abutilon sp., Allionia incarnata, Amaranthus cf. albus, Amaranthus fimbriatus, Amaranthus sp., Amaranthus/chenopodium,
Ambrosia confertifolia, Ambrosia sp., Amsinckia intermedia, Amsinckia sp., Amsinckia tesselata, Amsinkia/Cryptantha,
Amsonia sp., Argenome sp., Argythamnia lanceolata, Artemisia ludoviciana, Astragalus sp., Bahia absinthifolia,
Boerhaavia sp., Boerhaavia wrightii, Brickellia arguta, Chaenactis sp., Chenopodium (cf.), Chenopodium sp., Chorizanthe
brevicornu, Cirsium sp., Coldenia canescens, Cryptantha barbigera cf., Cryptantha maritima, Cryptantha racemosa,
Cryptantha sp., Cryptantha virginensis, Cucurbita sp., Daucus pusillus, Descurainia sp., Dithyrea californica, Erigeron
sp., Eriogonum fasciculatum, Eriogonum sp., Erodium sp., Eucnide urens, Euphorbia cf. polycarpa, Euphorbia
micromera/polycarpa, Euphorbia sp., Gilia cf. latifolia, Gilia sp., Gutierrezia lucida, Gutierrezia sarothrae, Gutierrezia
sp., Haplopappus brickelliodes, Hedeoma nana, Hedeoma sp., Helianthus sp., Heterotheca sp., Ipomoea or convolvulus,
Iva cf. ambrosiaefolia, Kallstroemia sp., Lappula redowskii, Lappula sp., Lepidium fremontii, Lepidium sp., Leptodactylon
pungens, Lesquerella sp., Lotus sp., Lupinus sp., Machaeranthera sp., Mentzelia multiflora-type, Mentzelia sp., Mirabilis
bigelovii, Mirabilis multiflora, Mirabilis sp., Oenothera (cf.), Oenothera pallida, Pectocarya cf. recurvata, Pectocarya
heterocarpa, Pectocarya sp., Penstemon sp., Perityle emoryi, Physalis crassifolia, Physalis sp., Plantago insularis,
Plantago sp., Polygonum sp., Portulaca oleracea, Psoralea sp., Salazaria mexicana, Salvia sp., Senecio sp., Sphaeralceatype, Sphaeralcea ambigua, Sphaeralcea sp., Stanleya pinnata, Stephanomeria exiqua, Streptanthus sp., Strepthanthus
cordatus, Thysanocarpus sp., Tidestromia oblongiflolia, Tidestromia lanuginosa, Verbena sp., Viguiera reticulata,
Viguiera sp.
Acacia greggii, Agave lecheguilla, Agave neomexicana, Agave sp., Agave utahensis, Aloysia wrightii, Ambrosia dumosa,
Atriplex canescens, Atriplex polycarpa, Berberis haematocarpa, Brickellia desertorum, Brickellia longiflora, Brickellia or
eupatorium, Brickellia sp., Buddleja utahensis, Cactaceae undet., Ceanothus greggii, Celtis pallida, Choisya dumosa,
Chrysothamnus nauseosus ssp. bigelovii, Chrysothamnus teretifolius, Coleogyne ramosissima, Coryphantha strobiliformus,
Cowania mexicana (cf.), Croton sonorae, Dalea fremontii, Dalea sp., Dasylirion sp., Dasylirion wheeleri, Ditaxis
lanceolata, Echinocactus horizonthalonius, Echinocereus sp., Encelia farinosa, Encelia frutescens, Encelia virginensis,
Ephedra aspera, Ephedra nevadensis, Ephedra sp., Ephedra torreyana, Ephedra trifurca-type, Ericameria larcifolia,
Eriogonum heermannii, Eriogonum sp., Fallugia paradoxa, Ferocactus-type, Ferocactus covellei, Forsellesia nevadensis,
Forsellesia spinescens, Fouquieria columnaris, Fouquieria splendens, Grayia spinosa, Gutierrezia microcephala,
Haplopappus cooperi, Haplopappus laricifolius, Horsfordia, Hymenoclea salsola, Hyptis emori, Janusia gracilis,
Koeberlinia spinosa, Krameria grayi, Krameria parvifolia, Larrea divaricata, Lycium andersonii, Lycium cf. berlandieri,
Lycium pallidum, Lycium sp., Machaeranthera tortifolia, Mammillaria tetrancistra, Menodora spinescens, Mortonia
scabrella, Neolloydia johnsonii, Nicotiana trigonophylla, Nolina micrantha, Opuntia, Opuntia acanthocarpa, Opuntia
arbuscula, Opuntia basilaris, Opuntia bigelovii, Opuntia chlorotica, Opuntia echinocarpa, Opuntia erincea, Opuntia
Biome reconstructions for western North America
ds2
frost-sensitive desert shrub or
succulent
ec
g
eurythermic conifer
grass
h
s
sf
heath
sedge
steppe forb
ss
steppe shrub
42
fulgida, Opuntia imbricata, Opuntia kunzei, Opuntia leptocactus, Opuntia phaeacantha-type, Opuntia ramosissima,
Opuntia sp., Opuntia versicolor, Peucephyllum schottii, Phoradendron californicum, Prosopis glandulosa, Prosopis
juliflora, Prunus fasciculata, Prunus sp., Psoralea sp., Psorothamnus fremontii, Rhus diversiloba, Rhus microphylla, Salvia
mohavensis, Salvia sp., Simmondsia chinensis, Solanum hindsianum, Tetradymia axillaris, Tetradymia spinosa,
Thamnosma montana, Thamnosma sp., Trixis californica, Viquiera stenoloba, Yucca angustifolia, Yucca angustissima,
Yucca baccata (cf.), Yucca baccata/torreyi, Yucca brevifolia, Yucca schidigera, Yucca sp., Yucca torreyi, Yucca whipplei,
Yucca whipplei ssp.caepitosa
Agave subsimplex, Bursera micophylla, Cactaceae undet., Carnegiea gigantea, Cercidium, Cercidium floridum, Cercidium
microphyllum, Cereus giganteus, Ferocactus-type, Ferocactus acanthodes, Ferocactus cylindraceus, Jatropha cuneata,
Lophocereus schottii, Olneya tesota, Pachycereus pringlei, Prosopis juliflora var. velutina, Prosopsis velutina, Sapium
biloculare, Stenocereus thurberi, Tetradymia sp.
Cupressaceae, Pinus sp.
Agropyron cf. spicatum, Agropyron sp., Andropogon barbinodis/saccharoides, Aristida adscencionis, Aristida sp.,
Avena/festuca type, Bouteloua aristidoides, Bouteloua barbata, Bouteloua curtipendula, Bouteloua eriopoda, Bouteloua
gracilis, Brickellia arguta, Bromus anomalus, Bromus rubens, Bromus sp., Bromus tectorum, Digitaria californica,
Echinochloa crusgallii, Elymus sp., Enneapogon desvauxii, Eragrostis sp., Erioneuron grandiflorum, Erioneuron
pulchellum, Festuca sp., Filago sp., Gramineae undet., Hilaria jamesii, Leptochloa dubia, Muhlenbergia microsperma,
Muhlenbergia monticola, Muhlenbergia pauciflora, Muhlenbergia sp., Oryzopsis hymenoides, Oryzopsis sp., Panicum
capillare, Panicum cf. hallii, Poa sandbergii, Poaceae undet, Setaria cf. leucophila, Setaria macrostachya, Sitanion
hystrix, Stipa arida, Stipa cf. lobata, Stipa comata, Stipa neomexicana, Stipa occidentalis, Stipa sp., Stipa speciosa, Stipia
arida cf., Tridens muticus
Arctostaphylos pungens, Arctostaphylos sp.
not present
Allionia incarnata, Amaranthus cf. albus, Amaranthus sp., Amaranthus/chenopodium, Ambrosia confertifolia, Ambrosia
sp., Amsinckia sp., Amsinkia/Cryptantha, Angelica sp. cf., Argenome sp., Artemisia ludoviciana, Artemisia sp., Astragalus
sp., Chaenactis sp., Chenopodium (cf.), Chenopodium sp., Cirsium sp., Cryptantha sp., Cryptantha virginensis, Cucurbita
sp., Daucus pusillus, Descurainia sp., Ericameria cuneata, Erigeron sp., Eriogonum fasciculatum, Eriogonum sp., Erodium
sp., Euphorbia sp., Gilia sp., Gutierrezia lucida, Gutierrezia sarothrae, Gutierrezia sp., Hedeoma nana, Hedeoma sp.,
Helianthus sp., Ipomoea or convolvulus, Lappula redowskii, Lappula sp., Lepidium fremontii, Lepidium sp., Leptodactylon
pungens, Lesquerella kingi, Lesquerella sp., Linum lewisii, Linum sp., Lithospermum sp., Lotus sp., Lupinus argenteus,
Lupinus sp., Machaeranthera sp., Mentzelia multiflora-type, Mentzelia sp., Mirabilis bigelovii, Mirabilis multiflora,
Mirabilis oxybaphoides, Mirabilis sp., Oenothera (cf.), Oenothera pallida, Pectocarya heterocarpa, Pectocarya sp.,
Penstemon breviflorus, Penstemon sp., Phacelia sp., Phlox sp., Physalis sp., Plantago insularis, Plantago sp., Polygonum
sp., Portulaca oleracea, Psoralea sp., Senecio sp., Sphaeralcea-type, Sphaeralcea sp., Stanleya pinnata, Stephanomeria
exiqua, Streptanthus sp., Strepthanthus cordatus, Verbena sp., Viguiera sp.
Amelanchier sp., Amelanchier utahensis, Artemisia frigida, Artemisia sp., Artemisia spinescens, Artemisia tridentata,
Atriplex canescens, Atriplex confertifolia, Atriplex polycarpa, Berberis fremontii, Berberis haematocarpa, Brickellia sp.,
Buddleja utahensis, Cactaceae undet., Ceanothus greggii, Ceratoides lanata, Cercocarpus ledifolius, Chamaebatiaria
millefolium, Chrysothamnus greenei, Chrysothamnus nauseosus, Chrysothamnus nauseosus ssp. bigelovii, Chrysothamnus
sp., Chrysothamnus teretifolius, Chrysothamnus viscidiflorus, Coryphantha strobiliformus, Cowania mexicana (cf.),
Biome reconstructions for western North America
ts
temperate summergreen
ts1
ts2
ts3
wc
cool-temperate summergreen
intermediate-temperate
summergreen
warm-temperate summergreen
woodland conifer
ws
woodland shrub
wtc
wte
warm-temperate conifer
warm-temperate broadleaved
evergreen
cool-temperate broadleaved
evergreen
warm-temperate sclerophyll shrub
wte1
wte2
43
Dasylirion sp., Dasylirion wheeleri, Echinocactus polycephalus, Echinocereus sp., Ephedra nevadensis, Ephedra sp.,
Ephedra torreyana, Ephedra trifurca-type, Ephedra viridis, Ericameria larcifolia, Eriogonum sp., Fallugia paradoxa,
Forsellesia nevadensis, Grayia spinosa, Gutierrezia microcephala, Lycium pallidum, Mammillaria-type, Mammillaria
microcarpa, Mammillaria sp., Menodora spinescens, Nolina micrantha, Opuntia, Opuntia arbuscula, Opuntia basilaris,
Opuntia bigelovii, Opuntia chlorotica, Opuntia erincea, Opuntia fulgida, Opuntia imbricata, Opuntia leptocactus, Opuntia
phaeacantha-type, Opuntia polyacantha, Opuntia sp., Opuntia spinosior, Opuntia whipplei, Petrophytum caespitosum,
Psoralea sp., Purshia tridentata, Rhus diversiloba, Ribes cereum, Ribes sp., Ribes velutinum, Salvia carnosa, Salvia sp.,
Shepherdia argentea, Symphoricarpo ssp., Symphoricarpos longiflorus, Symphoricarpos sp., Tetradymia axillaris,
Tetradymia sp., Tetradymia spinosa, Tidestromia oblongiflolia, Yucca angustifolia, Yucca angustissima, Yucca
harrimaniae, Yucca sp.
Amelanchier sp., Amelanchier utahensis, Ceanothus cuneatus, Ceanothus integerrimus, Celtis reticulata, Cercis
occidentalis, Cercocarpus sp., Fraxinus anomala, Berberis repens, Ostrya knowltoni, Physocarpus alternans, Prunus sp.,
Ptelea trifoliata var. pallida, Quercus arizonica x Q. grisea, Quercus gambellii, Quercus grisea, Quercus sp., Rhus
aromatica, Rhus aromatica or virens, Rhus diversiloba, Rhus sp., Rhus trilobata, Ribes cf. montigenum, Ribes sp., Rosa sp.,
Rubus sp., Sambucus neomexicana, Shepherdia sp.
not present
Robinia neomexicana
Prosopis glandulosa
Juniperus deppeana, Juniperus erythrocarpa, Juniperus monosperma (cf.), Juniperus occidentalis, Juniperus osteosperma,
Juniperus pinchotii, Juniperus scopulorum, Juniperus sp., Parthenium incanum, Parthenium sp., Pinus cembroides, Pinus
discolor, Pinus edulis, Pinus edulis/remota, Pinus flexilis, Pinus monophylla, Pinus monophylla x P. edulis
Aloysia wrightii, Amelanchier sp., Amelanchier utahensis, Aplopappus cuneatus, Berberis fremontii, Berberis
haematocarpa, Berberis repens, Berberis sp., Brickellia watsonii, Cactaceae undet., Ceanothus cuneatus, Ceanothus
greggii, Cercocarpus betuloides, Cercocarpus intricatus, Cercocarpus ledifolius, Cercocarpus montanus, Cercocarpus sp.,
Chamaebatiaria millefolium, Cowania mexicana (cf.), Ericameria cuneata, Eriogonum heermannii, Eriogonum sp.,
Fendlerella utahensis, Forsellesia nevadensis, Forsellesia pungens, Fraxinus anomala, Garrya flavescens, Haplopappus
laricifolius, Haplopappus nanus, Holodiscus dumosus, Holodiscus microphyllus, Jamesia americana, Mammillaria-type,
Mammillaria grahamii, Mammillaria sp., Menodora spinescens, Opuntia polyacantha, Opuntia sp., Pediocactus mesaeverdae, Philadelphus microphyllus, Physocarpus alternans, Prunus fasciculata, Prunus sp., Psoralea sp., Ribes cereum,
Ribes cf. montigenum, Ribes sp., Ribes velutinum, Rosa cf. stellata, Salvia carnosa, Shepherdia argentea, Shepherdia sp.,
Symphoricarpo ssp., Symphoricarpos longiflorus, Symphoricarpos sp., Tetradymia axillaris, Tetradymia spinosa,
Thamnosma montana
Cupressus arizonicus, Pinus cembroides, Pinus discolor, Torreya californica
Arctostaphylos pungens, Arctostaphylos sp., Cercocarpus betuloides, Cercocarpus sp., Quercus chrysolepis, Quercus
pungens, Rhamnus crocea, Rhamnus crocea var. Ilicifolia, Umbellularia californica
not present
Berberis sp., Ceanothus greggii, Quercus sp., Quercus turbinella, Quercus undulata, Vauquelinia californica
Biome reconstructions for western North America
Table 6 Assignment of PFTs to biomes used in the biomization for western North America.
Biome
Code
Plant functional types
cold deciduous forest
CLDE
bs, ec, h
taiga
TAIG
bec, bs, ec, h
cold mixed forest
CLMX
bs, ctc, ec, h, ts1
cool conifer forest
COCO
bec, bs, ctc, ec , h, ts1
temperate deciduous forest
TEDE
bs, ec, h, ts, ts1, ts2, wte1
cool mixed forest
COMX
bec, bs, ctc, ec, h, ts, ts1
broadleaved evergreen/warm mixed forest
WAMX
ec, wtc, h, ts, ts3, wte, wte1
xerophytic woods/scrub
XERO
ec, wc, ws, wte, wte2
steppe
STEP
g, s, sf, ss
desert
DESE
df, ds, ds2, g
tundra
TUND
aa, g, h, s
open conifer woodland
OC
ec, sf, ss, wc, ws
44
Biome reconstructions for western North America
45
Table 7 Climatic tolerances of biomes in western North America, based on (a) present-day potential natural
vegetation, and (b) present-day pollen and plant macrofossil assemblages. MTCO temperatures are
approximated to the nearest 2.5° C; GDD5 values to the nearest 500 growing-degree days; and α values to
the nearest 0.05. The values in 7 (a) were obtained through visual inspection and interpretation of Figs 5
and 6; the values in 7 (b) through visual inspection and interpretation of Fig. 5. Cold deciduous forests and
xerophytic woods/scrub are omitted from 7 (b) because they only occur once.
a)
Biome
MTCO
min
MTCO
max
GDD5
min
GDD5
max
α
min
α
max
cool conifer forest
-12.5°
5.0°
500
2500
0.45
0.95
xerophytic woods/scrub
2.5°
12.5°
2000
5000
0.30
0.70
grassland
-15.0°
7.5°
1500
5000
0.35
0.70
steppe
-10.0°
0.0°
1000
3000
0.15
0.55
desert
0.0°
12.5°
3000
6500
0.05
0.40
tundra
-12.5°
-2.5°
<500
1000
0.75
1.00
open conifer woodland
-10.0°
5.0°
1000
3500
0.20
0.65
Biome
MTCO
Min
MTCO
Max
GDD5
Min
GDD5
Max
α
Min
α
Max
taiga
-10.0°
-7.5°
500
500
0.75
0.85
cool conifer forest
-7.5°
5.0°
500
2500
0.60
1.00
grassland/steppe
-10.0°
0.0°
500
2000
0.40
1.00
desert
0.0°
7.5°
2000
6500
0.05
0.45
open conifer woodland
-10.0°
5.0°
500
3500
0.20
0.70
b)