H I STO RY
For Schiffman's contribution to this
section of the book, go to pp. 52 - 56
"Species composition at the time of first European settlement"
FOUR
Pleistocene and Pre-European Grassland Ecosystems
Late Quaternary Paleoecology of
Grasslands and Other Grassy Habitats
Peter E. Wigand
Today as in the past, grasslands, and grassy steppe and
chaparral, have been essential and dynamic elements of the
western North American ecosystems. Since the appearance of
grasses during the Eocene, they have provided both a crucial
role in the recycling of nutrients and an important habitat
for animal populations that have in many cases coevolved
with them.
Grasslands and grassy steppes are dynamic ecosystem components that are constantly responding to climate, fire,
animals, geomorphic change, and human impact. Grass
abundance within vegetation communities, as well as the
diversity of grass species, responds to changes in seasonal and
annual precipitation and to changes in evaporation rate due
to variations in annual or seasonal temperature. This can be
seen historically (e.g., the Dust Bowl) but especially
prehistorically in the paleobotanical record, where there is
abundant evidence that grass abundance, distribution, and
diversity have fluctuated significantly during the late
Quaternary. At times grasses have been much more, and at
other times much less, ample within vegetation communities
where they presently occur. In the West, and in California in
particular, both pollen and plant macrofossil records provide
evidence of the ebb and flow of grasses within late Quaternary vegetation communities.
Although there is some phytolith evidence from dinosaur
coprolites (Prasad et al. 2005) suggesting the presence of
grasses during the upper Cretaceous in central India, the first
well-documented appearance of grass pollen (spherical shape
and single pore) in the evolutionary record suggests an origin on the Gondwana continent (present-day South America
and Africa) shortly before the beginning of the Paleocene
(!65 million years ago) (Jacobs et al. 1999). The earliest
unequivocal grass fossils date to the Paleocene-Eocene
boundary, about 56 million years ago (Jacobs et al. 1999;
Kellogg 2001). After grasslands’ appearance (65 – 50 million years ago), their expansion seems to have been limited
until the middle and late Miocene (ca. 20–10 million years
ago), when grasslands and grass-rich ecosystems became
widely distributed as a result of either lower atmospheric
CO2 content, which gave grasses a photosynthetic advantage, or, more likely, climatic changes that created a fire
regime suitable for the replacement of woodlands by grasslands (Keeley and Rundel 2005). Keeley and Rundel (2005)
suggest that during the middle and late Miocene, climates
became more seasonal, resulting in an annual cycle comprising a wet season of high plant production followed by a
dry season during which these materials dried. A monsoonal
climate coupled with the dry season generated storms with
abundant lightning, which ignited fires that cleared the previously dominant forest habitats and paved the way for grassland expansion.
During the Miocene epoch, 20 million years ago, grass
species developed characteristics that are similar to those of
modern grasses, even identifiable to modern genera. In particular, they evolved with herbivore grazing. This is especially
true of western North America, where grassy habitats and herbivores characterized the Miocene of much of the region and
had coevolved through the Eocene. In addition, grasses seem
to have developed the capacity to respond during the same year
to dramatic increases in either winter or spring/summer
precipitation.
The history and environmental relationships of grassy habitats of the West during the late Quaternary is being revealed
by three kinds of paleobotanical evidence: pollen, plant
macrofossils (primarily seeds), and phytoliths. Each of these
provides different yet complementary kinds of evidence
regarding the distribution and abundance of grasses, and in
37
some cases of their importance to human and animal populations. Grasses, like all plants that rely upon the vagaries of
the wind for pollination, produce relatively large quantities of
pollen. These clouds of pollen settle across the landscape and
accumulate in lakes, bogs, or other locations favorable to their
preservation. Analysis of samples from such places provides a
potentially continuous record of the local and regional
relative abundance of grasses.
Phytoliths (silica concretions that form in the cells of grass
plants) provide a record of their actual distribution on the
landscape. Because they are deposited in the ground and are
buried when the plant dies and decays, phytoliths rarely
blow around the landscape. Therefore, they mark the places
where grasses actually grew.
Grass macrofossils usually occur in contexts where they have
been collected either by animals or humans. In most cases,
grass macrofossils are in relatively close proximity to the areas
where they were collected. This is especially true in the case of
small mammals, which have a relatively restricted foraging
area. In the case of small mammals, nesting sites are rarely preserved for very long, so they do not provide a long-term record
of grasses in a region. However, the indurated nests (middens)
of woodrats provide an exception. Urine-encrusted woodrat
nests can preserve plant macrofossils, insects, and pollen for
tens of thousands of years (Betancourt et al. 1990).
The relationship between plant macrofossils collected by
ancient peoples and the sources of these materials is a bit more
problematic. People can move across great distances in order to
collect raw materials and food for their survival. However, in
most cases plant materials are used, processed, or stored close
to the area of collection (Anderson, Chapter 5). Like both
pollen and phytoliths, plant macrofossils must be deposited in
places where their preservation is ensured. One exception to
this is if the plant materials are charred by fire. In that case they
become very resistant to destruction after burial.
General Characteristics of the Late Quaternary
History of Grasses
The paleoecological evidence of episodic increases and
declines in grass abundance and changing distributions during
the late Quaternary (we will restrict our purview to the last
20,000 to 30,000 radiocarbon years before present [rcyr BP]) in
the western Unitied States consists primarily of pollen data.
Paleoecological evidence from the late Quaternary of the
West suggests that grass dynamics are primarily the result of
changes in precipitation, though temperature may at times
also play a role. Ideally, for our examination of grassland history, we would examine pollen localities located in the Central
Valley in the midst of the grassiest habitats, i.e., the northern
portion of the Central Valley. Unfortunately, there are no
good palynological (study of pollen) records from such environments. Instead, our pollen records are obtained from
environments that, in most cases, do not correspond to grassdominated habitats in the West. Currently, late Quaternary
pollen records documenting the dynamics of late Quaternary
38
HISTORY
vegetation (including grass) have been obtained from a
variety of habitats; including coastal estuaries, large and
small lakes lying at a wide range of elevations on both sides
of the Sierra Nevada/Cascade mountain ranges, bogs and
higher mountain meadows, desert springs, caves, ancient
woodrat nests (middens), and archaeological sites. In most
cases, the archaeological records are either poorly dated or
undated and so cannot be used to provide much information
regarding the late Quaternary history of grass. However, several of the coastal estuary, mountain lake and meadow, and
desert spring records are dated and provide a continuous, and
at times detailed, record of grass expansion and contraction
in plant communities throughout the West and California in
particular. These are supplemented by well-dated and, in a
few cases, well-stratified ancient woodrat midden records
(Wigand and Rhode 2002).
TH E P LE I STO CE N E
Thus far, there are four long pollen sequences recording local
and regional vegetation change:
•
The three-million-year sequence from Tulelake on the
eastern edge of the Modoc Plateau in northeastern
California (Adam et al. 1989)
•
The pollen sequence from Owens Lake on the lower
portion of the Owens River east of the Sierra Nevada
Mountains in southeastern California, comprising an
approximately 180,000-year-long section (Woolfenden
1993, 2003), and a lower section extending from the
base of this section to over 870,000 years ago (Litwin
et al. 1993)
•
The !130,000-year-long sequence from Clear Lake in the
coast range of northern California (Adam 1981, 1988)
•
The ongoing analysis of a fragmentary million-year long
record from the Buena Vista Lake Basin southwest of
Bakersfield, California, at the mouth of the Kern River,
which will also eventually provide some information on
grass history (Wigand 2006, unpublished data)
These records provide an indication of the response of
grasses under California’s natural variation in precipitation,
temperature, and climate. Variance in the environment ranges
from annual (Reever-Morghan et al., Chapter 7) to decadal
patterns (e.g., El Niño/La Niña cycles) or to millennial scales
(e.g., glacial cycles). Temperature-driven changes based upon
orbital-scale climate change underlie many of these precipitation cycles (Ruddiman 2001; Liu and Herbert 2004), but they
can also drive grass response by reducing evaporation rates,
thereby increasing effective water availability.
At scales of tens and hundreds of thousands of years, the
earth’s orbital characteristics, including axial precession
(precession of the equinoxes), obliquity (or tilt of the earth’s
axis), and eccentricity, have resulted in differences in solar
insolation affecting global temperature (Milankovitch 1930).
Variations in the amount of heat accumulated in various
regions (oceans or land, Northern or Southern Hemisphere)
have driven the global climate system (Cronin 1999: 560).
Changes in millennial-scale solar insolation directly impact
the amount of moisture evaporated from the oceans, the paths
of this moisture across continents, its condensation as precipitation, and finally its accumulation as glaciers. Glaciers, once
they begin to grow, create their own local climates that further
impact the amount and nature of precipitation. Over the span
of the late Cenozoic the rise of the Sierra Nevada/Cascade
Mountains has further affected long-term grass distribution
and abundance through changing the distributions of moisture on opposite sides of their crest. A general rule of thumb
is that the Sierra Nevada Mountains have risen roughly 100
meters per 100,000 years since about 3 million years ago. Given
orographic forcing of precipitation on the western slopes of
these rising mountains, each successive glaciation forced more
moisture out of storms west of their crests and increased the
rain shadow effect on their lee. The eventual result of this
process might suggest the gradual decrease in grass abundance
east of the Sierra Nevada crest and its gradual increase at sites
on the west slope of the Sierra Nevada Mountains. This has not,
however, been observed in the pollen records from Tulelake
and Owens Lake.
The three-million-year record of grass abundance from
Tulelake in northeastern California’s Modoc Plateau clearly
reflects the trend to progressively cooler glacial cycles during
the last 3 million years (Adam et al. 1989). Currently Tulelake
is much smaller than it was during the Pleistocene. In presettlement California, it was surrounded by marsh habitats
dominated by sedges and cattails. Now, lowlands beyond the
marsh are composed of grassy sagebrush steppe. Whereas
intermediate elevations are covered with western juniperdominated woodlands, higher elevations are characterized by
pine- and fir-dominated montane forests. Compared with
areas further south and east in the Great Basin, the Modoc
Plateau region surrounding Tulelake is rich in grasses.
During the Quaternary, the pollen record indicates
increasing abundance of grass in the Tulelake core. This
appears to correspond directly to both increasingly cold and
long glacial cycles, as recorded in the !18O of Atlantic Ocean
sediment cores (Ruddiman 2001). In the deeper sections of
the Tulelake core, greater grass abundance is both low and
sporadic in its frequency (Adam et al. 1989). Grass abundance
appears to correlate only with the coolest of the glacial cycles
prior to 0.6 million years ago. However, at the top of the core
it is not only much more abundant, but abundant for much
longer spans of time (Adam et al. 1989). The correspondence
of increased grass pollen in the Tulelake record with the current timing of late Pleistocene glacial cycles is given additional support by coincident increases in pelagic lake algae
during these periods, suggesting a deeper lake (Adam et al.
1989). It appears that the early and later portions of the
Holocene (the modern interglacial) have been cool enough to
support more relatively abundant grass communities, compared to those found in previous interglacials (Figure 4.1a). At
Tulelake, abundant grass primarily reflects increased effective
precipitation due to reduced evaporation rates, although
there may have been a slight increase in real precipitation.
Especially during the last 600,000 years, grass has been more
abundant than in the previous 2.6 million years (Adam
et al. 1989). That transition occurred when 100,000-year glacial cycle dominance replaced 41,000-year glacial cycle dominance (Ruddiman 2001). Since 600,000 years ago, glacial
cycles have been both more severe and longer in duration,
factors that have encouraged the proliferation of grasses in
areas where they would normally be rare.
At Owens Lake the record of grass is less clear. Counts of
grass pollen are very low, and many samples do not even have
grass in them. This could indicate poor preservation or simply
its rarity. Despite this, there is a general correspondence of
grass pollen appearance in both the Tulelake and Owens Lake
records. The major difference is that the increase in grasses at
Owens Lake between 500,000 and 1,000,000 years ago is much
more dramatic than that at Tulelake. The two most abundant
periods of grass at Owens Lake during this period are centered
at !550,000 and at 750,000–800,000 years ago. Both of these
periods correspond to major glacial episodes, as indicated in
the !18O record from the eastern Pacific Ocean (Ruddiman
2001: Figure 12.16).
The pollen record from pluvial lake Buena Vista, southeast
of Bakersfield, contains a discontinuous pollen record spanning the last million years. Although analysis is ongoing,
the record indicates several episodes of increased grass pollen
during the last 250,000 years, between 370,000 and
380,000 years ago, about 440,000 years ago, 560,000 years
ago, and 680,000 years ago. (Wigand 2006, unpublished data).
The most dramatic increase occurred at 560,000 years ago,
roughly coinciding with the dramatic increase in grass in the
Owens Lake record centered at 550,000 years ago. This latter
event corresponds with the longest-duration cool, moist
episode, as recorded in the Owens Lake pollen record by
plants reflecting cooler, moister climate (our diagrams plotted
from data presented by Litwin et al. 1993).
Therefore, both the Tulelake and Owens Lake pollen records
indicate that cooler temperatures and increased effective precipitation, due to reduced evaporation rates during glacial
episodes, appear to have been crucial in dramatically increased
grass in semiarid woodland and shrub steppe habitats in the
currently dry interior regions of the West. Was this also true of
the region west of the crest of the Sierra Nevada Mountains?
Clear Lake in the coast range of northwestern California is
the only published record from western California that
extends beyond the latest Pleistocene (Adam 1988: 86).
Currently, this lake is surrounded by blue oak (Quercus
douglasii)/gray or foothills pine (Pinus sabiniana) forest with
small patches of chaparral near its southern shore. Grasses
occur primarily as understory in both the forest and
chaparral habitats, and as a result are not as abundant as in
more open habitats. A 130,000-year-long pollen record from
the lake provides a pollen record extending back into the
penultimate glaciation (Illinoian) (Adam 1988). Grass pollen
PLEISTOCENE AND PRE-EUROPEAN GRASSLANDS
39
F I G U R E 4.1a. Grass-to-sagebrush and grass-to-total-terrestrial (grass pollen percentage) pollen ratios for five of the longest pollen records from
California (the ratios from the other two long pollen records from California, Clear and Owens Lakes, are not illustrated here). The Exchequer
Meadow data are available online from the National Climate Data Center, NOAA, at the North American Pollen Database (NCDC 2007). The
Tulare Lake and Playa Vista core 1 data are available online from the Department of Geosciences at the University of Arizona (Davis 2002).
The Tulelake data were obtained from Adam and Vagenas (1990: 307). The axis on the right of each plot records the ratio of grass to terrestrial
pollen; the axis on the left records the ratio of grass to sagebrush. Courtesy of Dr. Peter E. Wigand.
has usually constituted less than 4% of the terrestrial pollen
at the site. During the late Quaternary, there is a clear correspondence between grass pollen abundance and vegetation
community structure that can be tied indirectly to climate.
Grasses seem to be slightly more abundant during interglacial
periods, when oak forests are dominant. This may reflect a
slightly more open structure in oak forests, as opposed to the
mixed conifer forests that characterized the glacial periods.
More light and precipitation was probably available for a
grass understory to develop. This is confirmed by greater
40
HISTORY
abundance during interglacial periods of alders, willows,
hazel, buckthorn, and composites (Adam 1988).
However, increased sagebrush during glacial cycles in the
Clear Lake record also suggests either that there were open
areas nearby or that the conifer forest may have been more
open in some areas. Because values of sagebrush pollen were
never greater than 10%, there probably was never a real sagebrush steppe community within the region (Adam 1988).
Episodes of greater moisture during glacial periods seem to
have resulted in sagebrush decline and grass increase. These
periods may also have been slightly warmer as well, as is suggested by declines of fir during such episodes. In summary,
over the span of the Pleistocene, grass abundance became
greater in shrubby steppe environments east of the Sierra
Nevada Mountain crest during each successive glacial period.
This seems to correspond to a shift toward cooler, wetter, and
longer-duration glacial cycles. Cooler temperatures during
these cycles resulted in greater effective soil moisture. In addition, the wettest portions of these cycles were not during the
glacial maxima, but during the slightly warmer onsets and
declines (Wigand and Rhode 2002). During glacial maxima,
reduced global temperatures caused orographic rainfall to
commence and be more intense at lower elevations than is
the case today on the western slopes of the Sierra Nevada and
Cascade mountains. As a result, much of the moisture that
reached the coast of California during the Pleistocene was
forced out of Pacific storms before they reached the crest of
the mountain ranges. This resulted in dramatically reduced
precipitation in the intermountain interior during the glacial
maxima, when increased precipitation might be expected.
However, reduced precipitation was somewhat balanced by
much reduced evaporation rates, due to the lower global
temperatures, as well. The ultimate result was a cold, dry
continental climate during glacial maxima in the intermountain West. However, just before and after the glacial
maxima were periods of warmer temperatures, when the rate
and amounts of precipitation wrung out of Pacific storms on
the western slopes of the Sierra Nevada and Cascade mountains was reduced. That is, because of warmer temperatures,
condensation of moisture occurred at higher elevations and
more slowly than during the glacial maxima. More and wetter
storms were able to cross the Sierra Nevada/Cascade mountain ranges and bring precipitation to the interior. Although
evaporation rates were slightly higher, there was a net gain
in effective precipitation. It is during these periods that we
see grass expansion in sagebrush steppe and juniper woodlands in the north and in the middle and higher elevation
shrub steppe communities in the south (Wigand and Rhode
2002). At the same time, pluvial lakes reached their highest
levels (Benson et al. 1990), and glacial advances in the
Sierra Nevada reached their greatest late Pleistocene extent
(Phillips 1996).
The pollen record from Tulare Lake at the southern end of
the Central Valley is the longest record currently available
from southern California west of the crest of the Sierra
Nevada Mountains (Davis 1999). There the pollen record
reveals three periods during the last 27,000 years when
grasses were relatively more abundant. During the last glacial
cycle of the Pleistocene it appears that grass was more abundant between 26,000 and 19,000 rcyr BP than during the
glacial maximum 18,000 to 19,000 rcyr BP (Figure 4.1a). This
is similar to the record from Clear Lake, where decreased
grass abundance corresponded to cooler, drier episodes
around the glacial maximum.
The transition from the Pleistocene to the Holocene is not
well documented. Unfortunately, the longer pollen records
discussed in the foregoing paragraphs have very low-resolution
(wide spacing between samples) during the late Pleistocene/
Holocene transition. In some cases, such as Tulare Lake, the
Pleistocene/Holocene transition is missing (Figure 4.1a).
The Playa Vista 1 pollen record from the Ballona Estuary
reveals a late Pleistocene increase in grasses between
!12,500 and 11,500 rcyr BP that corresponds to increased
pine, and moist-climate shrub pollen values from the same
core (Figure 4.1a; Wigand in press, a). We know from pollen
and woodrat midden macrofossil records from the northern Mojave Desert that climates were much wetter between
13,000 and 12,000 rcyr BP than they had been during the glacial maximum (Wigand and Rhode 2002). Unfortunately,
there is no preserved pollen record between 10,700 and 9,500
rcyr BP from the Playa Vista 1 locality, and only poorly
preserved pollen from the Playa Vista 8 locality for this period
(Figure 4.1a). Therefore, we do not have a good picture of what
may have been happening with grasses during the transition
from glacial to postglacial climates in southern California.
The record from Exchequer Meadow from the western
slope of the Central Sierra Nevada Mountains provides a
record of the transition from late glacial alpine/subalpine
grassy habitats to Holocene Sierran Montane Forest. Currently,
the meadow is dominated by sedges (Scirpus spp.) and rushes
(Carex spp.) (Davis and Moratto 1988). During the latest Pleistocene (between !13,000 and 12,000 rcyr BP), Exchequer
Meadow was dominated by grasses, sagebrush, and composites. This suggests a climate considerably cooler and drier
than that of today. Grasses were more abundant at Exchequer
Meadow at that time than at any time later during the
Holocene. This is in part due to the fact that slopes surrounding the meadow were still almost entirely dominated
by subalpine grasslands. By 10,500 rcyr BP, grass abundance
had declined precipitously as sedges and rushes became more
abundant in the increasingly wetter meadow. The grasslands
surrounding the meadow were slowly invaded by pine and
fir as they readvanced into higher elevations of the Sierra
Nevada to areas surrounding Exchequer Meadow. These
changes signaled both warmer and moister climates, as
storms that, during the glacial period, had lost much of their
moisture at lower elevations now carried increasing amounts
of precipitation to higher elevations.
The Tulelake record indicates that during the last glacial
cycle grasses were more abundant than during most of the
Holocene. In addition, a dramatic increase of grass that
occurred at !10,000 years ago at Tulelake may correspond
with an episode of effectively globally more moist climate due
to very cold temperature conditions that occurred between
11,200 and 10,200 rcyr BP. Allowing for the vagaries of radiocarbon dating and changing deposition rates, this cool
episode is recorded in the Tulelake record at 10,000 rcyr BP.
This cool episode may also appear in the record from Tulare
Lake, where it is recorded as the highest early Holocene grass
values centered at 10,000 rcyr BP (Figure 4.1a). At about the
same time, the European Younger Dryas event was bringing
dramatically lower global temperatures.
PLEISTOCENE AND PRE-EUROPEAN GRASSLANDS
41
FIGURE 4.1b. Grass-to-sagebrush and grass-to-total-terrestrial (grass
pollen percentage) pollen ratios for four pollen records from California
that span much of the middle and late Holocene. The San Joaquin
Marsh, Shellmaker, and John Wayne Freshwater Marsh records are
from coastal southern California and are available from the
Department of Geosciences at the University of Arizona (Davis 2002).
The Dinkey Meadow record is from the west slope of the central Sierra
Nevada Mountains and is available online from the National Climate
Data Center, NOAA, at the North American Pollen Data Base (NCDC
2007). The axis on the right of each plot records the ratio of grass to
terrestrial pollen; the axis on the left records the ratio of grass to
sagebrush. Courtesy of Dr. Peter E. Wigand.
TH E HOLO CE N E
The Holocene record of grass in California is documented
by substantially more pollen records than during the Pleistocene
(Figures 4.1a–d). It is clear that there are both long- (centuryscale) and short-term (multidecade) cycles in grass abundance.
42
HISTORY
FIGURE 4.1c. Grass-to-sagebrush and grass-to-total-terrestrial (grass
pollen percentage) pollen ratios for four pollen records from
California that span the late Holocene. These pollen records are all
from southern California west and northwest of the Los Angeles
Basin. The San Nicholas Island data are available from the
Department of Geosciences at the University of Arizona (Davis 2002).
The data for Zaca Lake, Carpinteria Marsh, and Cleveland Pond are
from Mensing (1993). The axis on the right of each plot records the
ratio of grass to terrestrial pollen; the axis on the left records the ratio
of grass to sagebrush. Courtesy of Dr. Peter E. Wigand.
However, it is only the highest-resolution pollen records that
can reveal what seem to be decade-long episodes of dramatic
grass increase (only one or two such records currently exist in
the West). In addition, the available pollen records show that
grass abundance is highly variable. These differences may reflect
microclimatic differences as well as the impact of local topography. Finally, it should be noted that the age assignments are
not exact. Differences in calculation of the deposition rate of
each site due to the position and number of radiocarbon dates
FIGURE 4.1d. Grass-to-sagebrush and grass-to-total-terrestrial (grass pollen
percentage) pollen ratios for three pollen records from California that
span the latest Holocene. The Playa Vista and Long Beach Campus pollen
records are both from southern California: Playa Vista from the southwestern corner of the Los Angeles Basin, and the Long Beach Campus
record from the Long Beach area. The pollen data from these sites are
available through the University of Arizona Department of Geosciences
(Davis 2002). The Woski Pond record is from the west slope of the central
Sierra Nevada Mountains, and the data are available through the North
American Pollen Database (NCDC 2007). The axis on the right of each
plot records the ratio of grass to terrestrial pollen; the axis on the left
records the ratio of grass to sagebrush. Courtesy of Dr. Peter. E. Wigand.
and the errors associated with them can result in slight
differences from site to site in the apparent position of specific
climatic events. In general, there appear to be several cycles of
grass increase during the Holocene that seem to reflect region
precipitation patterns, and mirror their change through time.
After the conclusion of the early Holocene Younger Dryas
event, there was a period of summer-shifted precipitation
between about 9,500 and 8,000 rcyr BP, coincident with the
post-glacial thermal maximum (Wigand and Rhode 2002).
Although this is most strongly manifested in the southern
portion of the intermountain West, it is evidenced as far north
as the northern Great Basin. A middle Holocene increase in
precipitation that occurred between 8,000 and 5,000 rcyr BP
was concentrated primarily in southern California during a
period of warmer temperatures (Wigand in press, a). A brief
cool, winter-wet event centered ! 5,500 rcyr BP, because of its
brevity, is only seen in higher-resolution pollen records from
the West (Wigand and Rhode 2002). The cool, winter-wet Neopluvial period occurred between !4,000 and 2,000 rcyr BP
(Wigand 1987; Wigand and Rhode 2002). The evidence for
these events is much more pronounced in pollen sites in the
northern half of the West than in the southern half. A dramatic increase in precipitation at the same time that much
cooler temperatures occurred !2,000 rcyr BP. The impact of
this event appears to have been similar throughout the West
(Wigand and Rhode 2002; Wigand in press, a). Between
!1,600 and 1,000 rcyr BP the West was characterized by
another period of wetter climate. However, this period was
characterized by warmer temperatures and increased late season (summer) precipitation (Wigand and Rhode 2002). A succeeding period of warm, moist winter climate between !800
and 700 rcyr BP resulted in a brief, though dramatic, increase
in biotic productivity in the northern intermountain West
and in some areas of the southwestern United States as well.
Finally, a period of cool, wet winters during the last phase of
the Little Ice Age, between about !350 and 180 rcyr BP, resulted
in the most recent surge of grass abundance in the West.
Pollen from Tulare Lake in California’s Central Valley provides one of the most sensitive proxy records of Holocene vegetation change for south-central California (Davis 1999). That
record is tied to changes in lake level and marsh history of the
lake as well (Negrini et al. 2006). At Tulare Lake there was an
early and middle Holocene episode of more abundant grass
between 10,000 and 5,000 rcyr BP, with a major break about
8,000 rcyr BP (Figure 4.1a). The middle Holocene grass event
clearly suggests more significant increases in precipitation than
does the early Holocene event. Evidence from the Mojave
Desert suggests that the early Holocene event is the result of an
increase in summer precipitation (Wigand and Rhode 2002).
An increase in grass between 8,000 and 6,000 rcyr BP corresponds to a hypothesized middle Holocene increase in summer
precipitation that is revealed in recent reanalyses of two cores
from the Ballona Estuary in the southwestern corner of the Los
Angeles Basin (Wigand in press, a). The Playa Vista 1 and
8 records from the Ballona Estuary provide some of the most
detailed information on both local and regional vegetation
change for coastal southern California currently available.
Although the timing and magnitudes of individual responses
in the two cores are variable, there is clear evidence of a middle
Holocene increase in grass around the Ballona Estuary between
8,000 and 6,000 rcyr BP (Figure 4.1a). This corresponds to
middle Holocene increases in oak and chaparral species as
well (Wigand in press, a). A late Quaternary synoptic climate
model reconstruction of southern California climate by
Dr. Reid Bryson of the University of Wisconsin suggests that
this period was characterized by warmer annual temperature
and increased annual precipitation, including increases in both
winter and summer precipitation (Wigand in press, a).
Farther north, at Exchequer Meadow, there are only slight
increases in grass pollen at that time (Figure 4.1a). However,
they are insignificant when compared with early Holocene
grass abundance at Exchequer Meadow. Yet farther north, at
PLEISTOCENE AND PRE-EUROPEAN GRASSLANDS
43
Tulelake, this event does not appear in the pollen record
(Figure 4.1a). This suggests that this event did not extend
much beyond the latitude of central California.
Evidence for increased grass in response to a brief, but dramatic cool, winter-wet episode centered around 5,500 rcyr BP
is evident only in the higher-resolution records from the
Shellmaker and Playa Vista 8 sites from southern California
(Figures 4.1a and b). An event at Dinkey Meadow that
appears to have occurred at about 4,700 rcyr BP may also be
this episode (Figure 4.1b). In this case, however, the episode
may simply be shifted as a result of the problems previously
mentioned in calculating an accurate chronology for the
core. In the Intermountain West the 5,500-rcyr BP event is
striking. It can be traced from the eastern Washington Plateau
to the northern Mojave Desert. It is manifested in data as
diverse as increased vegetation density in eastern Washington,
shifts from desert shrub to sagebrush steppe vegetation in
southern Oregon, dramatic rises of lake levels in Lake Tahoe,
and renewed spring activity in the northern Mojave Desert
(Wigand and Rhode 2002).
Although several small-scale increases in precipitation
have been recorded in the West between 5,500 and 4,000 rcyr
BP, the first significant ones occurred as part of what is called
the Neopluvial. Three episodes of cool, winter-wet climate
centered at !3,700, !2,700, and !2,200 rcyr BP characterize
the Neopluvial period (Wigand 1987; Wigand and Rhode
2002). The Tulare Lake, Playa Vista, Shellmaker, and, apparently, Dinkey Meadow sites may all record one or more of the
three episodes associated with this period (Figures 4.1a–c). In
the interior West these events are much more strongly
manifest in the northern half of the region (Wigand and
Rhode 2002). Expansions of woodlands and grassy habitats
during this period were the greatest since the end of the Pleistocene. These data suggest a cool, winter-wet precipitation
pattern that probably originated in the northern Pacific.
Although the impact of the dramatic, but brief cold, winterwet episode of climate centered around 2,000 years ago is most
dramatic in higher-resolution pollen records of the intermountain West, it also appears as an episode of increased grass
in the Tulare Lake, Shellmaker, and John Wayne Freshwater
Marsh records (Figures 4.1a and b). The event is also recorded
in the Playa Vista 1 and 8 records, but not as an increase in
grass. In the Ballona record it is characterized by dramatic
regional increases in both pine and sagebrush pollen (Wigand
in press. a). Because the increase in these pollen types was so
great, they may have masked a similar increase in grass pollen.
This episode is recorded in pollen sequences from Diamond
Pond in south central Oregon to Lower Pahranagat Lake in
southern Nevada (Wigand and Rhode 2002). This event may
have been caused by a major volcanic eruption that occurred
at that time and was recorded as the highest Holocene sulfur
values in ice cores from both Antarctica and Greenland
(Wigand in press, a). The dramatic increase of precipitation
associated with this event not only affected vegetation distributions but also resulted in dramatic changes in stream flow
regimes, desert lake levels, and even shifts of stream channels
44
HISTORY
on the Mojave River (Ely et al. 1993) and the Humboldt River
(House et al. 2001) for the next 150 years. In addition, although
the increase in grass in southern California does not seem as
dramatic in the pollen record as it does in the intermountain
West, it must have been significant. Its importance in southern
California is hinted at in the charred plant macrofossils recovered from Native American archaeological sites. A dramatic
increase in the number of radiocarbon-dated sites around the
Los Angeles Basin attest to a sudden increase in native population (Wigand in press, b). Grasses constituted 20 to 25% of
the seeds used by these Native Americans at that time (Wigand
in press, b). Although some of the grass seeds were of varieties
that might be found in sandy, or dune, areas, a significant proportion were of types that are typically associated with the vernal pools found on the Los Angeles Prairie (Wigand in press, b).
This suggests that vernal pools may have been less ephemeral
at that time and that the grasses around them may have been
more abundant. Both factors may have played a role in drawing Native Americans to the Los Angeles Prairie at that time.
Perhaps the most interesting event of the last two millennia
occurred between !1,600 and !1,000 rcyr BP. During this
period, which was contemporaneous with the European
Medieval Warm period, warmer climate and decreased winter
precipitation characterized the West. However, this period was
also characterized by increased late spring through early summer precipitation in the northern half of the region and
increased middle to late summer precipitation in the south
(Wigand and Rhode 2002). In the northern intermountain
West, grass abundance relative to winter-loving plant species
such as juniper increased dramatically (Wigand 1987). The
effect of this episode on vegetation, animals, and people was
nothing short of spectacular. In an area stretching from northern Nevada to eastern Washington, grasses became much more
abundant in the sagebrush steppe and semiarid woodlands
characteristic of the area at that time (Wigand and Rhode
2002). In response to this, bison expanded into the lush new
grazing habitats. Radiocarbon dates from archaeological sites
on bison remains records Native American pursuit of these animals into areas where bison had not been since the earliest
Holocene (Wigand and Rhode 2002). In the northwestern
Great Basin, late spring to early summer–shifted precipitation
resulted in the final northward expansion of piñon pine and
promoted a major shift from root crops to pine nuts in the
native economy. In the eastern Great Basin the shift in seasonal
precipitation enabled Fremont horticulturalists to raise maize
in areas where previously it could not be grown. Because these
people had no major irrigation systems but relied upon flood
farming on alluvial fans, they could grow corn only where
there was sufficient summer rainfall available. When this
period ended, !1,000 to 900 rcyr BP, these peoples disappeared.
In southern California the archaeological evidence for the use
of grasses from vernal pool areas noted in the previous
discussion of the 2,000 rcyr BP cool winter-wet episode continued through this period as well (Wigand in press, b). By
!1,000 rcyr BP it appears that Native American populations
declined precipitously around the southwestern corner of the
Los Angeles Basin, and evidence for their extensive use of
grasses wanes.
During the last millennium two wet-climate events are
evident in the paleoecological record. A warm, wet climatic
event !700 to 800 years ago is most strongly recorded in
pollen records currently being analyzed in the northern
Great Basin (Sardine Meadow in northeastern California west
of Reno, Nevada, and Summit Lake, north central Nevada).
This event also appears in the Exchequer and Dinkey
Meadow records and in the Tulare Lake record. It is also evident in the results of a recent study of lake levels in the
Tulare Lake Basin (Negrini et al. 2006). In northeastern California this period is marked by increased grass abundance
and greater spring discharge in the Sardine Meadows south
of the Sierra Valley. At Summit Lake there are dramatic
increases in aquatic algae productivity, indicating warmer
water temperatures, and increases in floating aquatic plant
abundance, indicating deeper water. At Grays Lake, Idaho,
floating aquatic plants became more abundant relative to
littoral plant species, indicating slightly deeper water conditions as well. This event seems to be one that is primarily
restricted to a relatively narrow swath across the northern
Great Basin stretching from the Sierra Valley in California to
Grays Lake.
Finally, it is tempting to associate an increase of grass at
Tulare Lake during the last 300 years to the cool, moist Little
Ice Age (Wigand and Rhode 2002; Figure 4.1a). Tulelake also
records the final Little Ice Age cool, moist climate event (Figure 4.1a). However, a record of the Little Ice Age event is
more difficult to confirm in the other pollen records available
from California. An accurate age assignment to a final
episode of grass expansion apparent in the upper sections of
many of the cores from southern California, and its correlation to the Little Ice Age event, is almost impossible. However,
it is highly probable that many of these grass increases do
correspond to the cool, winter-wet Little Ice Age. Further east
pollen records from the intermountain West contain abundant evidence of increased grass abundance and expansion
of winter moisture-loving plant species during the Little Ice
Age (Wigand and Rhode 2002).
Although the record of grass abundance from the Holocene
appear to be more variable from the West, there is good correspondence to episodes of wetter climate (associated with
both warm- and cold-temperature climate) and, in a few cases,
to shifts in seasonal distribution of precipitation. This correspondence is evident in late Quaternary grass pollen records
from east of the Sierra Nevada Mountains as well. In the more
arid regions of the West, plants (and their pollen records) are
more sensitive to slight variations in precipitation, so apparent responses can be very dramatic (Figure 4.2). There is also
clear evidence that fluctuating precipitation resulting in vegetation change also affected the distribution of the woodrats.
The timing, abundance, and spatial distribution of woodrat
middens are clear evidence of the impact of climate upon
woodrat populations (Wigand and Rhode 2002; Betancourt et
al 1990). A comparison of grass pollen from both sediment
cores and ancient woodrat middens reveals some of the same
major episodes of grass abundance that were discussed above
(Figure 4.2). In addition, the actual periodicity of ancient
woodrat midden occurrence reflects the recurrence of climates
favorable to woodrats. As previously discussed, the grass events
seen in Figure 4.2 were due to a variety of climatic factors.
Some of these episodes were related to slightly warmer, wetter
climates at the onset and conclusions of glacial maxima
(!40,000, 34,000, 21,000, and 10,600 years ago), others were
due to increased summer precipitation during episodes of
much warmer climate (!9,000 and 1,600 years ago).
Regional Climate Differences
From the review of the Holocene portion of this record it is
clear that there are regional differences in the response of grass
(and other plant species) that suggest regional differences in
climate. Differences in precipitation amount, and more importantly in the seasonality of precipitation, are the factors most
clearly associated with these changes in vegetation. A comparison of several average monthly precipitation records from
weather stations in northern and southern California on both
sides of the Sierra Nevada Mountains reveals several clear
patterns (Figure 4.3). In northern California January and
December tend to be the wettest months. East of the Sierra
Nevada Mountains a slightly increased May/June precipitation
is clearly evident. This provides an early growing season for
grasses. If precipitation increases significantly during this
period, as the paleovegetation record clearly indicates that it
has, it results in dramatic increases in grass abundance in shrub
steppe and semiarid woodland habitats east of the mountains
(e.g., between 1,600 and 1,000 rcyr BP). In south-central
California maximum monthly average precipitation occurs
later in winter than in the north and is centered around
February (Figure 4.3). This pattern is similar on both sides of
the Sierra Nevada Mountains at lower elevations. Closer to
the Sierra Nevada Mountains precipitation is again centered
around January, suggesting that the foothills have an earlier
onset of heavy winter precipitation than the surrounding
lowlands do. East of the Sierra Nevada Mountains at the
south end of the Owens Valley, an August/September
increase in average monthly precipitation (related to the
southwestern monsoon) is evident (Figure 4.3). At the
latitude of Blythe, California, in the central Mojave Desert,
August and September have the highest monthly average precipitation (Figure 4.3). In southern California west of the Sierra
Nevada, winter-dominated precipitation is again the typical
pattern (Figure 4.3).
This is roughly the current seasonal distribution of
precipitation in California. However, as the paleovegetation
evidence discussed in the preceding sections suggests, this
pattern has shifted significantly in the past. For example, at
Lower Pahranagat Lake in the northeastern Mojave between
1,600 and 1,000 rcyr BP, middle to late summer precipitation
may have increased significantly (Figure 4). Late spring and
early summer rainfall also allowed water levels in marshes to
PLEISTOCENE AND PRE-EUROPEAN GRASSLANDS
45
F I G U R E 4.2. Late Quaternary grass pollen records from east of the Sierra Nevada Mountains. Pollen records from the more arid side
of the Sierra Nevada/Cascade mountain crest are more sensitive to even small variations in precipitation. These records compare
pollen from both sediment cores and ancient woodrat middens. Periodicity in the occurrence of ancient woodrat middens has been
shown in the past to reflect recurrence of climates favorable to woodrats. The pollen from these records show clear periodicity in
grass abundance during the last 50,000 years. Some of these peaks are due to glacial advances or cooler climate episodes (!40,000,
34,000, 21,000, and 10,600 years ago), and others are due to increased summer precipitation (!9,000 and 1,600 years ago). Diagram
modified from one in Wigand and Rhode (2002).
remain deeper well into the summer months, promoting a
greater abundance of floating aquatic plants (Wigand and
Rhode 2002). Episodes of increased grass between 4,000 and
2,000 rcyr BP suggest that the cold, winter-wet climate
characteristic of the northwestern Great Basin may have
penetrated as far south as the northern Mojave during
the Neopluvial (Figure 4.4). Even the warm, wet episode of
precipitation between 800 to 700 rcyr BP is evidenced in the
Lower Pahranagat Lake record, suggesting that the May/June
increase in monthly average precipitation currently seen in
the northern Great Basin may also have extended into the
northern Mojave during that period.
Conclusions
In summary, late Cenozoic grass populations in the West have
responded to changes in both long- and short-term variations
in climate. On the millennial scale of glacial to interglacial
46
HISTORY
climates these variations have coincided with major shifts,
not only in grass abundance and species composition but also
in vegetation community composition and structure as a
whole. This process was driven by variations in solar insolation
(amount of solar radiation reaching the earth), resulting from
ongoing changes in the earth’s orbit and tilt as well as
variations in solar output (radiation output by the sun), and
mountain building. During the Holocene, variations primarily
in grass abundance (rather than in species composition) have
resulted from changes in precipitation on the scale of decades
to centuries caused by variations in solar output.
During the last 200 years the history of grass has been complicated by the impact of Euro-American activities and by their
animals, primarily cattle and horses. For example, east of the
Sierra Nevada/Cascade crest the impact of the horse may have
been felt as early as the 1680s, when Spanish horses escaped
during the Pueblo uprising. By the 1740s Native American
FIGURE 4.3. Average monthly precipitation records for weather stations near some of the pollen localities
discussed in the text. These provide an indication of the differences in the precipitation pattern within
California. (Source: Western Regional Climate Center, Desert Research Institute, UCCSN Reno, NV,
www.wrcc.dri.edu/climsum.html).
F I G U R E 4.4. Grass-to-sagebrush and grass-to-total-terrestrial (grass pollen percentage) pollen
ratios for a pollen record in the northern Mojave Desert northeast of Las Vegas, Nevada. It is
the highest resolution pollen record currently published for the western Hemisphere and
spans the latest Holocene. Sample spacing is decadal (Wigand, unpublished data).
horse cultures were well established, and grazing impacts upon
native grasses were in full swing. These impacts, together with
fire, have resulted in significantly changed habitats. Not only
have vegetation community compositions changed, but their
structure as well. Only in a few isolated places have relatively
intact native plant communities with rich grass understories
survived the onslaught of cheatgrass and other Eurasian
invaders (Figure 4.5). However, with the onset of global warming, even these few communities may be in jeopardy.
Rancholabrean Mammals of California and
Their Relevance for Understanding
Modern Plant Ecology
Stephen W. Edwards
The term Rancholabrean is rightly associated with the spectacular latest Pleistocene fossil assemblage recovered from
the Rancho La Brea tar pits in the Los Angeles Basin. But that
is only one among many assemblages of late Pleistocene age
in California, not to mention a host of localities producing
isolated mammal fossils. Savage (1951) defined Rancholabrean
as a continent-wide mammalian provincial age, recognized by
the appearance in North America, approximately 150,000 years
ago, of Bison, an immigrant from Asia (Woodburne and
Swisher 1995), and ending with the demise of “charismatic”
megafauna such as mammoths and sabrecats at the end of
the Pleistocene.
By Rancholabrean time the California flora consisted
almost entirely of genera (and, at least in woody plants, also
of species) that are still extant in the state. There were differences in distribution, and there were combinations of plant
taxa that no longer occur together, but the flora and vegetation were nevertheless distinctly Californian and recognizably
modern (Edwards 2004). The fossil flora recovered from the
Rancho La Brea tar pits, for example (Akersten et al. 1988;
Templeton 1964; Warter 1976) is basically of central-southern
California aspect. That paleoflora, as well as many others,
show that cismontane California was not a Pleistocene arctic
48
HISTORY
waste, but experienced a temperate-maritime climate that
served as a refuge for evergreen hardwoods (Johnson 1977).
The Rancholabrean fauna, looking at first encounter like
something belonging in Africa or Asia, was adapted to a
California flora. Mastodons browsed trees and shrubs, horses
grazed needlegrass and the other familiar perennial grasses,
which must have been more abundant and productive in
the more mesic Pleistocene climate than they are in today’s
fully Mediterannean regime. Although human hunters probably sealed the fates of many megafaunal species, it is likely
that a climatically induced type conversion from lush
Pleistocene grasslands to arid Holocene landscapes dominated by native annuals had already diminished megafaunal
populations. The relative contributions of climate change
and hunting are still debated, but the result is clear. The
Rancholabrean fauna, with so many large animals, representing the zenith of the Age of Mammals in North America and
what could be considered the true fauna of California, was
wiped out forever in the space of about 2,000 years. While
the fauna that had coevolved for millions of years with the
flora and vegetation of California thus disappeared, the flora
and vegetation fared better. Though there are many gaps in
the fossil record and in particular herbaceous taxa are very
poorly represented, all indications are that late Pleistocene
woody species persisted to make up modern associations,
and the same is true at least of herbaceous genera. Therefore,
when it is evident that a modern native plant species is well
adapted to grazing and/or browsing or even to fire, it makes
sense to recall the relations that were forged through millions
of years of coevolution between late Cenozoic (and especially
late Pleistocene) mammals and California native plants.
Adaptations affording grazing tolerance among living grassland plants include, among others, widespread occurrences
of toxic or unpalatable defense compounds (e.g., Fabaceae,
Madiinae, Ranunculaceae, Scrophulariaceae) and basal (or
near basal) meristems (probably in all Poaceae). Their presence through numerous genera within families reflects the
deep histories of such adaptations. It is unlikely that they
The Larger Rancholabrean Mammals of California
E DE NTATA (G ROU N D S LOTH S)
FIGURE 4.5. Grassy, semiarid woodland east of the Sierra Nevada crest.
Today overgrazing and fire have decimated much of the grass cover of
the Great Basin at lower elevations. At intermediate and higher
elevations, however, precipitation is high enough for native grasses to
recover from these impacts, and compete with invading plant species.
This photo was taken in the Virginia Mountains just northeast of
Reno, Nevada. The grasses are primarily bluebunch wheatgrass
(Pseudoroegneria spicata, formerly Agropyron spicatum). Pollen records
indicate that prior to the arrival of Euro-Americans most of the
vegetation communities in the Great Basin, ranging from the lower
sagebrush through the semiarid woodland and upper sagebrush
communities, were much richer in grasses. Photograph by P. Wigand.
Megalonyx jeffersoni: Variously reported as black bear to oxsized, and consistently regarded, on the basis of the simplicity
of its grinding teeth, as a browser.
Nothrotheriops shastensis: The smallest of the Californian
ground sloths, grizzly bear–sized at most. Dung deposits in
Utah, Nevada, Arizona, and New Mexico have been attributed to this animal. Studying deposits in Arizona, Hansen
(1978) identified 72 genera of plants in Nothrotheriops dung.
The most abundant taxa were Sphaeralcea ambigua (52%),
Ephedra nevadensis (18%), Atriplex spp. (7%), Acacia greggii
(6%), Cactaceae (3%), Phragmites communis (5%), and Yucca
spp. (2%). A similar study by Thompson et al. (1980) showed
a greater percentage of Ephedra (51%), with Rosaceae and
Agave the other prominent components.
Paramylodon harlani (Harlan’s Ground Sloth): Also known
as Glossotherium h., these were the largest edentates in
California, ox-sized and weighing up to 3,500 pounds. They
were capable of standing on their hind feet and manipulating tall vegetation with massive forelimbs armed with large
claws. Their simple, peglike grinding teeth have been interpreted as useful for grazing grass as well as for browsing, but
definitive research to elucidate their diet has not been done.
U R S I DAE (B EAR S)
suddenly appeared in response to decreased grazing pressures of the Holocene after the megafauna disappeared. But
they would have constituted excellent preadaptations for
fire, more intense in the arid Holocene, and grazing by
hypsodont microtine rodents, rabbits, hares, grazing avifauna,
and elk.
Much basic research needs to be done on dietary habits of
Rancholabrean mammals, but enough data are available
(Edwards 1996, 1998) for preliminary indications to be given.
This will be done, telegraphically, in the following list of the
larger Rancholabrean mammals of California.
There is no way accurately to estimate population numbers
of extinct mammals, but two lines of evidence suggest that
large Rancholabrean herbivores were very abundant and thus
must have had dramatic and pervasive impacts on vegetation
and flora. First, the array of carnivorous mammals and large
scavenging birds (two condors, four vultures, one teratorn, and
six eagles at Rancho La Brea) equals or exceeds that of the East
African Pleistocene, which exceeded that of the game reserves
of East Africa today. There must have been plenty of meat on
the hoof to support this diversity. Second, Davis and Moratto
(1988) found abundant spores of the dung-consuming fungus
Sporormiella in sediments dated to 11,600 rcyr BP at Exchequer
Meadow in the Sierra Nevada. They noted:
Sporormiella spores are abundant in modern sediments only
where introduced grazing animals are plentiful, and they are
even more profuse in sediments older than 11,000 yr B.P. in
several sites. (Davis and Moratto 1988: 146)
Arctodus simus (short-faced bear): These huge animals, outsizing polar bears and having longer limbs, making them
capable of bursts of greater speed, were perhaps the most
powerful predators of the Pleistocene world. Convergences in
skull structure with felids and cheek teeth less modified for
omnivory than those of other bears suggest that the large
ungulates of the day may have been prime targets (Kurten
1967; Shaw and Cox 1993).
Ursus americanus (black bear): These small bears (by Pleistocene standards) are omnivorous, with emphasis on plants
and insects.
Ursus arctos (grizzly bear): The diets of these animals are
well understood from studies in the northern United States
and Canada. They are omnivores, feeding on everything
from limpets in the intertidal zone to glacier lily bulbs in the
subalpine, with copious supplementation from fish and
mammals. In 1862 Brewer (1966) observed the extensive
rototilling effects of these animals resulting from digging for
bulbs in the south coast ranges. Whether in the Holocene
(when it is likely that California’s geophyte flora attained its
greatest prominence) or earlier, bulbs must always have been
a major food source for grizzlies.
CAN I DAE (D O G S)
Canis dirus (dire wolf): This wolf was similar in size to the modern gray or timber wolf but had a larger head, stronger jaws,
more massive teeth, and a heavier build, but shorter lower
limbs. It was presumably a pack-hunter like its modern relative,
PLEISTOCENE AND PRE-EUROPEAN GRASSLANDS
49
and thus capable of exciting and stampeding large ungulates,
thus increasing the impacts of the latter on the landscape.
Canis latrans (coyote)
Canis lupus (gray wolf)
F E LI DAE (CATS)
Felis concolor (puma): Pumas in California today will consume
practically any other mammal, but in terms of kill frequencies they are deer specialists, and that adaptation makes sense
for the Pleistocene, given the relatively gracile skeleton
(though powerfully muscled) of these extremely nimble cats.
Homotherium serum (scimitar cat): These lion-sized cats had
smaller sabers but longer limbs than their better-known relative Smilodon and were probably more cursorial. Evidence
from Friesenhahn Cave in Texas suggests that young mammoths were a favorite prey (Turner 1997).
Lynx rufus (bobcat): Like pumas, bobcats consume a wide
range of prey. They are fully capable of killing adult deer and
commonly do so.
Miracinonyx trumani (American cheetah): Fossils of these
gracile cats have been found in Nevada. These are the rarest
of fossils in a family that is rare in fossil form to begin with.
It is reasonable to suspect that fossils of Miracinonyx will show
up in California, especially because prey animals capable of
speeds far surpassing any other cats, namely pronghorns,
were present in the California Rancholabrean.
Panthera leo atrox (American lion): This was the largest felid
of the Rancholabrean. Males were about 25% larger than
African lions. Grayson (1991) speculates that these were
animals of open country, since they are absent from the
“forested east,” though the eastern United States was not as
forested then as it was historically (Guthrie 1990).
Panthera onca ( jaguar): Jaguars originated in Eurasia or
North America and later spread south across Panamania to
South America. Jaguars persisted in California into historic
time, and “roamed the South Coast Ranges between San
Francisco and Monterey up to at least 1826” (Jameson and
Peeters 1988). In South America their preferred prey include
peccaries and tapirs, and since both were represented in
Rancholabrean California, one may speculate that jaguars
hunted similar species in the north.
Smilodon fatalis (sabrecat): This saber-toothed cat was about
the size of a female African lion. Stock and Harris (1992) suggest on the basis of the skeleton that these animals were less
capable than other big cats of the time in chasing down prey
and hence would have depended more upon stalking and
ambush. One might speculate, then, that they did not specialize in the fleeter ungulates.
TAYA S S U I DA E ( P E C CA R I E S )
Platygonus compressus (flat-headed peccary): This animal is a
little larger than the extant peccary of the desert Southwest,
with longer limbs. Although Grayson (1991) considered this
an animal of open habitats, its dentition is low-crowned and
adapted for browsing. On analogy with living peccaries,
50
HISTORY
Platygonus may have been an opportunistic feeder, browsing
but also rooting for bulbs and taking small animals and
carrion. According to Simpson (1980), Platygonus is questionably distinct from Catagonus, the living Chacoan peccary of
South America that was only discovered in 1975. Catagonus
lives in dry thorn forest and feeds on cacti, bromeliad roots,
fruits, and forbs.
CAM E LI DAE (CAM E LS)
Camelops hesternus (western camel): This was a large camel,
with limbs up to 25% larger than those of the modern dromedary (Webb 1965). Its cheek dentition is higher-crowned than
that of modern tule elk, mixed grazer-browsers that consume
about 50% grass. Camelops cheek teeth often preserve cementum, which gives extra support for heavy mastication. Wear
profiles (mesowear) of cheek teeth are consistent with a
mixed-feeding strategy. Dompierre and Churcher (1996)
concluded on the basis of comparisons of snout shapes in
ungulates that Camelops was a mixed grazer-browser. North
African dromedaries are similarly mixed grazer-browsers.
Akersten et al. (1988) examined dental boli impacted in
molars from Rancho La Brea and found that they contained
Bouteloua, Bromus, Festuca, Hilaria, and Sporobolus, these
grasses collectively amounting to 10.7% of the identifiable
remains. The rest of the identifiable remains were dicotyledonous, but overall, 80% of the material in the boli was
unidentifiable as to monocot vs. dicot.
Hemiauchenia macrocephala (large-headed llama): A slenderlimbed, long-legged llama with relatively high-crowned cheek
teeth, this species has often been interpreted as a swift, opencountry grazer. The native living camelids of the Andes, vicuñas
and guanacos, are open-country animals with diets focused on
perennial bunchgrasses and forbs. Hemiauchenia is in or close
to their ancestry (Webb 1965). Dompierre and Churcher (1996)
interpreted Hemiauchenia as a browser based on premaxillary
morphology, while MacFadden and Cerling (1996) decided
this llama was a grazer, based on carbon isotopes in dental
enamel. More recent assessment of carbon isotopes by Feranec
(2003) led to the characterization of Hemiauchenia as an intermediate feeder with a preference for browse.
CE RVI DAE (DE E R AN D E LK)
Cervus elaphus (elk): Observations of tule elk at Pt. Reyes
National Seashore reveal that these large deer are mixed
grazer-browsers, consuming about 50% grasses and 50%
other, mostly forbs (Gogan and Barrett 1995). Their cheek
teeth are less high-crowned than those of cattle, and they
lack supporting cementum.
Odocoileus hemionus (mule deer): Deer are essentially
browsing animals that take very little grass, when the foliage
is young and tender.
ANTI LO CAP R I DAE (P RONG HOR N S)
Antilocapra americana (pronghorn): Historically, pronghorns
are browsers, and this is an important point, because in
popular literature they are regularly pictured as grazers that
impact grasses directly and substantially. In fact they prefer
shrubs and forbs; like deer, they consume grasses only sparingly and usually when the foliage is young (Yoakum 1980).
Capromeryx minor (dwarf pronghorn): This is a diminutive
creature, less than two feet (0.6 meter) tall at the shoulder.
Anderson (1984: 76) reports it as a grazer, but this is unlikely.
High surface-to-volume ratio would probably have necessitated a focus on higher protein values available in browse. If
one can take Thompson’s gazelle, an animal of similar size, as
an analogue, Capromeryx would have had no more interest in
grasses than Antilocapra does.
Bovidae (Cattle, Sheep, and Their Relatives)
Bison antiquus (Ice Age bison): These animals were morphologically very similar to living Bison bison and some authorities prefer to classify antiquus as a subspecies. On the basis
of plant tissues in dental boli from Rancho La Brea, Akersten
et al. (1988) considered this species a browser (only 13.4%
monocot). However, the relatively wide muzzle is more characteristic of grazers; mesowear on cheek teeth is consistent
with a grazing emphasis; dung attributed to this animal from
Cowboy Cave, Utah (Hansen 1980), is dominated by grasses;
skeletal evidence suggests bison visitied La Brea only for one
month in spring (Jefferson and Goldin 1989) when browse
may have been preferred locally; and isotopic analyses
reported by Feranec (2004) and Feranec and MacFadden
(2000) for Bison in Florida point to a diet ranging from grazing to mixed with an emphasis on grazing. The consensus
among those who have studied fossil Bison is that these were
predominantly grazing animals.
Bison latifrons (giant bison): The largest bison that ever
lived, with horn cores up to 7.5 feet (2.3 meters) across,
became extinct perhaps as much as 12,000 years before the
last antiquus. McDonald (1981) studied the cranial morphology and concluded these were browser-grazers; that is, they
preferred browse but did some grazing. In terms of vegetation
impact, an analogy can be drawn with moose. McInnes et al.
(1992) have shown that exclusion of these large ungulates
can lead to decline of herbaceous cover and invasion of clearings by shrubs and trees.
Euceratherium collinum (shrub ox): Some investigators have
classified these animals, about elk- or cattle-sized, as grazers;
but, according to Mead et al. (2003), dung pellets from sandstone rock shelters in the Glen Canyon region of Arizona
have been attributed to Euceratherium and contain mostly
browse species such as Quercus, Artemisia, and Chrysothamnus.
Mesowear on cheek teeth is consistent with a mixed diet
emphasizing browse.
Oreamnos americanus (mountain goat): Fossils of these animals have been recovered only in the far north, in Lassen and
Shasta counties.
Ovis canadensis (bighorn sheep): Even in the Pleistocene
this species was probably focused in transmontane
California.
Symbos cavifrons (woodland musk ox): This species was
widespread, from Alaska to Texas, though so far in California
it has been found only in Modoc County. Perhaps it is to be
expected at higher elevations in the mountains elsewhere.
Symbos was bison-sized, but more slender. Its dietary adaptations have not been studied.
EQU I DAE (HOR S E S)
Equus conversidens: This species was apparently restricted to
the desert counties east of the Transverse Ranges.
Equus cf. occidentalis (Western horse): According to Harris
and Jefferson (1985) the large sample at Rancho La Brea
affords a reconstruction about 4 .5 feet (1.4 meters) tall at the
shoulder. The cheek teeth of these animals are as hypsodont as
those of any other mammals known. Although isotopic studies suggest some browsing occurred, cheek teeth this hypsodont are intelligible only as adaptations for habitual grazing
of grasses.
TA P I R I DAE (TAP I R S)
Tapirus (tapir): Jefferson (1989) reports two species of tapir
in the Rancholabrean of California. Both occur along the
south coast, but T. californicus also has been found at several
localities in the central Sierra foothills, while T. merriami has
been recovered in Alameda and Contra Costa counties.
Living tapirs are wetland/woodland/forest browsers, and the
low-crowned dentitions of the fossils resemble those of
living species. According to Graham (2003), living tapirs of
the New World are very selective browsers that show some
preference for colonizing plants that are low in toxic defense
compounds.
P ROBOSCI DEA (E LE P HANTS AN D MASTOD ONTS)
Mammut americanum (American mastodon): These were
medium-sized elephant relatives, 6 to 9 feet (1.8–2.7 meters)
high at the shoulder. Their bunodont molars, lacking supporting cementum, were adapted for browsing.
Mammuthus columbi (Columbian mammoth): As large as
African elephants, these massive animals had high-crowned
cheek teeth with closely packed lamellae intermediate in
number between those of African and Indian elephants, both
of which are browser-grazers. Dung ascribed to mammoths
from Bechan Cave in Utah (Haynes 1991) contained over
95% by weight grasses, sedges, and rushes. Dung from Cowboy Cave, also in Utah (Hansen 1980), contained more than
95% grasses, mostly Sporobolus. At Grobot Grotto the dung
contained mostly Phragmites. These mammoths have traditionally been interpreted as open-country animals. The
immense size and length of tusks on some males, nearly
doubling the overall length of the animal, surely would have
limited their mobility in a forested environment. Judging
from behavior of modern African elephants, California mammoths may have opened up vegetation by trampling and
tree felling.
PLEISTOCENE AND PRE-EUROPEAN GRASSLANDS
51
Effects of Megafauna on California Grasslands
Grazing, browsing, and trampling are different but overlapping activities, and the habits of herbivorous mammals sort
out on a continuum. Horses, rabbits, and elephants graze,
browse, and trample with different foci and intensities. The
trampling element is increased as animals are harried by
predators, and Rancholabrean California had one of the most
awesome arrays of predators known since the days of Tyrannosaurus rex. Grazing, browsing, and trampling have all been
demonstrated by contemporary studies to contribute to protection of grassland from encroachment by woody plants. It
is likely that such impacts by Rancholabrean megafauna
helped to maintain large areas of grassland even during glacial
periods when cooler, more mesic conditions favored forests.
The origins of California grasslands ultimately cannot be
fully understood without considering the fauna that evolved
with them. Five million years and more of that history
involved a diverse megafauna, a fauna growing larger and
more diverse with time. Only 10,000 years involved the
anomalously depleted faunal remnants of the Holocene.
Study of Pleistocene megafauna is no idle pursuit. Parkman
(2002) has made a convincing case that brilliantly polished
surfaces high on raised seastacks along the Sonoma coast
were mammoth rubbing stations. He has also suggested that
some of California’s extant vernal pools may have originated
as wallowing basins of Rancholabrean herbivores, and this
hypothesis should be investigated. As for the native plants of
California’s grasslands, it is likely that they preserve a substantial genetic legacy of their relations with magnificent
animals that grazed, browsed, and trampled them not really
so long ago.
Species Composition at the Time of First
European Settlement
Paula M. Schiffman
A very basic question nags at ecologists and habitat managers: What was the species composition of California’s grasslands like at the time of European contact? More specifically,
which species were dominant? This question exists because
the grasslands were colonized by several invasive plant
species soon after European contact (Hendry 1931; Spira and
Wagner 1983; Sauer 1988; Blumler 1995; Mensing and Byrne
1998, 1999), and these species rapidly became incorporated
into natural landscapes. There are no descriptions of grassland species composition from that early time period, and,
amazingly, the invasion went unnoticed. When people finally
began to record detailed vegetation accounts in the mid-1800s
(e.g., Cronise 1868), invasive plant species were already geographically widespread and ecologically dominant.
Early Records
The historical spatial extent of California’s grassland area was
enormous (5.29 million hectares; Barbour and Major 1988:
3 – 10). That such a massive invasion could have occurred,
52
HISTORY
without anyone documenting it, is remarkable and perplexing. The Native Americans who lived in this ecosystem for
millennia used oral communication to share information.
When European diseases and brutality decimated their
populations (Preston 2002b), their in-depth knowledge of
historical grassland species compositions and community
dynamics was largely lost. The first European settlers were not
naturalists, and from the very start they tried to dominate,
rather than describe, their vast new environment. They
simply viewed it as land where opportunities for livestock
grazing and cultivation abounded. Moreover, California’s
grassland-covered plains and valleys were subtle landscapes
that, except in the spring when expanses of colorful
wildflowers appeared, were dry, stark places that lacked the
visual drama of California’s wave-crashed coastline or imposing mountain ranges. Perhaps the minutiae of grassland
species composition seemed trivial when juxtaposed with
such huge, wide-open landscapes. Harrison (1982) has noted
that as a group, early explorers and settlers were unusually
reticent to record their observations and impressions of the
North American prairie landscapes that they encountered. He
describes this odd phenomenon as “verbal blindness.”
Although detailed ecological accounts do not exist, a few
early observers did record general descriptions of California
grasslands. The writings of Juan Crespí, a Spanish priest who
journeyed from Baja California to San Francisco Bay in
1769–1770 and then from San Diego to Monterey in 1770,
were full of descriptions of places with “everything very
grass-grown” (Crespí 2001: 309). Spanish mission period
journals of other early Europeans such as Francisco Garcés,
Pedro Fages, Juan Bautista de Anza, Pedro Font, Josef Joaquin
Moraga, Francisco Palou, George Vancouver, Georg von
Langsdorff, and others also commented on the productive
pastoral environments that they encountered (Coues 1900;
Priestley 1937; Bolton 1930, 1931, 1966; Paddison 1999).
These observers’ accounts of the vegetation were extremely
general and it is clear that descriptions such as “good grass,”
“much grass,” and “level and grassy” terrain were not used
in a strict taxonomic sense. Rather, they were general portrayals of low green vegetation that could be exploited for
livestock grazing. In this context, simplifying a diverse assemblage of species — which would have included many
graminoids, forbs, geophytes, and even subshrubs — as
“grass” made sense. Even if some of the plants were not all
grasses, they grew alongside grasses and they were consumed
by grazing livestock as well as herds of native elk and pronghorn antelope. References to California’s grass-covered landscapes continued well into the 1800s, as did the botanical
imprecision. For example, in an 1847 observation, Edwin
Bryant, an American journalist, noted that “[T]he varieties of
grass are greater than on the Atlantic side of the continent,
and far more nutritious. I have seen seven different kinds of
clover, several of them in a dry state, depositing seed upon
the ground so abundant as to cover it, which is lapped up by
the cattle and horses and other animals” (Bryant 1848: 448).
Native clovers (Trifolium spp.) and grasses co-occurred, and
Bryant lumped these two completely different and unrelated
taxa together into a single functional group.
However, early observers were not so unobservant or naïve
to think that grasslands consisted only of grasses. It is evident
from their journal accounts that native forbs were abundant
in the grassland landscapes through which they passed. For
example, on May 7, 1770, when traveling near the Santa
Ynez River in what is now Santa Barbara County, Crespí
described “a great plenty of white, yellow, red, purple and
blue blossoms: a great many yellow violets or gillyflowers
such as are planted in gardens, a great deal of larkspur, a
great deal of prickly poppy in bloom, a great deal of sage in
bloom; but seeing all the different sorts of colors together was
what beautified the fields the most” (Crespí 2001: 711). Early
descriptions like this one did not include nearly enough
information for us to reconstruct the species composition of
these landscapes accurately today. Still, it is quite clear that
spring-flowering forbs were important, though ephemeral,
ecosystem constituents. A little more than a century after
Crespí, naturalist John Muir’s writings included reminiscences
of great profusions of annual wildflowers in the mid-1800s.
He wrote, “The Great Central Plain of California, during the
months of March, April, and May, was one smooth continuous bed of honeybloom, so marvelously rich that, in walking
from one end of it to the other, a distance of more than
400 miles, your foot would press about a hundred flowers at
every step. Mints, Gilias, Nemophilas, Castillejas, and innumerable Compositae were so crowded together that, had
ninety-nine percent of them been taken away, the plain
would still have seemed to any but Californians extravagantly
flowery” (Muir 1894: 339). Although Muir mentioned several
annual taxa, his descriptions primarily conveyed a vivid
sense of biodiversity rather than an ecologically meaningful
accounting of community composition. However, he did
remark that “all of the ground was covered, not with grass
and green leaves, but with radiant corollas” (Muir 1894: 342).
Clements’ Influence and Recent Interpretations
Because of their ephemeral nature, the ecological importance
of these annual and perennial forbs was not widely recognized. Frederic E. Clements’ (1934) relict analysis indicated
that the perennial bunchgrass, Nassella pulchra (Stipa pulchra
and S. setigera; Hamilton 1997a), had been the historical
dominant in California’s grasslands. He interpreted the
prominence of N. pulchra in some relict grassland fragments
as key. Clements’ reputation as a leading twentieth-century
ecologist led to the acceptance of his hypothesis among
California biologists (e.g., Piemeisel and Lawson 1937; Munz
and Keck 1959; Burcham 1961; Heady 1988). However, the
relatively mesic and periodically burned fragments that were
Clements’ exemplars did not constitute a good representation of the wide range of habitats that supported grassland
vegetation in California. In addition, as Hamilton (1997a)
convincingly explains, the scientific basis for Clements’
hypothesis was shaky because it relied upon little real data
and several erroneous assumptions. Nevertheless, relatively
recent references that discuss California grassland composition
and ecology in detail still usually identify N. pulchra as the
likely historically dominant species (Heady 1988; Schoenherr
1992; Holland and Keil 1995), and field studies, particularly
those focused on conservation and restoration, have continued to give more attention to N. pulchra than to any other
native grassland species. However, it has also been suggested
that several other perennial grasses (e.g., Poa secunda,
Leymus triticoides, Melica spp., Muhlenbergia rigens) were historically more important community constituents in some
environments (Keeley 1990; Heady et al. 1992; Holland and
Keil 1995; Holstein 2001).
But what about the historical importance of forbs? Historical accounts, though limited in ecological detail, did clearly
point to an impressive diversity and cover of colorful spring
wildflowers. Even Clements recognized perennial forbs as
“subdominants” and stated that “even more typical are the
great masses of annuals, representing more than 50 genera
and several hundred species” (Clements and Shelford 1939:
288). In fact, his description of the springtime vegetation of
1935 bore considerable resemblance to the much earlier
descriptions of Crespí and Muir: “the carpet of brilliant blues,
oranges, and yellows covered an area approximately 50 miles
wide and 100 miles long” (Clements and Shelford 1939: 288).
Like other observers, Clements noted the abundance of
native annuals and then glossed over their identities as if
they were unimportant. Despite his clear acknowledgement
of their tremendous percent cover, these plants’ transient
nature indicated to him that they had little real ecological
value. Clements’ endeavor to draw ecological linkages between
California’s grasslands and those of the midwestern United
States demanded that he emphasize perennials, especially
grasses (Hamilton 1997a), despite the ubiquity of so many
annual forbs.
The ruderal nature of annual plants (Grime 1979a) was
another feature of California’s native forbs that precluded
Clements from considering them to be ecologically important. By definition, he viewed climax communities as generally stable associations of species that developed through
succession (Hamilton 1997a). So, although vegetation made
up of weedy, invasive, non-native annuals including Avena,
Bromus, Hordeum, Festuca (Vulpia), and Erodium was considered a “proclimax” community, a stable community dominated by an association of disturbance-adapted native
annual plants completely violated his theoretical framework
and, therefore, went unrecognized. Today, it is well known
that native forbs repeatedly reappear on the same sites for
decades, though their covers vary with annual rainfall
amounts. In addition, soil disturbances by small burrowing
mammals, herbivory, periodic fires, and environmental
management by Native people were integral ecosystem
processes that had compositional consequences including
the promotion of annuals (Blumler 1992; Hobbs and
Mooney 1995; Painter 1995; Schiffman 2000; Reichman and
Seabloom 2002; Keeley 1990, 2002; Anderson 2005). Surely,
PLEISTOCENE AND PRE-EUROPEAN GRASSLANDS
53
Monterey
Monterey
San Luis Obispo
Santa Barbara
Orange
Riverside
ELK
HAS
CAR
SED
STA
ROS
The grasslands are plotted by code in Figure 4.7.
Alameda
San Mateo
Madera
LAW
JAS
JOA
NOTE :
Mendocino
Sacramento
Solano
Marin
HOP
COS
JEP
REY
Hopland Research and Extension Center
Cosumnes River Preserve
Jepson Prairie Reserve
Point Reyes National Seashore
Lawrence Livermore National
Laboratory Site 300
Jasper Ridge Biological Preserve
San Joaquin Experimental Range
Elkhorn Slough National Estuarine
Research Reserve
Hastings Natural History Reservation
Carrizo Plain National Monument
Sedgwick Reserve
Starr Ranch Sanctuary
Santa Rosa Plateau Ecological Reserve
County
Code*
Grassland
39o00’
38o25’
38o15’
38o05’
36o48’
36o22’
35o10’
34o45’
33o37’
33o31’
55.2
53.0
14.5
38.0
38.1
48.0
36.8
65.2
48.6
94.0
44.2
47.5
165.5
Latitude
37o42’
37o24’
37o05’
Mean rainfall
(cm/yr)
566
932
101,000
2,358
1,616
3,434
2,828
481
1,806
2,165
16,160
634
25,907
Area (ha)
TA B L E 4.1
Characteristics of the 13 Relict Grasslands Included in the Ordination
147
399
346
201
266
395
262
439
248
396
216
228
483
Total native
species
9.5
6.0
2.9
6.0
4.5
6.3
4.2
6.4
3.6
8.6
5.6
4.0
9.7
Percent
perennial
grasses
26.5
57.4
58.6
53.2
41.0
48.4
66.8
44.0
65.7
43.7
50.9
52.2
29.8
Percent
annual
forbs
the endurance of native annual forbs in California’s grasslands and their apparently adaptive interactions with other
organisms and processes reflects their historical ecological
significance.
In recent years, researchers have used evaluations of historical accounts, floristic surveys, relict analyses, and modern
experimental and comparative findings to propose alternatives to Clements’ vision of California’s grassland species
composition. Several of these reconstructions have suggested
that annual plant species, rather than N. pulchra or other
perennial grasses, had been the most ecologically important
species in much of southern California and relatively arid
inland environments including the Central Valley (Talbot
et al. 1939; Twisselmann 1967; Wester 1981; Blumler 1995;
Holstein 2000, 2001; Schiffman 2000, 2005). In more mesic
areas, annual forbs still constituted a diverse group of plants.
Sadly, it is now impossible to truly understand the ecological roles of individual plant species at the time of European
contact. Clues to the historical past have been blurred by
massive changes caused by the contamination of California’s
grasslands by invasive non-native annuals and a wide range
of human activities including cultivation, livestock grazing,
fire suppression, eradication of the grizzly bear (a keystone
species), and habitat fragmentation. So, the degree to which
the ecological dynamics in relict grasslands resemble those of
historical ecosystems is somewhat unclear. One thing is quite
certain, however. These habitats continue to support very
large numbers of native species, particularly forbs, just as
they did when Europeans first encountered them.
A Relict Analysis
Relict grassland floras typically include hundreds of native
species in addition to grasses. Therefore, a study of relict floras that focuses on grassland plants of all forms should yield
historically meaningful results. For example, this approach
can be used to estimate the degree to which the native species
compositions of historical grasslands in California resembled
each other. Did regional differences in latitude, proximity to
the Pacific coast, and rain shadow–producing hills mean that
the grasslands of the northern coast or Sacramento Delta
bore little resemblance to those of the San Joaquin Valley or
southern California? They undoubtedly had some species in
common, but how similar were these floras? Were they as
monolithic as much of the literature has implied?
To address these questions I have compared the native
floras of 13 different relict grassland preserves in California
(Table 4.1). Comprehensive plant species lists available for
each of the preserves were the data sources for the study. The
boundaries of these preserves encompass other vegetations in
addition to grasslands (e.g., wetlands, chaparral, oak woodlands, riparian forests, and coniferous forests), and they frequently intergrade. Therefore, grasslands typically share some
species with adjacent communities, and the communities
themselves can be difficult to differentiate. Because there is
ambiguity about the definition of “grassland,” my relict
F I G U R E 4.6. Frequency distribution of annual forbs, perennial
grasses, and other species in the 13 relict grasslands.
analysis was limited to low-stature native plants that could be
considered to be grassland species, at least in a broad sense
(grasses, graminoids, annual, biennial, and perennial forbs,
geophytes, and subshrubs as indicated by species descriptions
in Hickman 1993). Trees and shrubs were excluded from the
analysis, as were their parasites and nonwoody plants that,
according to Hickman (1993), occur primarily in forests. Multiple taxa differentiated below the species level (subspecies
and varieties) were also excluded from the analysis.
The analysis encompassed a remarkable number of plant
species and indicated that California’s extant grasslands are
extremely important reservoirs of biodiversity. A total of
1,348 native grassland species occurred at the 13 sites surveyed. This means that these relict grasslands collectively
support about 40% of the state’s total native plant species
richness (Hickman 1993). Many of the species in this study
occurred at only one or two of the sites, and most of these
species were annuals (Figure 4.6). Surprisingly, just 1% of the
species were present in all of the study’s grasslands. This small
group of ubiquitous species consisted of a perennial herb
(Achillea millefolium), 10 annual forbs (Amsinckia menziesii,
Calandrinia ciliata, Claytonia perfoliata, Crassula connata,
Eschscholzia californica, Lasthenia californica, Lotus wrangelianus,
Lupinus bicolor, Mimulus guttatus, and Trifolium willdenovii), an
annual graminoid (Juncus bufonius), and just one perennial
grass (Nassella pulchra). These findings strongly indicate that,
historically, California’s grasslands were habitat for an enormous number of different plant species and that the vast
majority of them were not perennial grasses.
PC-ORD (MJM Software Design, Gleneden, OR) was used to
compute Jaccard distances (Magurran 1988) for the 13 grasslands and to ordinate them in two-dimensional space
(Figure 4.7). Separation of the grasslands along the horizontal
axis (axis 1) was strongly correlated with percentages of
annual forbs and perennial grasses as well as with mean
annual precipitation (Table 4.2). Latitude was most highly
correlated with the distribution of grasslands along the
PLEISTOCENE AND PRE-EUROPEAN GRASSLANDS
55
TA B L E 4.2
Pearson Correlation Coefficients for the
Two-dimensional Ordination
of 13 Relict California Grasslands
Correlation coefficients (r)
Variable
Percent perennial grasses
Percent annual forbs
Mean annual precipitation
Latitude
Area
Total number grassland species
Axis 1
Axis 2
"0.918
0.905
"0.670
"0.233
0.372
"0.138
0.198
"0.114
0.218
0.819
"0.259
"0.245
F I G U R E 4.7. Ordination of 13 relict California grasslands produced
using Jaccard distances. Each grassland site is indicated by a threeletter code (Table 4.1). Variables correlated with most of the
separation of grasslands along the two axes are plotted as vectors
(percent annual forbs, percent perennial grasses, mean annual
precipitation, and latitude).
vertical axis (axis 2). These correlation relationships were
plotted as vectors (Figure 4.7).
Although Nassella pulchra did occur in all of the grasslands
included in this study, this very simple relict analysis of
species presence/absence data strongly suggested that, historically, grasslands located in different regions of California
had broadly differing species compositions. The ordination
showed four geographically distinctive grassland groupings
(Figure 4.7). San Joaquin Valley grasslands (represented by
Carrizo Plain National Monument, San Joaquin Experimental Range, and Lawrence Livermore National Laboratory Site
300), were characterized by high proportions of annual forbs
and relatively few perennial grasses. In contrast, the more
mesic coastal prairies at Elkhorn Slough National Estuarine
Research Reserve and Point Reyes National Seashore had high
percentages of perennial grasses and fewer annual forbs.
Latitude is associated with environmental and floristic
gradients, and the grasslands of the southern, central, and
northern coastal mountains (Starr Ranch Sanctuary,
Sedgwick Reserve, Santa Rosa Plateau Ecological Reserve,
Hastings Natural History Reservation, Jasper Ridge Biological
Preserve, and Hopland Research and Extension Center) were
56
HISTORY
distributed as a generally latitudinal group with moderate
proportions of annual forbs and perennial grasses. Finally, a
floristically distinctive grassland type occurred in the
Sacramento Delta (Cosumnes River Preserve and Jepson
Prairie). These northerly grasslands also had moderate levels
of annual forbs and perennial grasses.
Unfortunately, these relict grasslands now also include
many non-native plant species, and they no longer experience the disturbance regimes of the pre-European settlement
environment. So it is impossible to estimate the importance
(e.g., percent cover) of particular native species at the time of
first European settlement. Moreover, historical percents cover
of native plants, particularly annuals, would have varied with
the year-to-year variation in winter rainfall amounts and
other environmental factors. Despite the information limitations caused by such realities, this study’s comparative
approach to species presence/absence likely provides an accurate perspective on the historical species compositions of
California’s grasslands. If, however, composition is viewed
more narrowly (for example, in terms of the presence/
absence of perennial grasses or the presence/absence of
invasive non-native species), the relict grassland sites that
this study found to be floristically different would seem
much more homogeneous. It is clear that by fixating on a few
perennial grasses and invasive species, California biologists
have been distracted from what was actually an array of compositionally diverse and regionally distinctive historical
grasslands.