Journal of Sustainable Forestry, 34:30–48, 2015
Copyright © Taylor & Francis Group, LLC
ISSN: 1054-9811 print/1540-756X online
DOI: 10.1080/10549811.2014.973605
Exploring the Early Anthropocene Burning
Hypothesis and Climate-Fire Anomalies for the
Eastern U.S.
MARC D. ABRAMS1 and GREGORY J. NOWACKI2
1
Department of Ecosystem Science and Management, Penn State University, University Park,
Pennsylvania, USA
2
USDA Forest Service, Eastern Regional Office, Milwaukee, Wisconsin, USA
This review explores the long-term role of climate versus human
activity on vegetation and fire dynamics in the eastern U.S. Early
Holocene warming resulted in a conversion of Picea (boreal) to
temperate Quercus and Pinus forests when indigenous populations
were sparse but charcoal abundances were relatively high, underscoring the importance of climate. Pyrogenic trees also dominated
during the middle Holocene Thermal Maximum period, associated
with increasing indigenous populations and high charcoal abundance on most sites. During Neoglacial Cooling (3300 to 150 BP)
charcoal levels and pyrogenic trees remained high in the central
and southern regions apparently due to Native American and early
European burning trumping colder climate. In northern regions,
oak-pine and charcoal abundance were distributed on intermittent dry and/or Native American sites. High levels of charcoal
and pyrogenic species during the early Holocene and Neoglacial
Cooling represent important anomalies in the ecological history of
the eastern U.S. While the importance of warmer and drier climate
is evident throughout, the Early Anthropocene burning hypothesis
is plausible for the eastern U.S. where extensive lightning fires are
rare, outside of the southeast coastal plain. The cessation of Native
American burning and other early European disturbances were
transformative to the ecology of the eastern U.S.
KEYWORDS Native Americans, Holocene, fire, paleoecology,
paleocharcoal, witness trees
Address correspondence to Marc D. Abrams, Department of Ecosystem Science and
Management, Penn State University, 307 Forest Resources Building, University Park, PA 16802,
USA. E-mail: [email protected]
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Exploring the Early Anthropocene Burning Hypothesis
31
INTRODUCTION
A major debate throughout much of the history of vegetation science deals
with the relative importance of climate versus human impacts on plant communities (Clements, 1916; Loucks, 1970; Denevan, 1992; Munoz, Gajewski,
& Peros, 2010). Indeed, many scientists believe we are now in a new geologic epoch called the Anthropocene, during which humans have become
a dominant force shaping Earth systems, including climate (Ellis, Fuller,
Kaplan, & Lutters, 2013). However, the date when various regions of the
Earth entered the Anthropocene has not been resolved and ranges from a
century or two to many millennia. For instance, Ruddiman (2003) argues that
increases in greenhouse gases as a result of agricultural practices dating back
7,000 yr altered climate and represents the start of the Anthropocene. Others
have argued that this new epoch started with the Industrial Revolution in
the late 1700s, associated with huge exploitation of natural resources and
release of greenhouse gases (Steffen, Grinevald, Crutzen, & McNeill, 2011).
Alternatively, the epoch is considered to have begun when the most recent
abrupt warming started during the late 19th or early 20th century (Mann
et al., 2009). Moreover, the Anthropocene may not be limited to humaninduced climate change and may be expanded in theory to include the
profound impacts of early human hunting activities and their use of fire
(Pinter, Fiedel, & Keeley, 2011; Marlon et al., 2013).
The existence of cultural landscapes prior to European arrival has been
well documented throughout the eastern United States and attributed mainly
to human cultivation and use of fire (Day, 1953; Doolittle, 2000; Delcourt &
Delcourt, 2004; Guyette, Muzika, & Dey, 2002; Guyette, Stambaugh, Muzika,
& Dey, 2006b). The Early Anthropocene burning hypothesis was proposed
by Marlon et al. (2013) to address the possibility that human use of fire was
so extensive during the early Holocene that it became a dominant ecological
force on most continents. These authors, however, do not favor this hypothesis for the eastern U.S. where they believe climate played the leading role
in vegetation and fire dynamics. In contrast, many authors have described
the extensive use of Native American burning as a dominant ecological force
shaping vegetation in the East (Curtis, 1959; Heinselman, 1973; Pyne, 1983;
Abrams, 1992). In this article, we attempt to elucidate the degree to which
vegetation dynamics and fire (from charcoal and tree fire scar records) reflect
climate versus human activity before European settlement for the eastern
U.S. Where supporting evidence exists, we ascribe certain vegetation-climate
anomalies to disturbance and possible human origins. We suspect that a
temporal relationship existed between climate and human disturbance, with
climate being the primary driver of vegetation during the early portions of
the Holocene, when human populations (and their associated activities) were
low, after which human disturbance increased in importance with population expansion. This increase should be detectable in vegetation, fire, and
32
M. D. Abrams and G. J. Nowacki
climate anomalies. Moreover, we hypothesize that climate gradients from
north (cold) to south (hot) affected the human footprint in the eastern U.S.
due to its impact on the types and productivity of vegetation and, thus,
human population levels. We reviewed literature from paleopollen and charcoal, anthropology, direct dating of fire scars, and witness-tree composition
to explore the Early Anthropocene burning hypothesis for the eastern U.S.
and other vegetation-fire-climate anomalies.
METHODS
Because direct temperature records do not exist much beyond 120 yr, we
used Greenland temperatures derived from ice cores for generalized trends
(Alley, 2004). These data are considered a good proxy for northern hemisphere temperature trends during the Holocene (Alley et al., 1997) and have
been confirmed, in part, using dendroclimatology (tree ring analysis) of
regional trees (Mann et al. 2009). Nevertheless, we recognize that much temperature variation will exist at local and regional scales in the eastern U.S.
not captured by ice core data. A temperature database that recorded central Greenland temperatures was secured through NOAA Paleoclimatology
Program and World Data Center for Paleoclimatology, Boulder, CO (Alley,
2004). Three primary and contrasting temperature periods were identified
during the Holocene prior to 1900 (Figure 1): Early Holocene Warming
from 11,700 to 8,200 BP, Holocene Thermal Maximum from about 8,200 to
3,300 BP, and Neoglacial Cooling from 3,300 to 150 BP. The Thermal
Maximum period is noteworthy for having the highest estimated temperatures during the Holocene, whereas Neoglacial Cooling culminated with the
Little Ice Age (600–150 BP; Mann et al., 2009), lowering northern hemisphere temperatures to levels not seen for roughly 8,000 yr (Figure 1).
The Neoglacial Cooling period also had several warm phases, including the
Medieval Climatic Optimum from about 1,100–700 BP (Mann et al., 2009).
RESULTS AND DISCUSSION
The role of climate in vegetation dynamics during the late Pleistocene (in
the south) and early Holocene (farther north) is quite apparent. As temperatures increased rapidly, boreal-like forests dominated by spruce, aspen, and
northern pine were replaced by northern hardwoods in northern states; and
oak, hickory, and southern pine in central and southern states (Watts, 1979;
Shuman, Newby, Huang, & Webb, 2004; Gill, Williams, Jackson, Lininger, &
Robinson, 2009; Munoz et al., 2010; Figure 2). In the southern and central
portion of the U.S., the replacement species are not only adapted to a warming climate, but are also pyrogenic (Abrams, 1992; Thomas-Van Gundy &
Exploring the Early Anthropocene Burning Hypothesis
33
FIGURE 1 Climatic periods based on central Greenland temperature (GISP2 Ice Core; Alley,
2004) spanning 20,000 yr BP.
Nowacki, 2013). Concomitant variation with Early Holocene Warming was a
dramatic increase in fire, as evidenced by paleocharcoal data (Power et al.,
2008; Marlon et al., 2013; Figure 3). Enigmatically, this early and rapid rise
in charcoal occurred when Native American populations were relatively low.
This is a primary argument used against the Early Anthropocene (human)
burning hypothesis by Marlon et al. (2013). However, others have argued
that extensive areas can be burned by a relatively small number of people
(Guyette, Spetich, & Stambaugh, 2006a; Guyette et al., 2006b; Kay, 2007;
Pinter et al., 2011).
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M. D. Abrams and G. J. Nowacki
FIGURE 2 Broad and generalized latitudinal variation in Holocene vegetation dynamics from
paleoecological studies for the eastern U.S. until 1900.
Native Americans impacted forests of the eastern U.S. in many ways—
such as hunting, land clearing for settlements and agriculture, promoting
mast and fruit trees and shrubs, but most profoundly in the form of landscape
burning (Day, 1953; Doolittle, 2000; MacDougall, 2003; Abrams & Nowacki,
2008; Nowacki, MacCleery, & Lake, 2012). For example, the long-term sustainability of tallgrass prairie and oak-hickory or pine savannas and forests
has been directly attributed to Native American burning (Sauer, 1950; Curtis,
1959; Pyne, 1983). The Prairie Peninsula extends over much of Illinois and
southern Wisconsin, a region that receives enough precipitation to sustain
forests on most sites (Anderson, 2006). This eastern tallgrass prairie is thought
to have formed from former conifer-hardwood forests after 8700 BP as a
result of Thermal Maximum warming coupled with frequent Native American
burning (Gleason, 1913; Grimm, 1984; Anderson, 2006). After European settlement, a widespread conversion of prairies, including the Prairie Peninsula,
and savannas to closed-canopy oak forests occurred after cessation of Indian
burning (Gleason, 1913; Cottam, 1949; Abrams, 1992; Anderson, 2006).
In Indiana, oak dominance started about 10,500 BP associated with
a large increase in charcoal levels after megafaunal extinction (Gill et al.,
2009). The spread of fire- and weapon-toting humans across North America
left an indelible mark on the paleoecological record (Nowacki et al., 2012).
Disturbance regime changes from megaherbivores to human fire were practically seamless in North America, with the importance of fire increasing
during the period of megafaunal decline and ultimately their extinction (as
measured in dung fungal spores; Robinson, Burney, & Burney, 2005; Gill
et al., 2009). The switch to fire may have substantially lessened the ecological changes expected from megaherbivore loss since fire can mimic
Exploring the Early Anthropocene Burning Hypothesis
35
FIGURE 3 Generalized charcoal, temperature, pyrogenic tree species abundances, and Native
American population trends during the Holocene for the eastern U.S. (adapted from Power
et al., 2008, 2012; Gill et al., 2009; Marlon et al., 2013). The abrupt decline and then rebound
in the charcoal data during the Little Ice Age (LIA; derived from Power et al., 2012) may not
be indicative of all ecoregions of the eastern U.S.
herbivory by consuming vegetation via flame instead of by mouth (Bond
& Keeley, 2005). For instance, herbivory and fire are analogous in that both
are strong vegetation regulators, promoters of open conditions (grasslands,
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M. D. Abrams and G. J. Nowacki
shrublands, and savannas) and early successional plants at the expense
of closed-canopy forests and late successional plants. However, fire differs
from herbivory in several important aspects. First, herbivory selects against
highly edible/digestible plants within certain height windows (animal reach),
whereas fire indiscriminately consumes dead and living pyrogenic material
(regardless of protein value) at all heights from ground to tree top provided
there is flame access (Bond & Keeley, 2005). Nutrient cycling, including
nitrogen volatilization after fire, obviously differs from herbivory contingent on the digested remains (ashes vs. dung). Lastly, herbivory promotes
inedible and/or highly defended plants through avoidance, whereas fire promotes plants with specific physiological traits (e.g., thick bark, sprouting,
cone serotiny, stimulated seed germination). Altogether, the cascade from
megaherbivory to a human-based fire regime led to unique developmental
pathways and plant communities during the Holocene. Two different mechanisms operated in tandem, thus allowing (a) formerly herbivory-suppressed
plants to rebound, while (b) promoting fire-adaptive plants (Gill et al.,
2009).
The sustainability of oak, pine, and hickory forests during the Holocene
Thermal Maximum makes sense given the prevailing hot-dry climate
(Figures 1–3). Indigenous populations were steadily increasing during this
period and charcoal levels remained high in the eastern U.S. (Power et al.,
2008; Marlon et al., 2013). It seems likely that a combination of warm, dry
weather, Native American populations reaching significant levels, and their
increasing use of fire explains the dominance of pyrogenic vegetation over
much of the eastern U.S. (Shuman et al., 2004; Delcourt & Delcourt, 2004;
Munoz et al., 2010). Following the Holocene Thermal Maximum, however,
Neoglacial Cooling period from 3300–150 BP brought forth a very different
set of climatic conditions as temperature cooled about 2◦ C. Nevertheless,
the magnitude of change in vegetation (from paleopollen records) and fire
regimes (paleocharcoal) during Neoglacial Cooling were not nearly as dramatic on most sites (Foster, Clayden, Orwig, Hall, & Barry, 2002; Delcourt
& Delcourt, 2004; Patterson, 2006; Munoz et al., 2010). This includes many
northern sites thought to be under close climate control. In general, paleocharcoal levels remained relatively constant during most of the Neoglacial
Cooling, at least until the Little Ice Age when declines have been reported
for some regions of the eastern U.S. (Figure 4; Pederson, Peteet, Kurdyla,
& Guilderson, 2005; Power et al., 2012). This, however, is likely confounded by the simultaneous depopulation of Native Americans at that time
(see below).
Major issues associated with the climate-fire hypothesis (independent of
humans) are the ignition source and extent and timing of burning in relatively nonpyrogenic vegetation that dominates much of the eastern U.S.
now and in the past (e.g., summer-green, broadleaf forests). The climate
hypothesis assumes that lightning (natural fires) was the dominant ignition
source. However, there is little evidence to support this idea over the last
Exploring the Early Anthropocene Burning Hypothesis
37
FIGURE 4 Major forest biomes in the eastern U.S. at the time of European settlement (adapted
from Nowacki & Abrams, 2008). The three biomes dominated by oak and pine are considered
to be extensively pyrogenic. The two northern biomes are considered to be locally pyrogenic.
century or more in most locations due to a lack of dry lightning, which
may or may not be applicable to the early or middle Holocene. Most papers
report that lightning fires are rare in the northern and central U.S., including
the tallgrass prairie region (Grimm 1984; Kay, 2007; Guyette et al., 2006b;
Guyette, Stambaugh, Dey, & Muzika, 2012; Abrams & Nowacki, 2008). In the
southeastern coastal plain, by contrast, lightning (including dry lightning)
is frequent, the vegetation (conifers, ericaceous shrubs, grasses) is highly
flammable, and frequent and reasonably large fires may ensue (Huffman,
Platt, Grissino-Mayer, & Boyce, 2004; Stambaugh, Guyette, & Marschall,
2011). Most lightning, however, occurs during rainstorms and wet portions
of the growing season when green deciduous and many evergreen forests
are not prone to burning (Lafon, Hoss, & Grissino-Mayer, 2005; Stambaugh
et al., 2013). The vast majority of pre-European fires occurred during the
dormant season (detectable in tree rings fire scars), outside of the thunderstorm season, but within the ever-presence of human ignitions (Shumway,
Abrams, & Ruffner, 2001; Guyette et al., 2006a; Abrams & Nowacki, 2008),
but see Huffman et al. (2004) for a southeast Coastal Plain barrier island
experiencing recent growing season fires.
A relatively high occurrence of lightning strikes occurred during the last
century in Pennsylvania but the resultant fires were very small, due, in part,
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M. D. Abrams and G. J. Nowacki
to fire suppression activities (Ruffner & Abrams, 1998). Contemporary data
from the central and southern Appalachians indicate that human ignitions
vastly outnumber natural (lightning) fires, accounting for 82–99% (Barden &
Woods, 1973; Lafon et al., 2005; Lynch & Hessl, 2010). Of course, modernday fire suppression efforts did not exist prior to Europeans settlement,
suggesting that lightning fires may have been more extensive during the
early Holocene than modern records would suggest. Relative to the western U.S., with its dry climate and extensive highly flammable conifer forests,
the eastern U.S. is, in general, not highly pyrogenic because it is wetter
and dominated by summer green, broadleaf forests (Abrams, 2005). That is
not to say the East lacks lightning fires or pyrogenic vegetation, including
fire-adapted species that existed prior to the start of the Holocene, especially in the southeast coastal plain (Watts, 1979, 1980; Pausas & Keeley,
2009). Eastern grasslands, scrub vegetation, coastal pine (including serotinous) forests, central oak, and southern pine savannas are highly flammable
but have fairly limited distribution relative to closed canopy broadleaf forests.
Indeed, the foliage of most oak and pine is flammable due to its cellulose
and lignin content, oils, resins and terpenes, cones on the forest floor, dry
leaf thickness and curling, fast drying capacity, highly aerated fuel bed, and
low decomposition rate (Table 1). Nevertheless, lightning fires during moist
growing seasons would generally not burn as intensely and extensively as
human-set fires lit during dry dormant seasons. We conclude that the relative
extent of human versus lightning ignitions during the early Holocene remains
an open question that will hopefully be resolved with additional research,
TABLE 1 Leaf Litter and Fuel Bed Pyrogenicity Properties in Eastern U.S. Forests (Adapted
From Information in Mutch, 1970; de Magalhaes & Schwilk, 2012; Ganteaume, Jappiot, &
Lampin, 2012; Kreye, Varner, Hiers, & Mola, 2013; Plus Personal Observations)
Category
Microenvironment
Cellulose, lignin content
Mineral content
Oil, resin, fat content
Terpene content
Pine cones present
Energy content
Leaf thickness
Dry leaf shape
Fuel bed bulk density
Fuel bed aeration
Water absorbing capacity
Relative moisture content
Drying capacity
Decomposition rate
Time to ignition
Heat of combustion
Pyrogenic
Pyrophobic
Warm, dry, open
High
Low
High
High
Yes
High
High
Curled
Low
High
Low
Low
High
Low
Fast
Low
Cool, damp, shaded
Low
High
Low
Low
No
Low
Low
Flat
High
Low
High
High
Low
High
Slow
High
Exploring the Early Anthropocene Burning Hypothesis
39
including more information on the extent of fire and pyrogenic vegetation
prior to human habitation of the eastern U.S. For example, could lightning
fires alone have sustained a landscape of oaks, pines, and hickories prior to
human population?
The important role of climate on vegetation can also be seen in the spatial distribution of forest types at the time of European settlement (Figure 4).
There is an obvious north-to-south gradient from subboreal and conifernorthern hardwoods to southern pine-oak systems that follow a gradient of
increasing mean annual temperatures. This mimics the temporal variation
in vegetation that occurred during early Holocene warming. In addition,
a mapping of the pre-European fire return intervals for the eastern U.S.
shows a similar north-to-south trend, with generally longer fire frequency
in the cool north (>16 yr) and shorter in the hot south (<2 yr; Guyette
et al., 2012; Figure 5). However, some important exceptions exist, including relatively high fire frequencies in portions of the Great Lakes States
and the central and northeast Atlantic Coast associated that were associated with high Native American populations. For the purposes of this
article we divide the vegetation of the eastern U.S. into two major ecoregions based on fire relations: those that are extensively pyrogenic versus
locally pyrogenic. The process (climate-fire)-based “tension zone” line effectively divides these two ecoregions (Curtis, 1959; Cogbill, Burk, & Motzkin,
2002; Nowacki & Abrams, 2014). The extensively pyrogenic region includes
the vast central hardwoods, oak woodlands (savannas), oak-pine, southern
FIGURE 5 Native American population density (# per 100 km2 ) estimates for 1492 AD (shaded
areas in Legend; adapted from Driver, 1969; Map 6) coupled with pre-European settlement
fire return intervals in years (noted in boxes within lined boundaries; adapted from Guyette
et al., 2012).
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M. D. Abrams and G. J. Nowacki
pine, and tallgrass prairie (Figures 4–5). To the north, the locally pyrogenic
region occurs largely within a matrix of “asbestos” conifer-northern hardwoods and mixed-mesophytic and beech-maple forest types. North of the
tension zone the sporadic distribution of pine and oak, which constitute
the major pyrogenic flora, occur on dry, infertile outwash plains and areas
actively burned by Native Americans (Curtis, 1959; Dorney & Dorney, 1989;
Loope & Anderton, 1998). The north-south climate-fire gradient also reflects
successional relations, with generally late-successional forests (subboreal and
conifer-northern hardwoods; e.g., spruce, fir, hemlock, beech, maple) in the
north and fire prone, more light-demanding forests (oak-pine) in the south.
It is interesting to note the high dominance of oak and pine recorded as
witness trees during the Little Ice Age when these warm, pyrogenic species
should have declined (Figure 4).
A comparison of estimates of Native American populations at 1492 AD
with pre-European fire data suggests that climate is important (both generally
higher in the south) but was not the sole causal factor (Driver, 1969; Guyette
et al., 2012; Figure 5). Native American land modification followed a similar north-to-south gradient, generally increasing southward as temperature,
land productivity, and native populations increased (Driver & Massey, 1957).
Relatively high Native American populations existed in most areas with high
pre-European fire frequencies (Figure 5). This is most evident in the southeast and most of the entire Atlantic coast. In northern locations, cool-moist
climate and/or a lack of pyrogenic vegetation would generally act to suppress burning even when Indian populations were relatively high (e.g., along
the Great Lakes and St. Lawrence Seaway). Nevertheless, there is a strong
association among human populations, fire and oak-pine dominance along
the eastern Virginia, New Jersey, and the northeast coastline (e.g., Cape Cod).
This suggests that late Holocene burning can be attributed, probably to a
significant degree, to human ignitions within the context of regional climate,
including drought (Guyette et al., 2006b, 2012). How relevant this is for the
early Holocene is open for debate. Several documented examples of human
fire trumping climates comes from the Prairie Peninsula and localized areas
within the northern hardwood forest—including Crawford Lake, Ontario
(Grimm, 1984; McAndrews, 1988). Clark and Royall (1995) and Munoz and
Gajewski (2010) link late Holocene increases of human occupation and disturbance (farming and fire) to forest conversion from northern hardwoods
(beech and maple) to oak-pine forests. This happened from 1360–1650 AD,
which overlaps with the Little Ice Age. Their results support the notion that
Native American activities, especially burning, are capable of over-riding climate and producing dramatic vegetation change lasting centuries. The overall
impact within the matrix of northern hardwoods is probably restricted to
locations immediately in and around Native villages and trails (Dorney &
Dorney, 1989; Black, Ruffner, & Abrams, 2006). Campbell and Campbell
Exploring the Early Anthropocene Burning Hypothesis
41
(1994) suggest that less than 3.2% of southern Ontario was impacted by
Native American activities up to 1600 AD.
The highest pre-European fire frequency was recorded in present-day
Georgia and Florida (Figure 5). This can be attributed to many biotic and abiotic factors acting in concert—including hot weather, seasonal dry periods,
high incidence of lightning (including dry lightning), pyrogenic vegetation,
sandy and drought-prone coastal plain soils, high Native American populations and their cultural uses of fire (Huffman et al., 2004; Stambaugh
et al., 2011). A positive, self-promoting feedback also exists between fire and
pyrogenic vegetation (Mutch, 1970). The other regions of high Indian populations were in present-day eastern North Carolina and southeast Virginia
that had a fire frequency slightly below that of Georgia and northern Florida.
Population estimates for Native American Indians ranged from less than
100,000 at 12,000 BP to 3.2 million by 400 BP, after which Native American
depopulation (pandemic/removal) occurred following European colonization (Hart & Buchanan, 2012; Marlon et al., 2013; Figure 3). Snow (2001)
estimated that approximately 1.3 million American Indians, at their peak,
lived in forested biomes of the eastern U.S. (excluding central and southern grasslands). A decline in burning during the Little Ice Age reported in
some paleocharcoal studies appears to be due to colder climate coupled with
Native American depopulation (Pederson et al., 2005; Power et al., 2012;
Figure 3). However, the full magnitude and extent of the Little Ice Age on
paleocharcoal records is still unresolved (J. Marlon, personal communication,
March 2014).
Studies using the direct dating of fire scars on trees report very active fire
regimes during the Little Ice Age—including the influence of human population density, climate, and topography (Figure. 5; Guyette et al., 2006a,
2006b; Stambaugh et al., 2013). While high fire frequencies can be maintained independently of climate factors (Flatley, Lafon, Grissino-Mayer, &
LaForest, 2013), fire is also promoted by a warm and dry climate. Native
American population densities and their culture of burning were probably adequate to explain the high frequency and extent of burning during
the Little Ice Age (Guyette et al., 2006b). Some fire scar studies report that
fire frequency was roughly equivalent before and after European settlement,
while others report that it increased (Guyette et al., 2006b; Flatley et al.,
2013). This is particularly interesting in light of Native American depopulation, which was expected to have resulted in decreased fire frequency (Hart
& Buchanan, 2012; Stambaugh et al., 2013). Extensive cutting of forests,
controlled burning (e.g., of pastures) and catastrophic wildfires in the early
history of European settlement apparently maintained or elevated fire levels
and pyrogenic vegetation in most locations of the eastern U.S. (Figure 6;
Abrams, 2010).
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M. D. Abrams and G. J. Nowacki
FIGURE 6 Major land-use history changes in forests in the eastern U.S. following European
settlement.
CONCLUSION
The prominent role of climate in the spatial and temporal variation in vegetation is clearly evident for the eastern U.S. as forest types grade from
subboreal in the north-to-southern pines following a strong temperature gradient. Moreover, the change from boreal to oak-pine forest in the central
portions of the U.S. during early Holocene warming was evidently climatecontrolled. However, a strong argument can also be made for the role of
humans as an ecological factor and keystone species in the eastern U.S. A
large number of studies report on the frequent and extensive use of fire by
Native Americans resulting in the increased distribution of pyrogenic plant
species (Curtis, 1959; Pyne, 1983; Grimm, 1984; Abrams & Nowacki, 2008).
They were also responsible for the long-term maintenance of most subxeric
oak, hickory, and pine forests and tallgrass prairies, although southeastern Coastal Plain pine-oak woodlands/grasslands appear to have predated
humans (Watts, 1980) and, thus, were apparently maintained by lightning
fires. The high frequency of modern-day lightning and fires in the southeast
support this idea.
The ecological importance of American Indians is particularly evident following their depopulation from much of the eastern region, after
which trees invaded open systems (grasslands, savannas, and woodlands),
open systems became closed-canopy forests, followed by successional
Exploring the Early Anthropocene Burning Hypothesis
43
replacement by shade tolerant, fire sensitive tree species (Abrams, 1986,
1992; Nowacki & Abrams, 2008). Because fire regimes in many areas stayed
about the same or increased during early European settlement, the ubiquitous transformations of eastern vegetation did not extensively occur until
the start of active fire suppression in the 1930s (Figure 6; Abrams, 2003;
Nowacki & Abrams, 2008; Stambaugh et al., 2013). Other unique forms
of disturbances, which varied by region and date—including the charcoal
iron industry, clearcut era, catastrophic fire era, and the chestnut blight—
maintained or promoted the pyrogenic, disturbance-oriented species. The
southeastern Coastal Plain appears to be one region where frequent natural lightning fires, facilitated by the occurrence of dry lightning, may have
been adequate to sustain pyrogenic vegetation, pine forests, savannas, and
grassland and scrub, with or without Indian burning (Huffman et al., 2004;
Stambaugh et al., 2011). Indeed, a decrease in Native American populations from northern to southern Florida (from 150–375 to 25–60 people per
100 km2 , respectively) did not impact pre-European fire frequencies, with
the entire state having had a return fire interval of under 2 yr (Figure 5).
Nevertheless, Indian burning did occur in most southeast Coastal Plain
locations and probably elevated the natural fire cycle.
The interval between European settlement and government mandated
fire suppression in the 1930s (via the Forest Service, National Park Service,
Bureau of Land Management, and Civilian Conservation Corp) provides an
interesting opportunity to test the role of lightning fires. During this period,
there was widespread conversion of tallgrass prairie and oak savannas to
oak forests in the Midwest (Gleason, 1913; Cottam, 1949; Abrams, 1986).
Thus, lightning ignitions were inadequate to sustain these open systems
after the cessation of Indian burning, although the extent of these fires were
probably impacted by early European settlement land-use (e.g., agriculture
and livestock grazing). Grimm (1984) concluded that frequent (annual) fires
set by Native Americans shaped much of the vegetation of the Big Woods,
southeastern MN. The lack of Indian burning transformed the vegetation in
that region soon after European settlement from grassland to forest. In contrast, extensive clear-cutting of closed-canopy forests in the northeastern and
Appalachian regions and the catastrophic fire era (1870–1920) sustained most
oak-pine forest types into the early-middle 20th century (Figure 6; Abrams,
2010).
The Indian-fire hypothesis, however, is not without some large unanswered questions. For example, how do we explain the large amount
of burning (based on paleocharcoal evidence) that occurred in the early
Holocene when Native American populations were low? The three alternatives are that few people burned large expanses, lightning ignitions were
common and burned large areas, or that a combination of human and lightning ignitions were responsible. Moreover, charcoal evidence suggests that
the degree of burning peaked in the early to mid-Holocene well before
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M. D. Abrams and G. J. Nowacki
Native American populations peaked in the late Holocene (Figure 3; Power
et al., 2008). Alternatively, high fire occurrences were generally sustained
during Neoglacial Cooling and much of the Little Ice Age (recorded by high
resolution fire-scar dating) when they should have dropped significantly (if
primarily under climate control). These later fires can be attributed to the
repopulation of Native Americans and the use of fire by European settlers.
It seems reasonable to conclude that by the middle to late Holocene, if not
earlier, human use of fire became the dominant ecological factor over much
of the eastern U.S. (Delcourt & Delcourt, 2004; Abrams & Nowacki, 2008).
The lack of lightning ignitions for most of the eastern U.S. provides
support for the Early Anthropocene burning hypothesis, if modern records
are at all applicable to early Holocene. We believe that it is hard to explain
the vast amount of burning throughout the Holocene by lightning alone,
with the southeast coastal plain being a probable exception. Nevertheless,
climate played an important role. Even within the context of Indian burning, a longitudinal transition from closed oak forests in the Mid-Atlantic to
oak savannas and tall grass prairie in the Midwest existed across a gradient
of decreasing precipitation. Nevertheless, the fire return intervals were fairly
consistent across east to west longitudinal gradients, suggesting that increasing Indian population densities in the east overrode increasing precipitation
(Figure 5). Rather than choosing one or the other (climate versus human),
we believe that the interplay of several (or many) ecological and physical factors—including climate, wet and dry lightning, geographic location,
soil-site factors, vegetation, natural disturbances, and human land-use—more
fully explains the temporal and spatial variation of vegetation and fire history in the eastern U.S. This article attempts to improve our understanding
of these issues by integrating climate science, paleoecology, anthropology,
fire scar (dendrochronology), physical geography, and witness trees studies
and to stimulate further research of these intriguing questions.
ACKNOWLEDGMENTS
The authors wish to thank Chris Bouma for his help with the figures.
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