Exploring the Early Anthropocene Burning Hypothesis and Climate-Fire Anomalies for the Eastern U.S

Exploring the Early Anthropocene Burning Hypothesis and Climate-Fire Anomalies for the Eastern U.S.

MARC D. ABRAMS1 and GREGORY J. NOWACKI2 1Department of Ecosystem Science and Management, Penn State University, University Park, Pennsylvania, USA 2USDA 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, under- scoring 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 intermit- tent 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]

30 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 geo- logic 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 human- induced 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 hemi- sphere 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 hemi- sphere 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). 34 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 prac- tically 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, 36 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 contin- gent 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 mech- anisms 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 con- founded 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 thunder- storm 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, 38 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, modern- day 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 seroti- nous) 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 Pyrogenic Pyrophobic

Microenvironment Warm, dry, open Cool, damp, shaded Cellulose, lignin content High Low Mineral content Low High Oil, resin, fat content High Low Terpene content High Low Pine cones present Yes No Energy content High Low Leaf thickness High Low Dry leaf shape Curled Flat Fuel bed bulk density Low High Fuel bed aeration High Low Water absorbing capacity Low High Relative moisture content Low High Drying capacity High Low Decomposition rate Low High Time to ignition Fast Slow Heat of combustion Low 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 conifer- northern 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 effec- tively 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). 40 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 sup- press 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 abi- otic 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 coloniza- tion (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),fireisalsopromotedbyawarmanddryclimate.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). 42 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 climate- controlled. 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 ubiq- uitous 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 unan- swered 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 alterna- tives 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 44 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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