Tree Physiology 36, 421–427 doi:10.1093/treephys/tpw005

Commentary

An interdisciplinary approach to better assess global change impacts and drought vulnerability on forest dynamics

Marc D. Abrams1,3 and Gregory J. Nowacki2

1307 Forest Resources Building, Department of Ecosystem Science and Management, Penn State University, University Park, PA 16802, USA; 2Eastern Regional Office, USDA Forest Service, 626 E. Wisconsin Avenue, Milwaukee, WI 53202, USA; 3Corresponding author ([email protected]) Downloaded from

Received November 4, 2015; accepted January 9, 2016; published online March 3, 2016; handling Editor Danielle Way

Studies of tree physiology within and outside the USA have Significant accelerations of learning and scientific advances http://treephys.oxfordjournals.org/ historically focused on the impacts of environmental factors often take place when science of one field is combined with such as light, water and nutrients, and more recently on the another (­Linkov et al. 2014). In a recent study, we attempted to global change factors of increasing CO2, ozone, and air and soil expand the knowledge of climate change, human impacts and temperature, on various tree species (­Kramer and ­Kozlowski land-use legacy by merging the fields of tree physiology and 1979, ­Norby et al. 1999, ­Hoeppner and Duk­ es 2012, ­Chung forest ecology (­Nowacki and ­Abrams 2015). This came forth et al. 2013). One thing that has plagued tree physiology from the realization that the distribution and dominance of each research is the limited scope and breadth of most individual tree species corresponds to an ecophysiological expression and studies. This normally involves measuring a few physiological that long-term change in forest ecosystems is directly relatable parameters, in response to a few treatments or environmental to the underlying physiological attributes of component species. by guest on April 18, 2016 factors, over a relatively short period of time in nonreplicated This analysis can provide a more robust assessment of the role field or greenhouse settings with a small number of individuals and impacts of the most important drivers of forest dynamics, and species, although these issues are certainly not limited to namely climate change and land-use history. This method also tree physiology (­Hurlbert 1984). Nevertheless, these studies elucidates ecophysiological changes at the forest type and forest have been invaluable in characterizing the ecophysiological biome level by capitalizing on extensive and long-term forest attributes of many tree genera and species worldwide (­Abrams survey records. The results of that study indicate (i) that vegeta- 1990, ­Burns and ­Honkala 1990, ­Prasad et al. 2007–ongoing, tion changes since European settlement in the eastern USA are ­Peters et al. 2015). Fortunately, past limitations have been caused to a greater extent by anthropogenic alteration of distur- greatly improved upon over the last quarter century by network bance regimes (e.g., clearing for agriculture, wood harvesting, site initiatives like FACE, AmeriFlux and FluxNet, allowing global introduction of nonnative pests and diseases, and fire suppres- scaling of physiological and leaf data from many sites in various sion) than by climate change and (ii) that current vegetation forest/vegetation types across several continents (­Schulze et al. reflects human influence and disturbance history more than the 1994, ­Kattge et al. 2011, ­Norby and Zak­ 2011, ­Boden et al. current environment. These findings are a very important coun- 2013, ­Ali et al. 2015). In addition, analysis and synthesis of the terpoint to the idea of steady-state response to environment that numerous small-scale studies and the upscaling of these data is embedded of many current vegetation models, including cli- have led to huge advances of our understanding of plant form mate envelope models and Dynamic Global Vegetation Models and function at the global level (­Reich et al. 1997, ­Wright et al. (­Cramer et al. 2001, ­Iverson et al. 2008). The findings also 2004). Large data syntheses have produced more accurate contradict the conclusion of some that climate is the major driver predictions of global primary productivity and future climates, of post-European vegetation change in our study area (cf. and the role of plant physiology and dynamic vegetation ­Pederson et al. 2015). This includes evidence that increases ­feedbacks in the climate response (­Kattge et al. 2009, and decreases in moisture in the recent and more distant past ­Philippon-Berthier et al. 2010). affected forest composition and biomass (­Gustafson and

© The Author 2016. Published by Oxford University Press. All rights reserved. For Permissions, please email: [email protected] 422 Abrams and Nowacki

­Sturtevant 2013, ­Pederson et al. 2014). However, land-use Supplemental Table S2), who used community assemblages of alterations can mediate changes in forest composition that run eastern US tree species and drought tolerance characteristics counter to climate trends, including the increase in some assessed from literature. We chose three longevity classes, drought-tolerant trees during periods of increasing precipitation short-lived (<100 years), intermediate-lived (100–250 years) (­Abrams and ­Nowacki 2015). Indeed, at present, tree mortality and long-lived (>250 years) based on the range and distribution trends across the eastern USA are more closely linked to stand of the typical age of mortality for major eastern US tree species characteristics (products of forest history and subsequent stand compiled by ­Loehle (1988). These classifications were applied development; sensu ­Oliver and ­Larson 1996) than climate vari- to 190 tree-census datasets that compared pre-European settle- ables (­Dietze and ­Moorcroft 2011). ment (original land survey data dating back to the 1600s) and Our approach involves categorizing tree species/genera into current vegetation conditions in the eastern USA expressed as temperature, shade tolerance (intolerant, intermediate and toler- percent changes in the abundance (relative density or impor- ant) and pyrogenicity classes based on their known life history tance value) of arboreal vegetation (­Nowacki and Abrams­ and physiological characteristics (­Nowacki and ­Abrams 2015). 2015). Importance value is typically calculated by combining or Temperature classes were established using actual temperature averaging species’ relative frequency, density and dominance data from the Climate Change Tree Atlas (­Prasad et al. 2007– (basal area). Here, we expanded this comparative analysis to ongoing). Tree species were sorted by the average temperature five ecophysiological categories as a more robust set to relate within their ecological range (US distribution) and divided into compositional changes to known climate (temperature and Downloaded from four temperature classes (cold = 4.1–6.6 °C, cool = 6.7– drought) or disturbance (land-use) phenomena (Tables 1 and 2; 10.7 °C, warm = 10.8–13.9 °C and hot = 14.0–19.8 °C). To Figure 1). The Palmer Drought Severity Index (PDSI; Figure 1), assess level of disturbance, tree genera were categorized by a measurement of dryness based on precipitation and tempera- shade tolerance (intolerant, intermediate and tolerant) and pyro- ture (­Alley 1984), was used to assess past drought conditions. http://treephys.oxfordjournals.org/ genicity (pyrophilic and pyrophobic; ­Burns and Honkala­ 1990; Zero depicts normal conditions, whereas more negative num- Table 1). Tree survey data from pre-European settlement and bers indicate increasing drought and more positive numbers present-day were tallied by temperature (cold, cool, warm indicate increasing wetness (pluvials). and hot), shade tolerance (intolerant, intermediate and tolerant) The Euro-American settlement period (≈1500 onward) spans and pyrogenicity (pyrophilic and pyrophobic), and change per- two major climatic periods, the Little Ice Age (∼1400–1850) centages were calculated for each class. For the purpose of this and the Anthropocene (after 1850), as well as two major anthro- article, we expanded the ecophysiological classes to include pogenic disturbance periods, the catastrophic disturbance era drought tolerance and tree longevity (Table 1). Drought toler- from 1600 through the 1930s (moving east to west across ance classes were adapted from ­Peters et al. (2015; Appendix eastern North America) and the fire suppression era after 1940 by guest on April 18, 2016

Table 1. Common eastern North American tree genera classified by temperature, shade tolerance, pyrogenicity, drought tolerance and longevity (adapted and expanded upon from ­Nowacki and ­Abrams 2015).

Major tree genera Common name Temperature preference Shade tolerance Pyrogenicity Drought tolerance Longevity

Abies Balsam fir Cold Tolerant Pyrophobic Intolerant Short Acer Sugar maple Cool Tolerant Pyrophobic Intolerant Long Acer Red maple Warm Tolerant Pyrophobic Moderate Intermediate Betula Birch Cold Intolerant Pyrophobic Moderate Intermediate Carya Hickory Warm Intermediate Pyrophilic Tolerant Intermediate Castanea Chestnut Cool Intermediate Pyrophilic Moderate Intermediate Fagus Beech Warm Tolerant Pyrophobic Moderate Long Fraxinus Ash Cool Intermediate Pyrophobic Moderate Intermediate Juniperus Red cedar Warm Intolerant Pyrophobic Tolerant Long Larix Tamarack/larch Cold Intolerant Pyrophobic Moderate Intermediate Picea Spruce Cold Tolerant Pyrophobic Intolerant Intermediate Northern Pinus Northern pine Cold Intolerant Pyrophilic Tolerant Long Southern Pinus Southern pine Hot Intolerant Pyrophilic Tolerant Long Populus Aspen Cold Intolerant Pyrophilic Moderate Short Prunus Cherry Warm Intermediate Pyrophobic Moderate Intermediate Quercus Oak Warm Intermediate Pyrophilic Tolerant Long Thuja Cedar Cold Tolerant Pyrophobic Intolerant Long Tilia Basswood Cool Tolerant Pyrophobic Moderate Intermediate Tsuga Hemlock Cool Tolerant Pyrophobic Intolerant Long Ulmus Elm Warm Intermediate Pyrophobic Moderate Intermediate

Tree Physiology Volume 36, 2016 Global change impacts and drought vulnerability 423 1 1 3 − 1 12 20 15 Short 3 9 3 − 2 − 3 − 1 Inter − 12 3 − 8 − 15 − 16 − 13 − 16 − 10 Long Longevity Δ Longevity 5 − 2 − 18 − 20 − 23 − 16 − 16 Tol Δ 9 6 9 17 Inter 17 14 19 1 0 5 − 1 − 6 − 13 − 15 Intol Drought Drought 0 − 8 − 1 25 18 21 − 12 Pyrophobe ). The changes in drought tolerance and longevity were were tolerance in drought and longevity changes ). The Downloaded from 6 9 − 2 − 1 − 24 − 22 − 23 Fire Δ Fire Pyrophile ams 2015 8 4 − 7 18 10 11 Abr ­ Tol − 18 2 9 2 11 − 16 − 10 − 15 Inter http://treephys.oxfordjournals.org/ 3 5 0 − 3 − 1 ­ Nowacki and − 16 − 15 Intol Tolerance Δ Tolerance 0 0 0 0 0 0 − 4 Hot 5 2 − 1 − 6 10 − 15 − 11 Warm 3 9 1 0 4 − 8 17 Cool 1 0 2 − 2 − 1 − 8 by guest on April 18, 2016 − 13 Temperature Δ Temperature Cold

3) − 3) Pinus Castanea − Tsuga Fagus Larix Tsuga 17), 15), Betula ( − 3) Fagus ( − 4) − − Picea ( − 5) Betula ( − 6) Castanea ( Fagus ( 4), 6), − − ( ( ( − 8), ( − 7), ( − 13), ( − 10), Figure 1. Climatic data and two major land-use periods for the eastern Quercus ( Quercus ( Major decreasers Quercus ( − 24) Fagus ( − 23), ( − 18), Tsuga ( − 18), Pinus ( − 29), Pinus USA including temperature anomaly for the northeastern USA and PDSI from three sites north and three sites south of the tension line. The PDSI 1 . data from the three northern locations were in Vermont, Michigan and

(3) Wisconsin, while the three southern sites were in Virginia, south central Populus

Fraxinus Pennsylvania and Kentucky. The temperature and PDSI data were obtained from ­Mann et al. (2009) and the North American Drought Atlas Betula (5) Prunus (5), Populus Carya (3), Juniperus Acer (12)

(3) Fraxinus (http://iridl.ldeo.columbia.edu/SOURCES/.LDEO/.TRL/NADA2004/.pdsi- Acer (5) atlas.html), respectively. Temperature data were extracted from the paleoclimate dataset reconstructed by ­Mann et al. (2009) covering grid (3) Ulmus Betula / Quercus (4) (11), (12), (5), Acer (20), Acer (7), Major increasers Vegetation Δ Vegetation Acer (9), Acer (19), Acer (14), Quercus (13), (15), Populus cells in the eastern USA. thern thern nor ­ (Figure 1). The transition from the Little Ice Age to the Anthro- nor ­ pocene generally corresponds to progressively increasing annual temperatures and the lessening of drought based on the PDSI. R

The catastrophic disturbance era includes the disease-based 2. Native American pandemic that effectively started with Colum- hardwoods (10) hardwoods (29) hardwoods (33) hardwoods Table - oftolerance number (the drought and longev categoriescorresponding and the ecophysiological in the datasets ofchanges in parentheses) analyzed pyrogenicity, shade tolerance, temperature, eastern in the in species abundance) ity classes (based on changes upon USA (adapted and expanded from as listed in Table only, level atconducted the genus importanceor (density abundance in the regions present-day) to settlement of major forest value pre-European in parentheses) (from changes elative within seven vegetation arboreal Northeast oak-pine (10) Central oak-pine (33) Region Great Lakes oak-pine (18) Lakes Great Northeast conifer-northern Subboreal conifers (31) Subboreal Great Lakes conifer- Lakes Great pine- Lakes Great bus’ first voyage to the New World ­Ramenofsky ( 2003)

Tree Physiology Online at http://www.treephys.oxfordjournals.org 424 Abrams and Nowacki and largely ran its course through eastern tribes by 1800. It also warming, but is highly consistent with the wholesale cutting and co-occurred with active westward migration of Native American burning of northern hardwood forests (­Graham et al. 1963, populations. Emblematic of the catastrophic disturbance era was ­Cleland et al. 2001). The loss of cold-adapted conifers in north- the ‘Great Cutover’, which arose slowly along the Atlantic Sea- ern forests is consistent with the warming trend, but, we believe, board in the mid-1600s, to great expansion across the Midwest is better explained by their inability to sprout after clearcutting and Deep South during the 1700s and1800s, before terminat- and burning. The ubiquitous increase in Acer in all forest sys- ing along the western margins of the Eastern Deciduous Forest tems, whether cool-based (A. saccharum) or warm-based and within the most rugged parts of the Appalachians (West (A. rubrum), can be directly linked to changes in anthropogenic Virginia; ­Lewis 1998) in the early 1900s. Catastrophic wildfires disturbance regimes that have produced inconsistencies when that burned through abundant down and dry slash also charac- compared with the climate record. In oak-pine systems, the terized this period, especially during its later phase (1800s– increase in Acer is generally attributed to fire suppression, par- early 1900s; ­Guthrie 1936). Chestnut blight radiated outward ticularly in the case of the widely distributed A. rubrum (­Abrams from its origins in New York City in the early 1900s, effectively 1992, 1998­ , ­Nowacki and ­Abrams 2008). The increase in wiping out this genus from eastern forests by the 1940s warm-adapted Quercus and A. rubrum in the Great Lakes pine- (­Anagnostakis 1987). Aggressive fire suppression throughout northern hardwoods is consistent with warming, but is more the eastern USA (Smokey Bear Era) started in the 1930s and likely a response to the mass cutting of pines (prime timber continues to the present day (­Abrams 2010). tree), subsequent consumption of its fire-vulnerable regenera- Downloaded from From the time of European settlement to the present, the tion and release of hardwood understories—phenomen a well eastern USA experienced major compositional changes, includ- documented in the historical literature (­Elliott 1953, ­Whitney ing (i) a large overall decline in conifers (Pinus, Tsuga and Larix), 1987, ­Nowacki et al. 1990). The loss of warm-adapted, pyro- Fagus and Castanea, (ii) expansions of disturbance-oriented phobic Fagus in conifer-northern hardwood systems is also http://treephys.oxfordjournals.org/ Populus and Quercus species in former conifer-northern hard- inconsistent with the warming trend, being attributed to beech woods or subboreal forests and (iii) an ubiquitous increase in bark disease and its negative response to clearcutting and burn- fire-sensitive, shade-tolerant Acer across all regions (Table 2). ing of eastern forests (­Nowacki and ­Abrams 2015). When converted to and tracked by ecophysiological attributes, Across all oak-pine forest regions, the overall reduction of forest systems with similar initial compositions and stand dynam- drought- and fire-adapted trees and the corresponding increase ics had a similar response to European alterations of disturbance in mesophytic species, particularly Acer, are consistent with the regimes. In oak-pine forests, where the Quercus-to-Acer transi- lessening of drought after the 1930s (increasing PDSI trends: tion was most prominent, there was a distinct shift toward Figure 1; Table 2; ­McEwan et al. 2011). However, we believe greater shade tolerance and less fire and drought tolerance that this is better explained by the lack of fire or other interven- by guest on April 18, 2016 (Table 2). In Northeast oak-pine forests, cool-adapted Acer sac- ing anthropogenic factors. For example, the loss of mesophytic charum Marsh. was the principal benefactor, whereas in the cen- Tsuga is not consistent with the lessening of drought. Moreover, tral oak-pine forests, it was warm-adapted Acer rubrum L., the the twentieth-century increases in several important drought- latter representing a neutral temperature change from warm- tolerant or moderately drought-tolerant trees (e.g., Populus, adapted Quercus. In rich, mesic conifer-northern hardwood for- A. rubrum, Juniper (and Quercus in Great Lakes pine-northern ests (Northeast and Great Lakes), the loss of Fagus and Tsuga hardwoods)) run counter to the PDSI trend. The loss of Casta- had overwhelming effects on these systems’ collective ecophys- nea early in the twentieth century is due to the chestnut blight, iology, with recorded losses of warm adaptation (Fagus), shade and is unrelated to changes in climate. The decreases in longev- tolerance, pyrophobicity and drought intolerance. Here, the ity for most eastern forest types are due to the loss of long-lived replacement of warm-adapted genera by cold- and cool-adapted Quercus, Pinus, Fagus and Tsuga species, collectively the major genera during a period of global warming is noteworthy. The shift decreasers in eastern forests. The increase in shade tolerance in from cold, shade-intolerant Pinus to warm, shade-intermediate most forest systems was due to the wide-ranging decrease in Quercus and shade-tolerant A. rubrum in the drier systems of the Quercus and Pinus and increase in Acer, except for mesic conifer- Upper Midwest (Great Lakes pine-northern hardwoods and sub- northern hardwood forests that were previously dominated by boreal conifers) largely drove ecophysiological changes. Here, Fagus and Tsuga. It is important to note that most of these the replacement of long-lived Pinus by short-lived Populus pri- changes are unique to the clearcutting and catastrophic wildfire marily explains the reduction of tree longevity. and fire suppression eras and did not occur during previous cen- It appears that change in anthropogenic disturbance regimes turies with lessening drought events, based on paleoecological is a major driver of post-European forest dynamics in the study studies (Figure 1; reviewed in ­Abrams 2002). region. For example, significant increases in the cold-adapted, Converting forest compositional change to ecophysiological disturbance-oriented, clonal Populus species in conifer-northern derivatives to better understand and interpret forest dynamics hardwoods and subboreal forests runs counter to Anthropocene is in its infancy. As with any embryonic field, it may be met with

Tree Physiology Volume 36, 2016 Global change impacts and drought vulnerability 425 skepticism, which is part of any initial scientific endeavor. Disentangling disturbance versus climate change effects on Indeed, the relative simplicity of our method to analyze and vegetation and assigning importance is problematic for a num- interpret complex arrays of ecological factors that affect eco- ber of reasons, including spatiotemporal differences (e.g., dis- systems makes it particularly vulnerable to incertitude. To wit, turbance can have immediate, dramatic and long-term impacts, our interpretation that land-use change, specifically fire sup- whereas climate changes play out more subtly over longer tim- pression, principally drove succession to shade-tolerant, fire- escales due to ecological inertia and time lags; ­Pielou 1991, sensitive mesophytes in formerly pyrogenic oak-dominated ­Abrams and Now­ acki 2015, ­Wu et al. 2015). In any case, we ecosystems (­Nowacki and ­Abrams 2015) was questioned as are encouraged by recent research that acknowledges the this compositional conversion parallels increasing moisture over importance of disturbance regimes and other related factors on the last several centuries (­Pederson et al. 2015). But we main- vegetation dynamics, thus giving them fair and equatable treat- tain steadfast to our interpretation due to convergence of mul- ment relative to climate (Danneyrolles et al. 2016). For instance, tiple independent pointers, especially the alignment of the a mega-analysis of >1600 plots from across western Canada historical record of European land disturbance (including post- found that internal stand dynamics and competition were the 1930s decline in fire occurrence and size) with tree distribu- primary drivers of tree demographics compared with external tion-based trends in temperature (decreasing rather than climatic factors (­Zhang et al. 2015). In the absence of distur- increasing), shade tolerance (succession from fire-maintained bance since the Great Cutover, cold-adapted red spruce is moving sub-climax toward climatic climax conditions) and pyrogenicity downslope and reclaiming its former distribution in the face of Downloaded from (decrease in pyrophiles) (­Abrams and Now­ acki 2015). Higher global warming in New England (­Foster and ­D’Amato 2015). This temperatures often negate increases in precipitation (through phenomenon has been found throughout the entirety of red increased evapotranspiration) as expressed in ­Brzostek et al. spruce’s range (­Nowacki et al. 2010) and is consistent with (2014), so the latter does not necessarily explain the shift human land use being the primary factor driving forest dynamics http://treephys.oxfordjournals.org/ toward shade-tolerant mesophytes per se. Moreover, the for this species. Multiple peaks in mineral sediments suggestive of abruptness of the oak-to-maple transition is not consistent with hydroclimate variability do not consistently correspond to shifts in ever-increasing moisture trend spanning centuries (­Pederson Fagus grandifolia abundances in the Great Lakes Region during et al. 2015), but is consistent with the universal and rather the Holocene (­Wang et al. 2015). ­Flatley et al. (2015) demon- instantaneous removal of fire from the eastern landscape. Now, strated that fire exclusion, and not climate, is driving changes increased moisture, deer herbivory, etc., may have facilitated within mixed oak stands in the southern Appalachian Mountains. some of this transition (­McEwan et al. 2011), but were more The conclusions of this site-specific study pair nicely with our ancillary in nature than primary drivers. broadscale research (­Nowacki and ­Abrams 2015), whereby dis-

The immediacy and extent by which European disturbances turbances (and/or the absence of formerly important disturbance) by guest on April 18, 2016 affected ecosystems (relative to slow climatic changes over may actually possess long-term ‘trumping power’ over climate- time) cannot be underscored enough, vastly overriding climatic driven changes with regard to vegetation expression. This does changes that have occurred over the same period. The fact that not discount the fact that climate is having some important the rates of vegetation change assigned to European activities impacts on forests and other vegetation types worldwide, as stud- are roughly a magnitude above those of the past millennia is ies of drought- and heat-induced tree mortality indicate otherwise, particularly telling. In the Upper Great Lakes Region, the average including the eastern USA (­Pederson et al. 2014, ­Allen et al. amount of vegetation change recorded in pollen was 2.4 times 2015). Whether this will have a profound effect on future forest greater during the last 150 years than the preceding 850 years dynamics remains to be seen and will greatly depend on forth- (­Cole et al. 1998). Here, post-European vegetation changes coming drought severity and duration and changes in temperature were best assigned to human disturbance as changes in climate and disturbance regimes (­Gustafson and ­Sturtevant 2013, ­Peters were comparatively small (­Webb 1973). In the Central Hard- et al. 2015). However, the results of this and other studies indi- woods Region, ­Cole and ­Taylor (1995) report that recent (last cate that the loss of drought-tolerant Quercus, Pinus and Carya 150 years) rates of vegetation change are at least an order of and the increase in mesophytic trees in a process known as magnitude higher than in the prior 4000 years. This extreme mesophication (­Nowacki and ­Abrams 2008) are increasing the rate of change is largely attributed to fire exclusion possibly drought vulnerability of eastern US forests (­Hart et al. 2014, enhanced by other factors, including human-driven increases of ­Frelich et al. 2015). This vulnerability may exacerbate tree mortal- nitrogen deposition (­Vitousek et al. 1997). Only the rapid and ity and water-stress declines in growth, specifically for water- profound changes in human land use (specifically fire suppres- demanding mesophytic species (­Brzostek et al. 2014). sion) can explain the huge oak-to-maple compositional shift, The world’s climate is warming in ways not seen for several such as the 4000+ percent increase in maple–beech forests thousand years (­Alley 2000). The post-1850 increases in from 1962 to 1985 in Illinois (­Iverson et al. 1989, ­Fralish greenhouse gases and abrupt global warming are clearly affect- 2004). ing tree physiology and many other ecosystem processes, and

Tree Physiology Online at http://www.treephys.oxfordjournals.org 426 Abrams and Nowacki provide vast research opportunities for tree physiologists and growth and the carbon sink of deciduous hardwood forests. Glob other environmental scientists (­Norby et al. 1999, ­Karnosky Change Biol 20:2531–2539. Burns RM, Honkala BH (1990) Silvics of North America: volumes 1 and et al. 2007, ­Way and O­ ren 2010). These topics are now the 2, hardwoods and conifers, Agriculture Handbook 654. Department dominant focus worldwide, but have the potential, in our opinion, of Agriculture, Forest Service, US Government Printing Office, Wash- to minimize the role of other important factors, particularly ington, DC, 877 p. among those who are intent on finding climate signals Pederson­( Chung HH, Muraoka M, Nakamura N, Han S, Muller O, Son Y (2013) Experimental warming studies on tree species and forest ecosystems: et al. 2015). Human populations and their role as a disturbing a literature review. J Plant Res 126:447–460. agent in ecosystems have also changed dramatically along with Cleland DT, Leefers LA, Dickmann DI (2001) Ecology and management of the recent changes in climate (­Ellis 2015). Comprehending past aspen: a Lake States perspective. In: Proceedings, sustaining aspen in and future impacts of climate change on vegetation requires a western landscapes. RMRSP-18. US Department of Agriculture, Forest Service, Rocky Mountain Research Station, Fort Collins, CO, pp 81–99. more complete understanding of the human–climate–vegetation Cole KL, Taylor RS (1995) Past and current trends of change in a dune dynamic. Vegetation models should include changes in distur- prairie/oak savanna reconstructed through a multiple-scale history. bance regimes, and not rely solely on climate change or bio- J Veg Sci 6:399–410. geochemical factors. New research endeavors that combine Cole KL, Davis MB, Stearns F, Guntenspergen G, Walker K (1998) His- torical landcover changes in the Great Lakes region. In: Sisk TD (ed) multidisciplinary data, as outlined in this article, in other loca- Perspectives on the land-use history of North America: a context for tions and with other disciplines should greatly increase our understanding our changing environment. Biological Science Report understanding of global change impacts as we move further into USGS/BRD/BSR-1998-0003, U.S. Geological Survey, Biological Downloaded from the twenty-first century. Resources Division, Springfield, VA, pp 43–50, 104 pp. Cramer W, Bondeau A, Woodward FI et al. (2001) Global response of

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