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An Indicator of Tree Migration in Forests of the Eastern United States Forest Ecology and Management 257 (2009) 1434–1444 Contents lists available at ScienceDirect Forest Ecology and Management journal homepage: www.elsevier.com/locate/foreco An indicator of tree migration in forests of the eastern United States C.W. Woodall a,*, C.M. Oswalt b, J.A. Westfall c, C.H. Perry a, M.D. Nelson a, A.O. Finley d a USDA Forest Service, Northern Research Station, St. Paul, MN, United States b USDA Forest Service, Southern Research Station, Knoxville, TN, United States c USDA Forest Service, Northern Research Station, Newtown Square, PA, United States d Michigan State University, East Lansing, MI, United States ARTICLE INFO ABSTRACT Article history: Changes in tree species distributions are a potential impact of climate change on forest ecosystems. The Received 18 August 2008 examination of tree species shifts in forests of the eastern United States largely has been limited to Received in revised form 20 November 2008 simulation activities due to a lack of consistent, long-term forest inventory datasets. The goal of this Accepted 12 December 2008 study was to compare current geographic distributions of tree seedlings (trees with a diameter at breast height 2.5 cm) with biomass (trees with a diameter at breast height > 2.5 cm) for sets of northern, Keywords: southern, and general tree species in the eastern United States using a spatially balanced, region-wide Climate change forest inventory. Compared to mean latitude of tree biomass, mean latitude of seedlings was significantly Tree migration farther north (>20 km) for the northern study species, while southern species had no shift, and general United States Forest species demonstrated southern expansion. Density of seedlings relative to tree biomass of northern tree Seedlings species was nearly 10 times higher in northern latitudes compared to southern latitudes. For forest Latitude inventory plots between 448 and 478 north latitude where southern tree species were identified, their biomass averaged 0.46 tonnes/ha while their seedling counts averaged 2600 haÀ1. It is hypothesized that as northern and southern tree species together move northward due to greater regeneration success at higher latitudes, general species may fill their vacated niches in southern locations. The results of this study suggest that the process of northward tree migration in the eastern United States is currently underway with rates approaching 100 km/century for many species. Published by Elsevier B.V. 1. Introduction catastrophic wildfires in regions of the United States (Westerling et al., 2006). These effects on individual tree fitness are forecasted Due to a doubling of pre-industrial atmospheric carbon dioxide to subsequently affect tree response to stress agents such as insects concentrations, the world’s climate is forecasted to change and disease (Volney and Fleming, 2000; Logan et al., 2003). The significantly over the next century, resulting in an increase in combination of numerous climate change effects on forest mean surface temperatures of 2–4.5 8C, more episodic precipita- ecosystems may ultimately be the migration of tree species tion events, and a lengthening in growing seasons (IPCC, 2007). (Opdam and Wascher, 2004; Walther et al., 2002). These climate change effects are predicted to be especially There is evidence of past forest migration rates exceeding 50 km prominent at middle and higher latitudes (IPCC, 2007). Climate per century during episodes of climate change (Schwartz, 1992; is an important driver of forest ecosystem functions (Stenseth Noss, 2001; Parmesan and Yohe, 2003). An important question is et al., 2002), thus changes in climate should change forest whether predicted future climate change will be at a rate that ecosystem attributes and functions. Increases in carbon dioxide exceeds a tree species’ capacity to migrate resulting in species concentration is expected to increase tree biomass increment extirpation/extinction or the conversion of forests to grasslands or through fine root and woody biomass growth (Ainsworth and Long, other systems (Iverson and Prasad, 2002; Woodwell et al., 1998; 2005; Norby et al., 2002, 2004). Fitness of trees is expected to be Davis and Shaw, 2001). Forests may need to migrate one order of impacted by changes in absolute temperatures and the timing/ magnitude faster than in past migrations in order to adequately amount of precipitation events (Saxe et al., 1998; Nabuurs et al., respond to current rates of warming (Schwartz, 1992). However, 2002; Sacks et al., 2007), along with a higher probability of modern day fragmentation of forest ecosystems may inhibit the movement of tree species, potentially reducing tree migration capacity by one order of magnitude (Schwartz et al., 2001; Davis and * Corresponding author at: 1992 Folwell Ave, St. Paul, MN 55108, United States. Shaw, 2001; Walther et al., 2002; Opdam and Wascher, 2004). Tel.: +1 651 649 5141; fax: +1 651 649 5140. Examination of tree species migration largely has been E-mail address: [email protected] (C.W. Woodall). conducted by investigating historic ranges during the past 0378-1127/$ – see front matter. Published by Elsevier B.V. doi:10.1016/j.foreco.2008.12.013 C.W. Woodall et al. / Forest Ecology and Management 257 (2009) 1434–1444 1435 millennia (for example see Davis and Shaw, 2001; Malcolm et al., gradient has been empirically demonstrated for numerous tree 2002; McLachlan et al., 2005; Pearson, 2006) and simulating future species during the past century (Grace et al., 2002); perhaps a tree species shifts (for example see Schwartz et al., 2001; Iverson similar movement of tree species may be demonstrated northward and Prasad, 1998; Iverson et al., 1999, 2008; Malcolm et al., 2002; along a latitudinal gradient. The goal of study was to develop a new McCarty, 2001). These studies have been invaluable for not only indicator of tree migration through comparison of tree seedlings raising awareness regarding climate change impacts on forest and biomass density/latitudinal attributes using an annual, ecosystems, but also highlighting knowledge gaps. However, national-level forest inventory. Specific objectives were to: holistic assessment of these climate change effect models continues to call for refinement of modeling techniques with (1) Compare the current geographic ranges of selected eastern U.S. little or no empirical validation of these models with current data tree species using FIA data to past range maps from Little (for example see Botkin et al., 2007). Therefore, there is substantial (1971). need for developing techniques to validate extensive simulations (2) Compare the mean latitude for seedlings and biomass for tree of potential tree species shifts, which are based on poorly species using the most recent FIA annual inventory data. understood tree migration dynamics (Malcolm et al., 2002). (3) Compare the mean ratio of seedlings/ha and mean biomass/ha Remote-sensing products and field-based forest inventories relative to all other species on each study plot by classes of provide data for monitoring forest attributes across large regions. latitude (38). Unfortunately, remote sensing products are not well suited for (4) Compare the mean seedlings/ha and mean biomass/ha in outer identifying individual tree species across large geographic extents, ranges of the species ranges (northern range greater than 90th especially in the understory. The alternative is to use forest percentile latitude of biomass and southern range less than inventories to track geographic ranges of tree species over a period 10th percentile latitude of biomass) and by 28 latitude classes of decades. Prior to 1999, the national inventory of United States across the entire eastern United States. forest land was conducted only periodically, using sample designs (5) Develop recommendations for interpretation of study results and data management systems that varied by state and inventory and development of future indicators of tree species migration period (Gillespie, 1999). Attempts to compare historic forest from FIA data. inventories to contemporary inventories results are confounded by the lack of digital data (pre-1970), lack of consistent inventory 2. Methods methods both spatially and temporally, and sparse methods/ database documentation (Woodall et al., 2008). These historic 2.1. Data periodic forest inventories provide limited utility for accurately tracking tree species locations over time. For more than 75 years, the USDA Forest Service Forest In 1999, an annual forest inventory was initiated by the United Inventory and Analysis (FIA) program has been charged by States Department of Agriculture, Forest Service’s Forest Inventory Congress to ‘‘make and keep current a comprehensive inventory and Analysis (FIA) program. All eastern states currently have an FIA and analysis of the present and prospective conditions of and annual forest inventory (for more information see USDA, 2007). requirements for the renewable resources of the forest and Due to the inconsistency of periodic forest inventories, comparing rangelands of the United States’’ (McSweeney-McNary Act of 1928) the contemporary annual forest inventories to the older periodic (Gillespie, 1999; Bechtold and Patterson, 2005). FIA is the primary inventories for the purpose of tracking species shifts is confounded source for information about the extent, condition, status and by numerous factors (Woodall et al., 2008). First, only trees with a trends of forest resources across all ownerships in the United diameter at
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