WESTERN RANGE BOUNDARIES OF FLOODPLAIN TREES IN THE SOUTHEASTERN UNITED STATES

The distribution and abundance of many trees in the southeastern United States are determined by climate (Davis 1981; Bartlein, Prentice, and Webb 1986; Woodward 1987). Latitudinal range boundaries at a regional scale are generally associated with seasonal differences in temperature and growing season that affect phenological processes (Iverson and Prasad 1998; Chuine and Beaubien 2001; Mes-saoud, Bergeron, and Leduc 2007). Western range boundaries are most commonly attributed to differences in precipitation and soil moisture (Graybeal 1999). Forest cover and tree-species richness decline to the west, as the luxuriant forests of the Southeast shift gradually toward mixed forests and grasslands, and ultimately to pure grasslands. The longitudinal distributions of many Coastal Plain trees, however, are not closely related to climate. Many studies addressed the effects of climate change on tree migration and abundance in eastern North America (Overpeck, Bartlein, and Webb 1991; Davis and Zabinski 1992; Malanson 1993; Iverson, Schwartz, and Prasad 2004). Forest-species migration models are generally based on the assumption that vegetation patterns at a region scale depend on climate. These models also consider dispersal mechanisms, land use, habitat fragmentation, and other landscape characteristics (Dyer 1995; Iverson and Prasad 1998; Morin, Augspurger, and Chuine 2007), but they do not take into account the presence or absence of suitable habitats based on hydrogeomorphic processes necessary for regeneration. A moderately drier climate will have little or no effect on many Coastal Plain trees with western range boundaries that are either indirectly related to or largely independent of climate. Understanding the western range limits for these trees depends on knowledge of the physiography and underlying geologic conditions affecting earth surface processes. In this article we argue that hydrogeomorphic processes control the western range boundaries of common floodplain trees in the Coastal Plain of the southeastern United States. Many bottomland tree species reach their western range boundaries at the edge of the Coastal Plain. This distribution pattern largely reflects the location of suitable floodplain habitats, not the existence of favorable climatic conditions. Some riparian trees species show another distribution pattern, one that is associated more directly with hydrogeomorphic processes than with climate. These species extend westward along streams from the Coastal Plains into the drier Great Plains, where groundwater availability largely determines whether the trees can survive. Species with range limits linked to physiographic boundaries or riparian habitats contrast with the many upland trees with western range boundaries that are linked more directly to precipitation and soil moisture. First we analyze western range boundaries of common floodplain trees in the Coastal Plain using distribution maps by Elbert Little and data from the Forest Inventory and Analysis (FIA), the U.S. Department of Agriculture (USDA), and the U.S. Forest Service data (Little 1971; Prasad and Iverson 2003), Next we note the range of tree species with boundaries corresponding to physiographic regions and describe hydrogeomorphic conditions that account for these range boundaries based on phenological characteristics and the preferred habitats for these species. We then group species into two categories based on habitat requirements and regeneration characteristics. First are bottomland species restricted mostly to Coastal Plain alluvial river valleys that, to varying degrees, tolerate a high water table and root submersion, and have boundaries related to physiographic regions. Second are riparian species that grow well upstream of the Coastal Plain and westward into the dry Central Lowlands, where they are sustained by a high water table. PHYSIOGRAPHIC REGIONS The southeastern section of North America is divided into distinct physiographic regions (Figure 1). …