Ecological constraints on the distribution of the Chinese tallow tree (Sapium sebiferum) in the southeastern United States

We used the climatic matching model (CLIMEX) and field studies to predict the potential distribution of the invasive tree Sapium sebiferum in the United States. Sapium sebiferum (Euphorbiaceae) is a deciduous native of China that is rapidly invading habitats throughout the southern U.S.A. Greenhouse and growth chamber results of seedling and seed tolerance to cold temperatures along with published information on the species' distribution were used to define species parameter values for the CLIMEX model. CLIMEX predictions were closely correlated with seed germination rates (r 2 = 0.98 P < 0.001), but not seedling growth rates (r2 = 0.21, P > 0.05) from a field study at sites throughout the southeastern U.S.A.; Seedling survivorship varied across microhabitats and sites; some seedlings survived, however, in all microhabitats two years after transplanting. With few exceptions survivorship was lowest in closed canopy microhabitats at all sites. Survival varied little among open-canopy microhabitats across sites. Higher survival correlated with reduced competition in only two instances.; The highest values for seedling relative growth rates (RGR) were found for seedlings in open-canopy microhabitats at sites well beyond the current distribution of S. sebiferum. With few exceptions RGR values were higher in open-canopy than closed canopy microhabitats. Relative growth rates of seedlings in closed canopy-upland microhabitats were often negative (a loss of biomass over time). Few seedlings were grazed, and any negative RGR values in closed canopy environments were most likely caused by the inability of seedlings to maintain positive carbon balances in these environments. While reduced competition increased the values for RGR in 7 microhabitatlsite combinations, there was no indication that competition limited distribution at either local or regional scales.; The distribution of S. sebiferum at local scales (in closed canopy microhabitats) appears to be limited by low seed germination and by the inability of seedlings to maintain positive carbon balances. Distribution and rates of spread at regional scales is likely limited by seed dispersal, habitat availability (open-canopy habitats) and site specific conditions, such as soil characteristics. These results suggest that S. sebiferum is likely to spread well beyond its current distribution if control is not implemented.