Background/Question/Methods . Recent climate predictions for the end of this century suggest that soil moisture will decrease and the number of drought days will increase over large parts of the temperate forests in North America. Tolerance of moisture stress is likely to become increasingly important in mediating competitive interactions among tree species, mostly because seedlings are particularly vulnerable to water stress. The goal of our study was to determine how the physiology and growth of seedlings of four temperate tree species were altered by a gradual decrease in soil water content. Ten seedlings of Betula alleghaniensis, Platanus occidentalis, Quercus rubra and Ulmus americana were randomly distributed in a five-block design within a greenhouse. In each block, one seedling per species served as a well-watered control while the other was subjected to drought. As soil gravimetric water content and soil water potential gradually declined, we measured leaf photosynthetic capacity (A_max), stomatal conductance (g_s) and fluorescence of dark-adapted leaves (Fv/Fm). Biomass allocation and growth were measured at the end of the experiment.

Results/Conclusions . Leaf A_max and g_s in P. occidentalis declined by 77% and 87% after 10 days drought, respectively; this species lost most of its leaves after 16 days and had a 79% reduction in growth by the end of the drought treatment (one month). The A_max and g_s of B. alleghaniensis declined more slowly but A_max was 100% lower and g_s 88% lower than controls on day 30 of the experiment; final biomass was 63% lower than the control. Quercus rubra and U. americana had higher levels of stress tolerance than B. alleghaniensis and P. occidentalis. The final biomass of Q. rubra and U. americana were 41 and 46% lower than the controls, respectively. After 30 days of drought, Q. rubra A_max had declined by 82% and gs by 80% relative to controls, while for U. americana A_max had decreased by 69% and g_s by 62%. Water stress tolerance in P. occidentalis and U. americana was markedly different despite both being floodplain species. Betula alleghaniensis and Q. rubra, both upland species, had intermediate water stress tolerance, although Q. rubra tolerance was similar to U. americana. For all species, leaf Fv/Fm remained close to control values, although small changes in Fv/Fm mirrored large changes in leaf gas exchange. There were clear stress-tolerance differences between species that could have a large influence on the spatio-temporal distributions of species under a drier climate.