Relatively small temperature increases may alter species survival and distribution by increasing the potential for leaf-level water loss via increased transpiration. This is particularly important at the moisture driven ecotone (tension zone) between forest and grassland where changes in species distributions will alter the range of biomes and fundamentally restructure ecological processes related to carbon, water, and nutrient cycling. Our objective was to determine how temperature mediated increases in plant water use and water stress affect leaf-level gas exchange and survival of tree seedlings commonly found in Oklahoma along the ecotone.  Specifically, we tested whether a 3 ^oC increase in temperature is enough to increase plant water stress and decrease survival during periods of drought. We exposed seedlings of Populus deltoides, Qurecus shumardii, Diospyrus virginiana, Cercis cannadensis, and Platanus occidentailis to two day and night temperature regimes in growth chambers.  Temperature regimes were 33/28 ^oC and 30/25 ^oC (day/night). Dewpoint was maintained at 19 ^oC at all times by altering relative humidity in the chambers such that daytime VPD was 39% higher in the chambers maintained at 33/28 ^oC than the chambers maintained at 30/25 ^oC.  Leaf gas exchange was measured using the Li-Cor 6400 portable photosynthesis system (n=8). 

Results/Conclusions

A 3 ^oC increase in temperature and commensurate 39% increase in VPD increased transpiration by between 24 and 42% for the different species and did not have a significant effect on net photosynthesis. This higher rate of transpiration hastened mortality of seedlings from which water was withheld. Seedlings in the low temperature/low VPD chambers lived between 27 and 66% (species averages) longer than seedlings in the high temperature/high VPD chambers. These results indicate that increased water use associated temperature increases within the range predicted by global climate change models have the potential to decrease survival of tree seedlings and contribute to changes in species distributions.