Using a Physiological Approach to Improve Predictions of Climate Change Effects on Temperate Forests

Evidence of climate change on Earth is unequivocal. Around the globe, air temperatures and ocean heat content are increasing, the extent of snow and ice is decreasing, sea levels are rising, and extreme climate events are more frequent. Temperature in the eastern US is expected to increase by 2--6 °C by 2100 and drought is projected to become more frequent and severe, yet it is uncertain how terrestrial ecosystems will respond to these climate changes.;This dissertation used a physiological approach to analyze plant species responses to increased temperature and drought in North Carolina. I utilized an experimental warming site in the understory of a temperate deciduous forest (Duke Forest, NC), where air temperature inside large open-top chambers was increased from 1.6--5.3 °C above ambient. Warming of 5 °C extended the growing season of four tree species by 20--28 days. Nonlinear responses of budburst phenology and failure to fully track warmer temperatures suggests that current high rates of phenological change are unsustainable and will decrease with warming throughout the coming century. Warming advanced flowering by 6--25 days in three of seven study species, but warming delayed flowering of Tipularia discolor by 10 days and had no effect on three species that flower in early spring. Warming of 2 °C inhibited reproduction of Chimaphila maculata and T. discolor, suggesting temperature thresholds that could severely limit the distribution of these species in the future.;Experimental warming negatively affected growth of the terrestrial orchid T. discolor, which is leafless in summer and acquires carbon primarily in winter. Like many plant species, the optimum temperature for photosynthesis in T. discolor is higher than the maximum temperature throughout most of its growing season, and therefore growth should increase with warming. Warming of 4.4 °C resulted in nearly 60% less growth than under ambient conditions, however, likely due to restrictions in stomatal conductance ( gs). Leaf-toair vapor pressure deficit (VPD) over 1.3 kPa restricted gs of T. discolor to 10--40% of maximum conductance. Sensitivity of orchid stomata to VPD was higher than for most species in mesic ecosystems. These results highlight the need to account for changes in VPD when estimating temperature responses of plants under future warming scenarios.;Mean VPD was increased by 0.16--0.96 kPa inside the chambers as a consequence of heating. I used miniature sap flow gauges to measure the effect of atmospheric drying on transpiration and gs of four common deciduous trees (Acer rubrum, Carya tomentosa, Quercus alba, Quercus rubra) throughout the growing season. Experimental warming for 3 years resulted in growth increases in C. tomentosa and Q. alba but not the other two species. Warming and increased VPD significantly decreased midday leaf water potential while increasing midday transpiration and daily water use, indicating that future climate change will increase the potential for temperature-induced drought stress.;I measured the physiology of transgenic black cottonwood (Populus trichocarpa) trees growing at two sites in NC: a cooler mountain site and a warmer piedmont site that experienced frequent water stress. Two low-lignin genotypes had significantly lower mean leaf water potential, gs, transpiration, hydraulic conductivity, and leaf-specific whole-plant hydraulic conductance relative to the wild-type. The water transport capacity of vascular tissues in transgenic genotypes was severely impaired. Stunted growth of low-lignin trees at the piedmont site was caused by restricted carbon gain due to water stress and lower biochemical and biophysical photosynthetic processes. Despite the fact that transgenic trees did not maintain lower lignin content at the mountain site, the genetic transformation resulted in a total water savings of roughly 1 kg tree-1 day-1 without sacrificing productivity.