Ecosystem Response to a Changing Climate: Vulnerability, Impacts and Monitoring

Rising temperatures with increased drought pose three challenges for management of future biodiversity. First, are the species expected to be vulnerable concentrated in specific regions and habitats? Second, are the impacts of drought and warming varying across regions? Third, could recent advances in remote sensing techniques help us in monitoring the impacts in real-time? This dissertation is an effort to address the above questions in the three chapters.

First, I used foliar chemistry as a proxy for drought vulnerability. I used soil and moisture gradients to quantify habitat variation that could be critical for alleviating drought. I used a large dataset of forest plots covering the eastern united states to understand how community weighted mean foliar nitrogen and phosphorus vary across climate and soil gradients. I exploited trends in these variables between species, traits, and habitats to evaluate sensitivity. Critical to our approach is the capacity to jointly model trait responses. Our data showed that nutrient demanding species strongly respond to environmental gradients. I identified a wide range of sites across low to high latitudes threatened by drought. The sensitivity of species to high temperatures is largely explained by soil variations. Drought vulnerability of nutrient and moisture demanding species could be amplified depending on local soil and moisture gradients. Although local soil moisture may dampen drought-induced stress for species with large leaves and high water use, nutrient demanding species remain vulnerable in wet regions during droughts. Phosphorus demanding species adapted to dry sites are drought resilient compared to communities in wet sites. This research is consistent with the studies that supports declining nutrient demanding species with increasing temperature and decreasing moisture. I also detected strong soil effects on shaping community weighted traits across a large geographical and environmental range. Our data showed that soil effects on controlling foliar traits strongly vary across different climates. The findings are critical for conservations and maintaining the biodiversity.

Next, I used space-borne remotely sensed vegetation indices to monitor the process of leaf development across climate gradients and ecoregions in the southeastern United States. A hierarchical state-space Bayesian model was developed to quantify how air temperature, drought severity, and canopy thermal stress contribute to changes in leaf opening from mountainous to coastal regions. I synthesized daily field climate data with daily remotely sensed vegetation indices and canopy surface temperature during spring green-up season. The study was focused on observation of leaf phenology at 59 sites in the southeast United States between 2001 to 2012. Our results suggest strong interaction effects between ecosystem properties and climate variables across ecoregions. The findings showed that despite the much faster spring green-up in the mountains, coastal forests express a larger sensitivity to inter-annual anomaly in temperature than mountain sites. In spite of the decreasing trend in sensitivity to warming with temperature in all regions, there is an ecosystem interaction: Deciduous-dominated forests are less sensitive to warming than are those with few deciduous trees, possibly due to the presence of developed leaves in evergreen species throughout the season. The findings revealed mountainous forests are more susceptible to intensifying drought and moisture deficit, while coastal areas are relatively resilient. I found that increasing canopy thermal stress, defined as canopy-air temperature difference, slows the leaf-development following a dry year, accelerates it after a wet year.

Finally, I demonstrate how space-borne canopy “thermal stress”, i.e. surface-air temperature difference, could be used as a surrogate for drought-induced stress to estimate forest transpiration. Using physics-based relationships that accommodates uncertainties, I showed how changes in canopy water flux may be reflected in surface energy balance and in remotely-sensed thermal stress. Validating with field measurements of canopy transpiration in the southeastern US, I quantified sensitivity of transpiration to thermal stress in a range of atmospheric and climate conditions. I found that a 1 mm change in daily transpiration may cause 3 to 4 °C of thermal stress, depending on site conditions. The cooling effect is large when solar radiation is high or wind speed is low. The effect has the highest control on water-use during warm and dry seasons, when monitoring drought is essential. I applied our model to available satellite and metrological data to detect patterns of drought. Using only air and surface temperatures, I predicted anomaly in water-use across the contiguous United States over the past 15 years, and then compared with anomaly in soil water content and conventional drought indices. Our simple model showed a reliable accuracy in compare to the state-of-the-art general circulation models. The technique can be used in varying time-scales to monitor surface water-use and drought in large scales.