1a. Objectives (from AD-416):
Tree mortality rates are increasing globally, partly as a result of changes in weather patterns (climate change). Mortality may result in the loss of large amounts of CO2 to the atmosphere. Objectives of this work are to 1) quantify effects of the recent drought on tree mortality on central Texas and Oklahoma rangelands, 2) determine causes of mortality (carbon starvation, hydraulic failure, or some combination of the two), and 3) predict tree mortality under continued drought and the consequences of increased mortality for landscape-scale carbon dynamics.
1b. Approach (from AD-416):
We will use high spatial resolution (0.5-5 m) multispectral imagery to evaluate mortality of adult trees during the period from 2008 to 2014. High resolution images are available from the USDA for pre-drought (2008) analysis. Satellite imagery will be acquired for post-drought analysis (2012-2014). Mortality and loss of live crown area will be validated by field inspection. We will measure vulnerability to hydraulic failure in roots and stems, transpiration, plant water status, and net photosynthesis and respiration for six dominant tree species. A soil–vegetation–atmosphere carbon and water transfer model will be used to evaluate whether mortality under drought results from hydraulic failure or inadequate carbon. The model will be applied to predict future patterns of water use, carbon exchange, and tree mortality.
3. Progress Report:
Field sampling was initiated to address the specific objective of this Agreement – use platform-based measurements of optical reflectance to identify trees with enhanced susceptibility to continuing or recurrent drought. Our basic assumption is that mortality in response to drought will increase as relative 'vigor' decreases among trees of a given species growing on similar soils. Reflectance at the red edge of the visible range of the light spectrum (690-730 nm) is negatively correlated with photosynthesis capacity, leading to our working assumption that high reflectance at the red-edge is indicative of plants with relatively low physiological activity and growth rate and heightened sensitivity to drought stress. We have completed measurements of size, optical reflectance (using a portable spectroradiometer), leaf area, and xylem potential of 12-20 trees each of three species common in the Edward Plateau region of central Texas where tree mortality was high following the record drought during 2011.