Tree Mortality and Decomposition Dynamics Following an Extreme Drought in East Texas, USA

Abstract

Throughout 2011, the state of Texas, USA, experienced an extreme drought that broke statewide temperature and precipitations records, causing extensive tree mortality. No study comprehensively examined impacts to the heavily forested and important economic and ecologic region of east Texas. This dissertation aimed to fill that knowledge gap by: 1) examining tree species mortality responses multiple years postdrought; 2) evaluating the impacts of management and stand structure on pine species mortality; 3) quantifying and describing the dynamics of standing dead trees; and 4) refining understanding and estimation of structural volume changes in standing dead pine trees using terrestrial light-detection-and-ranging (LiDAR). The first three objectives made use of U.S. Forest Service Forest Inventory and Analysis data for east Texas. The final objective was accomplished using LiDAR and a novel volume calculation algorithm. Oak species experienced significant immediate mortality, presumably crossing a threshold by which they could not continue transpiring. Pine species mortality was the lowest of all examined and did not increase significantly until two years post-drought, suggesting pines successfully employed physiological strategies to avoid rapid mortality. Planted loblolly pines were generally maintained at lower densities and moderate tree sizes than naturally-regenerated loblolly and shortleaf pines. This management effect appeared to offer favorable competitive conditions allowing planted loblolly pine to resist drought mortality. Standing dead trees experienced high probability of falling. within five-years, driven primarily by stem size and decay class. Reconstructed standing dead tree volumes derived from LiDAR produced robust allometric models for volume estimation and provided for empirical assessment of structural changes across decay classes. These findings highlight the resistant nature of managed pines to extreme drought mortality and the vulnerability of oaks to die-off in future extreme droughts. Future work should strive to identify the physiological mechanisms driving drought mortality and specific silvicultural targets for mitigating extreme drought mortality. Biomass and carbon that transitions to the standing dead wood pool following mortality becomes downed dead wood very rapidly in east Texas. Tools developed herein for predicting fall rates and quantifying standing dead wood via LiDAR will help to refine future understanding of carbon dynamics, wildfire risk, and habitat management.