Globally, trees are increasingly dying from extreme drought, a trend that is expected to increase with climate change. Loss of trees and canopy cover has significant ecological, biophysical, and biogeochemical consequences. In 2011, a record drought caused widespread tree mortality in Texas. Using remotely sensed imagery, we quantified canopy loss during and after the drought across the state at a 30-m spatial resolution, from the eastern pine/hardwood forests to the western shrublands, a region that includes the boundaries of many species ranges. First, we classified ~200 multi-temporal orthophoto sets (1-m) to create fine-scale canopy loss maps for a total area of ~8000 km2. Then with random forest models, we used the fine-scale maps to train coarser Landsat imagery (30-m) to create wall-to-wall binary maps of canopy loss covering an area of ~600,000 km2. We used these canopy loss maps to determine which ecological systems were most impacted by the 2011 drought and to identify climatic anomalies that, when surpassed, led to enhanced canopy loss. We then examined whether these 2011 climatic anomalies were likely to be surpassed again in the future using downscaled projected climate data from representative concentration pathway, RCP, 4.5 and 8.5 trajectories.
Canopy loss occurred across all major ecoregions of Texas, with an average canopy loss of 9.5% across the state. The drought had the highest impact in post-oak woodlands, pinyon-juniper shrublands, and Ashe juniper woodlands. Focusing on a 100-km by ~1000-km transect spanning the state’s 5-fold east-west precipitation gradient, we found that much of the canopy loss occurred in areas that passed specific anomaly thresholds from historical climate: warm season mean temperature (+1.6 °C), warm season vapor pressure deficit (VPD, +0.66 kPa), annual precipitation (-38% deviation), and 2011 annual difference between precipitation and potential evapotranspiration (-1206 mm). Although similarly low precipitation occurred during the landmark 1950s drought, the VPD and temperature anomalies observed in 2011 were even greater. Furthermore, future climate data (RCP 8.5) project that average values will surpass the 2011 VPD anomaly during the 2070-2099 period and the temperature anomaly during the 2040-2099 period. Identifying vulnerable communities to drought stress and climatic thresholds associated with canopy loss will aid in predicting how forests will respond to a changing climate and how ecological landscapes will change in the near term.