Who Died, Where? Quantification of Drought-Induced Tree Mortality in Texas

Abstract:

During 2011, Texas experienced a severe drought that killed millions of trees across the state. Drought-induced tree mortality can have significant ecological impacts and is expected to increase with climate change. We identify methods to quantify tree mortality in central Texas by using remotely sensed images before and after the drought at multiple spatial resolutions. Fine-scale tree mortality maps were created by classifying 1-m orthophotos from the National Agriculture Imagery Program. These classifications showed a high correlation with field estimates of percent canopy loss (RMSE = 2%; R²=0.9), and were thus used to calibrate coarser scale 30-m Landsat imagery. Random Forest, a machine learning method, was applied to obtain sub-pixel estimates of tree mortality. Traditional per-pixel classification techniques can map mortality of whole stands of trees (e.g. fire). However, these methods are often inadequate in detecting subtle changes in land cover, such as those associated with drought-induced tree mortality, which is often a widespread but scattered disturbance. Our method is unique, because it is capable of mapping death of individual canopies within a pixel. These 30-m tree mortality maps were then used to identify ecological systems most impacted by the drought and edaphic factors that control spatial distributions of tree mortality across central Texas. Ground observations coupled with our remote sensing analyses revealed that the majority of the mortality was Juniperus ashei. From a physiological standpoint this is surprising, because J. ashei is a drought-resistant tree. However, over the last century, this species has recently encroached into many areas previously dominated by grassland. Also, J. ashei tends to occupy landscape positions with lower available water storage, which could explain its high mortality rate. Predominantly tree mortality occurred in dry landscape positions (e.g. areas dominated by shallow soils, a low compound topographic index, and a high heat index). As increases in extreme drought events are predicted to occur with climate change, it will become more important to establish methods capable of detecting associated drought-induced tree mortality, to recognize vulnerable ecological systems, and to identify edaphic factors that predispose trees to mortality.