COS 175-2 - The Effects an Exceptional Drought on Tree Mortality and Recovery in the National Forests of East Texas

Background/Question/Methods

In 2011, Texas experienced a record-breaking drought resulting in the driest year in recorded state history. This devastating event killed an estimated 301 million trees statewide, including 65 million in the East Texas forest region. Previous research shows an increase in tree mortality and biomass loss throughout the East Texas region following the drought, but the effects of this event on the national forests of the region have not been fully investigated. Our objective is to determine the effects of the exceptional drought on tree mortality in the four national forests of East Texas, and the major factors influencing mortality and recovery patterns. Tree mortality metrics consist of the analysis of 322 Forest Inventory and Analysis (FIA) plots that have been remeasured four times (once per FIA cycles 7, 8, 9, and 10) from 2001 to 2019. Multispectral analysis of Landsat imagery highlights the spatial distribution of tree mortality among the national forests. To define drought severity, we utilized a Standardized Precipitation-Evapotranspiration Index (SPEI). Repeated measures ANOVA and a Generalized Linear Mixed Model (GLMM) were used to examine drought impacts and pre-drought stand conditions on tree mortality among forest types and tree genera at the scale of each national forest.

Results/Conclusions

Preliminary results indicate that average annual mortality volume in the four national forests was 2.07 m³/ha (1.1-3.6 m³/ha) at its highest after the drought, with tree mortality rates reaching an average of 1.7% (1.1-2.2%) among the national forests immediately following the drought. Our results indicate that the most significant losses in total standing volume were predominantly Pinus species, while Quercus species experienced higher mortality rates. During the drought, mortality due to weather (drought and hurricanes) increased up to ~50%, followed by wildfire (~22%) and disease (~16%). Our multispectral image analysis shows a significant decrease in infrared reflectance two-years after the severe drought, with an average of 45.4% of landcover in the national forests reporting a decline in forest canopy vigor. Preliminary results also indicate a degree of recovery following the aforementioned weather disturbance. Recovery is shown by a decline in standing mortality volume coupled with an increase in basal area during the final years of our study period (FIA cycle 10, 2015-2019). A comprehensive understanding of drought impacts on forest mortality patterns in the national forests of East Texas can help provide useful information to important stakeholders and natural resource managers in the region for forest management planning.