Widespread Tree Mortality from the 2011 Texas Drought: Consequences for Forest Structure and Carbon Stocks
Abstract
Larger and more frequent drought-related tree mortality events can alter the carbon cycling of terrestrial ecosystems; however, the carbon cycling implications of drought in forest ecosystems is poorly understood because the effects are not discrete in time, tree species have varying survival tolerances, and tree mortality from drought is often diffusely distributed across the landscape masking its effect from satellite observation. Widespread tree mortality was caused by the exceptional 2011 drought in Texas. In the summer following the drought, we used a statewide survey of 599 plots and satellite imagery (pre/post drought) to estimate the impact of the 2011 Texas Drought on forest C storage and cycling. In each 0.16 ha plot, dead trees were identified to the genus level and diameters were recorded. Normalized Difference Vegetation Index (ΔNDVI = May 2012 NDVI - May 2011 NDVI) derived from the MODIS satellite sensor was calibrated to the field plots to produce a 2011 tree mortality map for Texas. We estimate that 301 million trees died in Texas from the exceptional drought of 2011 (6.2% of live trees), which resulted in a conversion of 24-30 Tg C from live to dead tree carbon pools. This event was notable in that it affected a wide assemblage of species throughout the state across distinct ecoregions. The largest trees experienced disproportionately higher mortality; large angiosperms from historically wetter regions suffered the greatest losses. Gymnosperms thought to be drought-hardy also experienced unexpectedly high levels of mortality, with significant regional variation in whether the mortality was concentrated in large trees. The concentration of mortality in large trees likely had a disproportionate effect on forest net primary productivity, which is a key constraint on estimating the full effect of this drought on future forest C capture. In an effort toward full accounting of this event, we modeled its cumulative effects on regional C cycling and identified key unknowns for properly constraining the model. Our results suggest that ground based measurements are critical for documenting the shifts in tree size and species composition associated with drought mortality in forests and for predicting the consequences of drought for terrestrial ecosystem carbon cycling. This drought investigation in Texas is noteworthy because within the United States, policies to reduce atmospheric CO2 levels, such as offsetting emissions through bioenergy harvests or afforestation, will likely be implemented at the state level and thus require a detailed and region-specific understanding of drought impacts on forest C cycling.
- Publication:
-
AGU Fall Meeting Abstracts
- Pub Date:
- December 2013
- Bibcode:
- 2013AGUFM.B33J0599M
- Keywords:
-
- 0428 BIOGEOSCIENCES Carbon cycling;
- 1812 HYDROLOGY Drought;
- 1630 GLOBAL CHANGE Impacts of global change;
- 1813 HYDROLOGY Eco-hydrology