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Measuring canopy loss and climatic thresholds from an extreme drought along a 5-fold precipitation gradient across Texas

Authors: 
Schwantes AM, JJ Swenson, M González-Roglich, DM Johnson, JC Domec, RB Jackson
Year: 
2017
Journal: 
Global Change Biology
Journal Volume/Pages: 
23:5120–5135

Globally, trees are increasingly dying from extreme drought, a trend that is expected to increase with climate change. Loss of trees 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 30-m spatial  resolution,  from  the eastern pine/hardwood forests to the western shrublands, a region that includes the boundaries of many species ranges. Canopy loss observations in ~200 multitemporal fine-scale orthophotos (1-m) were used to train coarser Landsat imagery (30-m) to create 30-m binary statewide canopy loss maps. We found that canopy loss occurred across all major ecoregions of Texas, with an average loss of 9.5%. The drought had the highest impact in  postoak  woodlands,  pinyon-juniper  shrublands and Ashe juniper woodlands. Focusing on a 100-km by ~1,000-km transect spanning the State’s fivefold east–west precipitation gradient (~1,500 to ~300 mm), we com- pared spatially explicit 2011 climatic anomalies to our canopy loss maps. Much of the canopy loss occurred in areas that passed specific climatic thresholds: warm sea- son  anomalies  in  mean  temperature  (+1.6°C)  and  vapor  pressure  deficit  (VPD, +0.66 kPa), annual percent deviation in precipitation (-38%), and 2011 difference between precipitation and potential evapotranspiration (-1,206 mm). Although simi- larly low precipitation occurred during the landmark 1950s drought, the VPD and temperature anomalies observed in 2011 were even greater. Furthermore, future cli- mate data under the representative concentration pathway 8.5 trajectory, project that average values will surpass the 2011 VPD anomaly during the 2070–2099 per- iod and the temperature anomaly during the 2040–2099 period. Identifying vulnera- ble ecological systems to drought stress and climate 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.

KEY W ORD S
change detection, climate change, disturbance, extreme event, forest die-off, random forest, tree mortality, vapor pressure deficit