Persistence of MODIS evapotranspiration impacts from mountain pine beetle outbreaks in lodgepole pine forests, south-central Rocky Mountains

Abstract

The current extent and high severity (percent tree mortality) of mountain pine beetle outbreaks across western North America has been attributed to regional climate change, specifically warmer summer and winter temperatures and drier summers. This study paired multiple mountain pine beetle outbreak location datasets, both current and historical, with Moderate Resolution Imaging Spectroradiometer (MODIS) and Global Modeling and Assimilation Office (GMAO) Modern Era Retrospective-Analysis for Research and Applications (MERRA) products in order to quantify the full seasonal evapotranspiration impact of outbreak events for decades after outbreak (0 to 60 years). Following mountain pine beetle outbreaks in lodgepole pine (Pinus contorta) stands in the Colorado Rockies we observed an 18.7±1.4% (p<0.01) reduction in the rate of summer evapotranspiration at 14 to 20 years since outbreak. We also observed a 21.6±2.2% (p<0.01) increase in the rate of summer evapotranspiration, relative to non-attacked stands, in intermediate-aged stands 30 to 40 years since outbreak. Changes to growing season evapotranspiration correlated positively with changes in stand density, stand leaf area index (LAI) and the fraction of absorbed photosynthetically active radiation (fPAR), while high incoming solar radiation during the summer months acted to amplify changes to evapotranspiration even given relatively minor changes to summer fPAR and LAI due to the rapid regeneration of understory vegetation. Lodgepole pine mortality from mountain pine beetle outbreaks showed lasting effects on stand-scale evapotranspiration which could have important implications for regional water resources.