Drought has been implicated as a major driver of forest ecosystem change globally. Variation in drought exposure (e.g., duration and magnitude) and forest sensitivity jointly determine drought consequences on forest structure and composition, but these consequences are not yet fully understood. In this study, our objectives were to assess (1) the effect of drought s on the timing of remotely sensed forest decline (RSFD) and (2) examine the impacts of RSFD on forest structure and composition in several forest types across the western USA from 1984 to 2013. RSFD was identified using manual interpretation of Landsat multispectral and aerial photographic imagery. We quantified lagged precipitation and vapor pressure deficit effects (VPD) on the timing of RSFD events at locations across the western USA in five broadly distributed forest types using stochastic antecedent modeling, a statistical method for identifying and quantifying temporally lagged effects. For a portion of the study area (California, Oregon, and Washington), we compared co-occurring RSFD events and repeated measurements of forest inventory plots to quantify changes in tree species basal area at 500 forest inventory plots measured between 1993-2013.
Results/Conclusions
Our results indicated that drought likely contributed to RSFD across the western US, but the temporal and seasonal patterns of precipitation and VPD preceding drought varied substantially among forest types. Drought was more often related to RSFD in dry forest types (pinyon-juniper and ponderosa pine forests) compared to moist forest types (Douglas-fir, lodgepole pine, and subalpine forests). Low winter precipitation in dry forests and low summer precipitation in wet forests contributed to drought effects on RSFD. Winter temperature contributed to drought effects in all forests, indicating the importance of growing season length. In many cases, climatic conditions 1-3 years in the past contributed substantially to the drought signal. Despite a clearer effect of drought on RSFD in dry forests, forest structural and compositional changes were of greater magnitude in moist forests, especially subalpine forest where several fir species decreased in live tree basal area coincident with RSFD events. Forest dynamics are temporally complex, relying on multi-year patterns of drought that vary among forest types. Our results highlight the complexity of drought effects on RSFD and forest dynamics and demonstrate a need to better understand the connections between tree level responses and emergent effects on stand structure.