Climate, forest growing season, and evapotranspiration changes in the central Appalachian Mountains, USA.

We analyzed trends in climatologic, hydrologic, and growing season length variables, identified the important variables effecting growing season length changes, and evaluated the influence of a lengthened growing season on increasing evapotranspiration trends for the central Appalachian Mountains region of the United States. We generated three growing season length variables using remotely sensed GIMMS NDVI3g data, two variables from measured streamflow, and 13 climate parameters from gridded datasets. We included various climate, hydrology, and phenology explanatory variables in two applications of Principle Components Analysis to reduce dimensionality, then utilized the final variables in two Linear Mixed Effects models to evaluate the role of climate on growing season length and evapotranspiration. The results showed that growing season length has increased, on average, by ~22 days and evapotranspiration has increased up to ~12 mm throughout the region. The results also suggest that a suite of climatic variables including temperature, vapor pressure deficit, wind, and humidity are important in growing season length change. The climatic variables work synergistically to produce greater evaporative demand and atmospheric humidity, which is theoretically consistent with intensification of the water cycle and the Clausius-Clapeyron relation, which states that humidity increases nonlinearly by 7%/K. Optimization of the evapotranspiration model was increased by the inclusion of growing season length, suggesting that growing season length is partially responsible for variations in evapotranspiration over time. The results of this research imply that a longer growing season has the potential to increase forest water cycling and evaporative loss in temperate forests, which may lead to decreased freshwater provisioning from forests to downstream population centers. Additionally, results from this study provide important information for runoff and evapotranspiration modelling and forest water management under changing climate.