Assessing the sensitivity of evapotranspiration to shifts in plant traits, changing climate, and earlier snowmelt: Long-term observations from a 1,000m elevation gradient in the East River Watershed, Colorado, USA

Abstract:

Plants are an important driver of variation in Evapotranspiration (ET). However, recent global-climate models predict increased temperature, earlier snowmelt, and vapor pressure deficit (VPD) in western North America. We test the hypotheses that (i) changing climate; (ii) shifts in snowmelt date; (iii) and variation in plant traits are key drivers of evapotranspiration. We used a long-term (17 year) dataset of peak carbon and water flux measures of alpine, meadow, and shrubland vegetation along a 1,000m elevational gradient in Colorado, USA. Daytime light, dark, and nighttime water flux measures based on an instantaneous field method using a closed chamber field ‘tent’ allowed us to estimate night and daytime transpiration, T, evaporation, E, and ET at peak phenological stage. While both daytime ET and nighttime E increased with elevation, we observe no elevational change in daytime ’dark’ evaporation rates suggesting that shifts in ET is due to shifts in T and not E. Over the last 17 years we show a decrease in interannual peak season ET but no change in nighttime E or daytime E with time again showing a strong response of vegetation transpiration. Mid-elevation sites showed stronger decreases in ET with time. Assessing the independent effect of snowmelt date shows that peak season ET increases with later snowmelt dates suggesting that forecasted earlier snowmelts will further reduce peak ET. Long-term monitoring of vegetation evapotranspiration A sizeable fraction, 38.1%, of the interannual variability in peak ET can be explained by just site, year, and snowmelt date effects. However, additional analyses indicate a possible larger role of variation in vegetation traits and biomass in controlling peak season ET. Since the start of our monitoring we have observed a ~10% decrease in daytime peak season ET across the gradient. Hotter and drier conditions (greater VPD) appear to be rapidly impacting plant water use but shifts in plant traits and biomass may disproportionately impact ET. Our results suggest that increasing VPD due to climate change and changing snowmelt as well as shifts in plant trait composition together may have large effects on ET in the western USA. As plant water use influences recharge rates in the watershed these results suggest that over the past decade plant impacts on recharge rates have been impacted.