Evapotranspiration flux dynamics in a changing climate

Global change in the Anthropocene will impose various combinations of warming, atmospheric CO₂ levels , and moisture availability on terrestrial ecosystems. A warming climate may increase vapor pressure gradients near plant and soil evaporation fronts driving higher rates of evapotranspiration (ET), whereas atmospheric CO₂ enrichment (eCO₂) can trigger stomatal closure, suppressing transpiration. Our best depiction of future water resources comes from controlled climate-manipulation experiments; however, climate change factors (e.g., eCO₂, warming, and drought) are primarily studied in isolation, limiting the scope of inference. Here we use a series of chamber measurements taken throughout the 2020 growing season to quantify the individual and combined effects of elevated atmospheric CO₂ (+300 ppm), warming (+ 3°C) and recurrent drought on evapotranspiration and plant water use in a mountain grassland. Though water use efficiency (WUE) was nearly identical between “future” (+300 ppm CO₂ and 3°C) and “current” (ambient conditions) systems during drought simulations, the future plots maintained a 2-3 fold higher WUE with the twice the inter-measurement variance during the post-drought recovery period. The isotopic signatures of droughted plots were generally isotopically depleted compared to their non-drought counterparts at peak drought, and the future drought systems had 20 ‰ lighter bulk ET δ²H compared to plots receiving warming alone. Altogether these preliminary results suggest that 1) drought under a future warmer climate and eCO₂ may drive grassland ecosystems to conserve water; 2) when warmed, mountain grasslands may preferentially return recently fallen precipitation to the atmosphere, whereas 3) drought can induce preferential withdrawal of older water storage. Future work will include the use of StorAge Selection (SAS) modeling to characterize the preference of water residence time to atmospheric fluxes under a changing climate.