EOMF-13. Advancing understanding of plant-drought interactions in North American ecosystems

Plants and their strategies to deal with heat and moisture stress may play significant roles in the evolution, maintenance, and severity of drought. Water and carbon are exchanged through plant stomates, and plants can regulate these exchanges in response to increased moisture stress. Plants may become more efficient at using water and leading to less transpiration and increased retention of soil moisture at root depth, thus dampening the effect of drought on the plants, but possibly intensifying it in the atmosphere. On the other hand, during short dry periods trees can deliver soil moisture from deeper roots to the atmosphere, moderating atmospheric dryness. Current methods to monitor and forecast droughts have highly simplified representation of these vegetation feedback mechanisms. Mechanistic land surface process models with detailed representations of vegetation can explicitly diagnose which plant responses alter stomatal regulation and ecosystem function under varying environmental conditions. Here we present results from the Simple Biosphere Model (v4.2) analyzing the interactions of drought and carbon cycle changes in North American ecosystems. We evaluated drought events from 2000-2020 across 76 individual flux tower sites as well as 0.5-degree gridded simulations across the CONUS region. Drought events were identified using county-level data from the University of Nebraska Lincoln Drought Monitor (UNL DM). We examined anomalies in plant uptake against anomalies in soil moisture and vapor pressure deficit (VPD), binned by drought severity class as identified by the UNL DM’s Drought Severity Coverage Index. Preliminary analysis indicates uptake and moisture anomalies become more negative during droughts, as expected. However, the magnitudes differ across different ecosystems, with forest sites exhibiting more resiliency to drought versus grasslands. Similar results are seen for a newly added tracer – carbon-13 of CO2. Carbon-12 is favored during plant uptake for energetic reasons, and modeled positive delta-13C anomalies indicate plant stress and less ability to discriminate. Finally, soil-moisture-VPD interactions show that increased VPD can offset decreased stomatal conductance at some sites, implying that atmospheric water demand can outcompete a plant’s ability to conserve water.