The North American Monsoon and the Megadrought: How Precipitation Influences Forest Responses to Drought Conditions

Drought conditions have been projected to increase globally as atmospheric CO2 (ca) and vapor pressure deficit (VPD) increase. Experimental and modelling studies have suggested that forest responses to drought may be bolstered by increased ca, but it remains unclear how prevalent this response is in natural forest ecosystems. We developed a framework using intrinsic water use efficiency (WUEi = assimilation (A)/ stomatal conductance (gs)) and evaporative water use efficiency (WUEE = A/transpiration (E)) to evaluate how a dominant species’ drought response within the Western US, Pinus ponderosa, has been affected by increased ca during a multi-decadal megadrought (MD). Using a hierarchical clustering approach we identified two distinct populations within our sites, those within the core region of the North American Monsoon (NAM), and those on the periphery of the NAM. Across all populations, WUEi increased from 1960-2017, corroborating published studies, however, WUEE did not increase at a commensurate rate with meaningful discrepancies between the two populations. WUEE diverges from WUEi during the MD as WUEE accounts for the impact of increasing VPD on E, but WUEi does not. Despite reduced gs via higher ca during the MD, increased VPD drove larger gradients of water vapor pressure between leaves and the atmosphere, leading to higher E. These findings suggest that reductions in gs may be offset by increasing VPD & E, leading to increased water loss in forests through evapotranspiration. Enhanced E rates despite reduced gs are important to consider for forests globally as temperatures, and VPD, continue to rise with climate change.