COS 15-5 - Evidence for antecedent influences of winter snowpack on the use of monsoon rain in montane coniferous forests

Abstract: A two decade-long megadrought, with likely anthropogenic causes, has impacted forest growth and mortality across the Southwestern U.S. Given this event, and the future likelihood of similar climate challenges, it is important to understand how water resources are utilized by semi-arid forests in this region. Here, we studied seasonal water-use in eight different P. ponderosa montane forests distributed across a climate gradient with varying contributions from winter and summer precipitation within the geographic domain of the North American Monsoon climate system. We collected oxygen isotopes from precipitation, soil, and xylem water during two different and contrasting hydrologic years to determine how trees differentially use seasonal precipitation sources. We observed that both the geographic climate gradient and the differential effects wet and dry years influenced how trees use winter and summer precipitation. As expected, sites at the drier end of the climate gradient used a greater fraction of summer rain than winter snowpack, to support summer growth, whereas sites at the wetter end of the gradient used more snowmelt water to support summer growth. Unexpectedly, however, during low snowpack years, trees at all sites trees used less summer rain during the summer growth period, demonstrating an antecedent influence of winter precipitation on the availability of summer rain. Possible mechanisms to explain the antecedent effect include: (1) an influence of deeper, later-melting snowpacks to enhance hydrologic connectivity within the soil profile, thus increasing the soil penetration of summer rains, and (2) the maintenance of shallow, fine-root biomass during the hyper-arid period that separates the winter and summer precipitation seasons during years with deeper, later snowpacks. These findings provide crucial insights into how the seasonal interactions between major climate systems influence forest tree water use in the face of an extreme megadrought.