Changes in Gross Primary Productivity and Biogeochemical Fluxes in a Mountainous Watershed as Affected by Warming in Recent Decades

Studies of plant water use have historically focused on water stored within shallow, near-surface soils ignoring deeper moisture stores and dynamics. However, observations across a range of mountainous watersheds have shown that plant roots reach well below the soil, into fractured and weathered bedrock, where plant available water greatly exceeds shallow, near-surface storage. While these deeper water pools are critical to primary production, they are also more sensitive to climate warming than shallow stores of moisture. With an aim to reduce this uncertainty related to the importance of deeper moisture stores in a changing climate, we focus on a mountainous watershed observatory located at East River, Colorado. The specific objectives of this study are to: (a) evaluate how changes in snowpack depths and precipitation regimes impact deeper water stores and dynamics, and (b) investigate the consequences of these changes on vegetation productivity and biogeochemical fluxes at the hillslope scale. Historical data indicates that variation in winter precipitation is one of the defining features of the East River site. Water availability varies significantly within seasons and across years where summer monsoons may contribute 17 to 35% of the total annual precipitation, increasing in recent years and representing a shift in a typically snow dominated system. Numerical modeling results indicate that warm winters cause an earlier and greater pulse of nitrate from microbial processes at hillslopes, representing a potential source of exports to the river. However, warmer winters and early snowmelt can enhance primary productivity through an extended growing season depending on the timing and magnitude of monsoon events. Moreover, sensitivity analysis indicates that leaf-out and root trait parameters are the most sensitive to changes in precipitation. We conclude that plant water use and primary productivity depend not only on total annual precipitation, but also on how much winter precipitation can be stored in the deeper layers and the timing of monsoons in these mountainous environments.