Winter Climate Change Promotes Altered Timing of Spring Water and Carbon Fluxes in Piñon-juniper Woodlands, New Mexico, USA

Abstract:

Piñon-juniper woodlands constitute a large proportion of land area of the southwestern United States and may be highly susceptible to climate change. These ecosystems have experienced widespread tree mortality during regional drought events over the past 100 years, and piñon pines have been especially affected by these events. Climate change projections for the southwestern US call for greater temperature-driven aridity, but the potential effects of winter climate change are unclear. Based on recent research that suggests winter climate anomalies may influence growing season productivity in many semiarid systems, we explored the potential for winter climate change to influence piñon-juniper woodlands in the coming 50 years. We developed a low-dimensional ecohydrological model of lowland and upland piñon-juniper tree stands in northern New Mexico, USA, and simulated the probabilistic response of ecosystem water availability, surface conditions, and water and carbon flux dynamics to climate change projections of increased temperature (+1.6 °C) and decreased winter precipitation (-0.11 mm month^-1). The climate change scenario reduced average winter snowcover, decreased surface albedo, increased net radiation, and altered the timing of spring evaporation (E) towards earlier dates. Lowland piñon and juniper trees experienced small reductions in transpiration (T) and carbon assimilation (A), and upland sites experienced small but relatively larger reductions in T and A, as well as relatively higher daily variance in soil moisture (θ) and E. The peak of spring T occurred on average 6 days earlier in lowland and 10 days earlier in upland simulations. The timing of A shifted towards earlier March dates for both lowland and upland, and A was reduced during April and May. Upland piñon pines experienced greater proportional reductions in T and A than junipers. Our results suggest that winter climate change will promote an earlier growing season in piñon-juniper woodlands, will increase variance in θ and E, and will have greater negative impacts in upland compared to lowland sites, and on piñon compared to juniper trees. Earlier growing season onset lengthened and intensified the late spring dry period in April and May, and we hypothesize that this trend may also amplify the effects of early summer drought events.