Winter climate change promotes an altered spring growing season in piñon pine-juniper woodlands

Piñon pine-juniper (Pinus edulis-Juniperus monosperma) woodlands constitute a large proportion of land area in the southwestern United States and have experienced widespread vegetation mortality during regional drought events over the past century. Piñon pines have been especially affected by these events, and drought severity is predicted to increase in this region in the future. Based on research that suggests winter climate may influence growing season productivity in semiarid ecosystems, we evaluated the potential for small changes in average winter climate to affect spring growing season conditions in piñon-juniper woodlands, New Mexico, USA. We developed a low-dimensional ecohydrological model of piñon-juniper woodland ecosystems on moderate slopes (5%) and on steep slopes (25%) and simulated the responses of ecosystem water availability, surface conditions, and water and carbon flux dynamics to a climate change scenario of increased temperature and decreased winter precipitation. 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. Moderate slope piñon and juniper trees experienced small reductions in transpiration (Tr) and carbon assimilation (A), and those on steep slopes experienced small but relatively larger reductions in Tr and A, as well as higher increases in soil moisture (θ) variance and E variance. As a result of climate change, the peak of spring Tr occurred on average 6 days earlier on moderate slopes and 10 days earlier on steep slopes, the timing of A shifted towards earlier March dates, and A was reduced during April and May. Steep slope piñon pines experienced greater proportional reductions in Tr and A than junipers. Our results suggests that winter climate change will promote an earlier growing season in piñon-juniper woodlands, will increase daily variance in θ and E during spring, and will produce slight reductions in A in woodlands with steep slopes and a large proportion of piñon pines. In a more arid future climate, a shift towards lower soil moisture availability and carbon assimilation in April and May may intensify the effects of early summer drought events for piñon-juniper woodlands, thus exacerbating the impacts of larger changes in climate dynamics.