Climatic change has resulted in an altered precipitation regime in the southwestern US. The frequency and the severity of drought have been increasing and are expected to continue increasing in this region. These changes in the hydrologic cycle are altering key ecosystem functions directly by decreasing available water, and indirectly by causing long-term shifts in species distributions where species that are more drought resistant persist. This pattern is particularly striking in piñon-juniper woodlands where increases in drought frequency has resulted in a loss of piñon pines (Pinus edulus)–resulting in a juniper (Juniperus monsperma) dominated landscape. To understand how changes in precipitation are altering piñon-juniper woodlands, we assessed an important ecosystem function, nitrogen cycling, in an existing, large scale, precipitation manipulation (50% reduction drought treatment, a water removal and reapplication control, 50% increased precipitation treatment, and an unmanipulated control) in central New Mexico. We measured nitrogen availability and mineralization across all treatments and beneath both piñon and juniper trees to understand both the direct and indirect effects of precipitation change on nitrogen cycling.
Results/Conclusions
We found that nitrogen cycling in this woodland was more sensitive to plant identity than to direct changes in precipitation. There was 29% more nitrogen available beneath piñon relative to juniper (F = 4.4, p = 0.04). This pattern held across 2 growing seasons and across all treatments. In addition, we saw increased mineralization beneath piñon relative to juniper trees (F = 4.6, p = 0.03). These data suggest that as piñon die across the landscape, alterations in the nitrogen cycle may result in a decrease in available, mobile nitrogen in an already nutrient limited ecosystem.