Print PageForecasts predict that precipitation regimes in the western US will become more variable, with dry periods becoming more frequent and with individual rainfall events becoming more extreme. In water-limited ecosystems, increased event size may reduce soil moisture stress and increase net primary production (NPP), N mineralization (Nmin), and soil water content (Q) and potential (Y). In more mesic systems, the increased time between rain events may increase soil moisture stress and reduce NPP, Nmin, Θ, and Ψ. To test this hypothesis, we experimentally altered the timing and size of rainfall events and reduced ambient rainfall during the growing season for xeric, low-elevation sites and mesic high-elevation sites. Research was conducted at the Great Basin Experimental Range in Ephraim Canyon, UT, USA. The experimental treatments were (1) ambient rain, (2) 30% reduction in ambient rain, (3) 70% reduction in ambient rain, (4) reapplication of ambient rain weekly, and (5) reapplication of ambient rain every 3 weeks. We monitored soil temperature and Θ, leaf area index and NDVI, N-mineralization, soil respiration, and aboveground NPP. Here we report the results of the first three years of this experiment.
Results/Conclusions
We found that net primary production was highly sensitive to growing season precipitation amount, even in the mesic, snow dominated subalpine. Large rainfall events increased the duration where Y is above critical water thresholds. A key insight is that precipitation amount and timing influenced canopy greenness late in the growing season more than during maximum biomass. There were no significant changes in N-availability with altered precipitation, but the modeling results suggest that drought is a much stronger control over N-availability than precipitation timing. Our results demonstrate that the timing and amount of precipitation alter both total fluxes and their seasonal dynamics.
phone 202-833-8773