Alpine environments are already responding to recent climate change, with vegetation range shifts upwards in elevation and increased abundance of warm adapted species. Over the last five decades the alpine communities near the White Mountain Research Center (Inyo County, California) have experienced both rising temperatures and encroachment from lower elevation communities. In particular, sagebrush (Artemisia rothrockii) has increased in both elevational range limit and abundance, and several alpine cushion plants have declined in abundance. Will these shifts in community composition influence ecosystem-level responses to climate change? The effect of warming and sagebrush encroachment on litter decomposition and nutrient mineralization rates in an alpine environment was the focus of this experiment. We hypothesized that experimental warming would increase decomposition and nutrient mineralization, while both processes would be slowed in sagebrush dominated plots. Experimental warming was achieved with passive warming chambers both with and without sagebrush at two elevations: 3,100m and 3,700m. A separate shading treatment evaluated the impact of lower light availability under shrubs. Ion exchange resin bags (measuring nitrogen mineralization rates) and litterbags containing either sagebrush (A. rothrockii) or cushion plant (Trifolium andersonii) were deployed in October 2013 and collected annually (litterbags) or biannually (resin bags) for two years.
Results/Conclusions
As expected, rates of mass loss and nitrogen mineralization were lower at the cooler, higher elevation site. However, both experimental warming and the presence of sagebrush lowered rates of mass loss. These effects were additive, with the lowest rates of decomposition in the warmed treatments where sagebrush was present. Thus, elevational range limits of woody species in this region could significantly change ecosystem carbon dynamics compared with predictions that do not account for shifting species distributions. There were lower levels of nitrate with shrub presence at the higher elevation, and lower soil moisture in the warmed plots at the lower elevation; hence both moisture limitation and nutrient limitation could have contributed to lower microbial activity and therefore lower decomposition rates in the plots with warming and sagebrush. There was no significant effect of the shading treatment, thus a lack of abiotic UV degradation was unlikely to be the mechanism driving lower decomposition in the plots dominated by sagebrush. There was no significant difference between decomposition rates of the two litter types. The plots at the higher elevation had the greatest mean differences between treatments, suggesting higher elevation sites may have the greatest sensitivity to both warming and shrub encroachment.