Do OTC warming effects on ecosystem processes depend on moss species identity, precipitation, and moss removal?

Sammanfattning: Long-term warming experiments in arctic tundra have resulted in reduced moss cover and increased vascular plant cover. As mosses have a major impact on microclimatic conditions, changes in community composition can potentially alter direct and indirect drivers of productivity and decomposition, which are low in arctic ecosystems. This can potentially change the carbon balance. Therefore, it is of importance to investigate how the effects of warming on ecosystem processes depend on the identity of the moss species dominating the vegetation. Furthermore, because subarctic climate is expected to get warmer and wetter, it is essential to examine how warming effects depend on variation in precipitation as well. Thus, Gross Primary Production (GPP) and Ecosystem Respiration (Reco) fluxes, moss growth, and decomposition rate, were measured in plot pairs along a natural precipitation gradient in subarctic tundra Sweden. Each pair was dominated by one of three common moss species (Hylocomium splendens, Pleurozium schreberi and Sphagnum spp.), and were subjected to moss removal and Open Top Chamber (OTC) warming treatment combinations. Moss growth measurements were taken as a measure of productivity and Tea Bag Index was used as a measure of decomposition rate. Warming effects on GPP depended on both species and precipitation; Hylocomium and Sphagnum had highest productivity in warmed high-precipitation plots, whereas the effect was negative in all cases for Pleurozium. No significant interactions were found for the other response variables, but there was a positive effect of warming on Reco across all treatments, as well as a negative effect on decomposition. Warming induced increases in GPP and respiration, and decrease in decomposition rate, could imply that the increase in CO2 emissions from arctic ecosystems will be smaller than predicted, as the sequestration of CO2 in that case would exceed what is being emitted. Still, it is a complex matter and therefore crucial to further investigate the role of moss species in relation to warming effects on ecosystem processes.