Effects of Warming Climate and Land Use Change on Ecosystem Processes in Icelandic Rangelands as Mediated by Plants

The Arctic is warming around four times faster than the global average, leading to visible ecosystem changes such as Arctic greening and browning, due to shifts in plant community composition and productivity (NDVI). Those shifts are accompanied by altered community functional trait composition, potentially, influencing associated ecosystem processes. Functional trait composition may either shift toward resource acquisition or conservation, depending on other environmental factors such as land use. Understanding how warming influences functional traits, ecosystem processes and their relationship is critical in predicting ecosystem responses to climate change. The aim of this study was to understand how community-weighted mean (CWM) plant functional traits relate to ecosystem processes (CO₂ fluxes, decomposition and greenness) and how fenced sheep grazing cessation alone (F) and warming combined with grazing cessation (F+W) influence the relationship. I used a long-term warming experiment site at Auðkúluheiði in the Icelandic highlands, where a one ha fence excludes grazing and warming has been simulated for almost 30 years with open-top chambers. Ecosystem process responses differed between treatments, with fenced plots (both F and F+W) increasing ecosystem respiration and F+W plots enhancing decomposition. F+W plots had a stronger effect on CWM plant functional traits than F, shifting communities toward a more conservative yet competitive trait composition (larger plants). The relationship between CWM plant functional traits and ecosystem processes was weak, but significant, and no trait independently explained variation in processes independent of treatments. This suggests that by removing the grazing disturbance that warming along with grazing cessation enhanced the CWM trait-process relationships.