A meta-analysis of the response of soil respiration, net nitrogen mineralization, and aboveground plant growth to experimental ecosystem warming

Climate change due to greenhouse gas emissions is predicted to raise the me an global temperature by 1.0-3.5 degreesC in the next 50-100 years. The dir ect and indirect effects of this potential increase in temperature on terre strial ecosystems and ecosystem processes are likely to be complex and high ly varied in time and space. The Global Change and Terrestrial Ecosystems c ore project of the International Geosphere-Biosphere Programme has recently launched a Network of Ecosystem Warming Studies, the goals of which are to integrate and foster research on ecosystem-level effects of rising tempera ture. In this paper, we use meta-analysis to synthesize data on the respons e of soil respiration, net N mineralization, and aboveground plant producti vity to experimental ecosystem warming at 32 research sites representing fo ur broadly defined biomes, including high (latitude or altitude) tundra, lo w tundra, grassland, and forest. Warming methods included electrical heat-r esistance ground cables, greenhouses, vented and unvented field chambers, o verhead infrared lamps, and passive nighttime warming. Although results fro m individual sites showed considerable variation in response to warming, re sults from the meta-analysis showed that, across all sites and years, 2-9 y ears of experimental warming in the range 0.3-6.0 degreesC significantly in creased soil respiration rates by 20% (with a 95% confidence interval of 18 -22%), net N mineralization rates by 46% (with a 95% confidence interval of 30-64%). and plant productivity by 19% (with a 95% confidence interval of 15-23%). The response of soil respiration to warming was generally larger i n forested ecosystems compared to low tundra and grassland ecosystems, and the response of plant productivity was generally larger in low tundra ecosy stems than in forest and grassland ecosystems. With the exception of aboveg round plant productivity, which showed a greater positive response to warmi ng in colder ecosystems, the magnitude of the response of these three proce sses to experimental warming was not generally significantly related to the geographic, climatic, or environmental variables evaluated in this analysi s. This underscores the need to understand the relative importance of speci fic factors (such as temperature, moisture, site quality, vegetation type, successional status, land-use history, etc.) at different spatial and tempo ral scales, and suggests that we should be cautious in "scaling up" respons es from the plot and site level to the landscape and biome level. Overall, ecosystem-warming experiments are shown to provide valuable insights on the response of terrestrial ecosystems to elevated temperature.