Global assessment of experimental climate warming on tundra vegetation: heterogeneity over space and time.

Understanding the sensitivity of tundra vegetation to climate warming is critical to forecasting future biodiversity and vegetation feedbacks to climate. In situ warming experiments accelerate climate change on a small scale to forecast responses of local plant communities. Limitations of this approach include the apparent site-specificity of results and uncertainty about the power of short-term studies to anticipate longer term change. We address these issues with a synthesis of 61 experimental warming studies, of up to 20 years duration, in tundra sites worldwide. The response of plant groups to warming often differed with ambient summer temperature, soil moisture and experimental duration. Shrubs increased with warming only where ambient temperature was high, whereas graminoids increased primarily in the coldest study sites. Linear increases in effect size over time were frequently observed. There was little indication of saturating or accelerating effects, as would be predicted if negative or positive vegetation feedbacks were common. These results indicate that tundra vegetation exhibits strong regional variation in response to warming, and that in vulnerable regions, cumulative effects of long-term warming on tundra vegetation - and associated ecosystem consequences - have the potential to be much greater than we have observed to date.

Global negative vegetation feedback to climate warming responses of leaf litter decomposition rates in cold biomes.

Whether climate change will turn cold biomes from large long-term carbon sinks into sources is hotly debated because of the great potential for ecosystem-mediated feedbacks to global climate. Critical are the direction, magnitude and generality of climate responses of plant litter decomposition. Here, we present the first quantitative analysis of the major climate-change-related drivers of litter decomposition rates in cold northern biomes worldwide. Leaf litters collected from the predominant species in 33 global change manipulation experiments in circum-arctic-alpine ecosystems were incubated simultaneously in two contrasting arctic life zones. We demonstrate that longer-term, large-scale changes to leaf litter decomposition will be driven primarily by both direct warming effects and concomitant shifts in plant growth form composition, with a much smaller role for changes in litter quality within species. Specifically, the ongoing warming-induced expansion of shrubs with recalcitrant leaf litter across cold biomes would constitute a negative feedback to global warming. Depending on the strength of other (previously reported) positive feedbacks of shrub expansion on soil carbon turnover, this may partly counteract direct warming enhancement of litter decomposition.

Genetic and environmental effects on morphology in clonal sedges in the Eurasian Arctic.

We studied the variation in morphological characters of importance for resource acquisition and storage in 21 populations of four clonal sedge taxa in arctic Eurasia, Carex bigelowii, C. ensifolia subsp. arctisibirica, C. lugens, and C. stans, and the response to transplantation to a common garden in Tromsø, Norway. The morphology of C. stans was distinct from the other three taxa, all of which belong to the C. bigelowii species complex. However, differences among populations within taxa were even greater than differences among taxa, and environmental variables explained 40-50% of the among-population variation in the morphological characters. Stomatal size decreased with temperature while stomatal density increased. Shoot height and leaf width were smaller at peak lemming population phase, while rhizome length was shorter at higher longitudes. Transplantation to a common garden affected stomatal density in all taxa, stomatal size and shoot height only in some taxa, while leaf width was not affected. We found a weak, but highly significant correlation between geographical, morphological, and genetic distances. We concluded that although genotypic differentiation in arctic rhizomatous Carex species is reflected in their morphology, they are also capable of plastic morphological responses to the environment and that these responses are specific for each taxon.