Spatial and temporal variability in the responses of Arctic terrestrial ecosystems to environmental change

This paper compares the responses of two contrasting Arctic ecosystems to c limate change simulations: a polar semi-desert (in Svalbard) and a dwarf sh rub heath (at Abisko, northern Sweden). These ecosystems are located close to the northern- and southernmost extremes of the Arctic region, respective ly. Impacts of simulated climatic changes were determined through factorial perturbation experiments, where growing season temperature, nutrient avail ability and water supply were manipulated. The results are compared with th e impact of interannual variation in climate on the growth of a keystone mo ss species, Hylocomium splendens, from the wider circumpolar area. The pert urbation studies revealed that current interannual variability in temperatu re and the temperate tolerance of many species may exceed predicted changes in mean summer temperature over the next century. Arctic ecosystems differ ed in their responses to environmental manipulations, with the structure of the dwarf shrub heath being affected through shifts in competitive hierarc hy, potentially leading to lower biodiversity, and the polar semi-desert be ing affected through invasion, potentially leading to higher diversity. H. splendens showed negative responses to perturbation at the sub-Arctic site, in contrast to the positive relationship between temperature and growth ob served in the natural environment. This apparent discrepancy may result fro m: (i) artefacts arising from the perturbations, such as lower atmospheric relative humidity; (ii) non-equilibrium responses during the relatively sho rt-term perturbation studies and/or (iii) ecotypic variation in the moss po pulation. Thus, caution should be employed when extrapolating from perturba tions studies to both longer time-scales and different ecosystems within th e Arctic.