The response of peat-rich permafrost soils to human-induced climate change may be especially important in modifying the global C-flux. We examined the Holocene developmental record of a High Arctic peat-forming wetland to investigate its sensitivity to past climate change and aid understanding of the likely effects of future climate warming on high-latitude ecosystems. The microhabitat of mosses was quantified in the present-day polygon-complex at Bylot Island (73 degrees N, 80 degrees W) and used to interpret the radiocarbon-dated macrofossil record of three cores, comprising c. 3500 years of wetland development. Recurrent wet and dry phases in the reconstructed palaeohydrological record indicated pronounced temporal variability. Wet and dry phases were compared between cores and with palaeoclimatic proxy values, measured as percentage melt and delta O-18 in nearby ice cores. Periodic wet and dry phases appear unrelated to past climate over c. 50% of the combined stratigraphic records, and are attributable instead to geomorphological mechanisms. At other times, association of wet and dry phases with significantly lower and higher values of percentage melt and delta O-18 indicate a possible effect of past climate change on polygon hydrology and vegetation, although inconsistencies between cores suggest that local geomorphological processes continued to modify a regional climatic effect. However, during a period incorporating the Little Ice Age (c. 305-530 cal. years BP), reconstructed moisture and vegetation change is pronounced and consistent among all three cores. The results provide strong evidence for the sensitivity of a High Arctic terrestrial ecosystem to past climate change during the Holocene. The estimated magnitude of changes in soil moisture between wet and dry phases is sufficient to imply recurrent shifts in wetland function, periodically impacted upon by pronounced climatic variability, although controlled principally by autogenic processes. The structure and function of such wetlands may therefore be susceptible to predicted, human-induced climate warming.

Previous studies in tundra ecology provide evidence for sensitivity of the vegetation-soil complex to climate. Short-term experiments (less than or equal to10 yr) suggest that climate change may have a decade-scale effect on soil moisture, decomposition and nutrient availability, plant phenology, and plant growth. In contrast, there exists little evidence to confirm or refute the role of climate in structuring tundra vegetation over longer time scales (10 to 1000 yr). This study accordingly examines similar to1500 yr in the stratigraphy of two permafrost sediment cores from a High Arctic, polygon-patterned, graminoid-moss tundra. Present-day bryophyte-environment relationships are quantified, and the radiocarbon-dated macrofossil record of bryophytes is used to reconstruct past changes in soil moisture. The paleoecological record is characterized by pronounced variability during polygon development. As the hydrology of tundra polygons is controlled by known climatic and geomorphologic mechanisms, the recurrent development of polygon vegetation (cf. hydrologic change) is compared to an independent paleoclimatic proxy for net radiation (R-n). Based on this comparison, the vegetation provides support for a pronounced shift to colder and wetter conditions during the Little Ice Age (similar to300-465 yr BP), though the long-term response to past climate change is otherwise equivocal. We suggest accordingly that autogenic geomorphologic-vegetation processes may have been generally more important than climate in the long-term development of the polygon-patterned wetland examined. A framework for such processes is presented. We caution that previous research to simulate and describe the effects of climate warming might not have properly accounted for the dynamic role of geomorphology in regulating tundra microclimate.