Holocene vegetational history of the central Arctic Foothills, northern Alaska: Pollen representation of tundra and edaphic controls on the response of tundra to climate change.

摘要

As arctic tundra responds to climate change, does its response vary spatially because of small-scale edaphic heterogeneity? This research addresses this question by reconstructing the past vegetational response to climate change of ecosystems on adjacent, but contrasting, glaciated surfaces in the central Arctic Foothills of northern Alaska. Vegetation history is inferred from pollen records from lakes on the Sagavanirktok surface (glaciated >125,000 BP), which has smooth topography and fine-textured soils, and the Itkillik II surface (glaciated 24,000 to 11,500 BP), which has irregular topography and coarse-textured soils. Modern pollen-vegetation relationships are examined using regional- and landscape-scale data sets. These studies (1) improve our understanding of how pollen represents tundra vegetation, (2) refine multivariate methods for comparing entire pollen assemblages, and (3) explore the ability of pollen data to distinguish between the dwarf-shrub tundra (DST) and prostrate-shrub tundra (PST) communities that dominate the Sagavanirktok and Itkillik II surfaces, respectively. Consistent with the tundra community composition of DST and PST, Sagavanirktok samples had relatively high percentages of Rubus chamaemorus and Ericales, and Itkillik II samples had relatively high percentages of Equisetum and Thalictrum . The findings of these pollen-vegetation calibration studies were then applied to Holocene records from Upper Capsule Lake (Sagavanirktok surface) and Red Green Lake (Itkillik II surface). During the early Holocene (11,300 to 10,000 cal BP) the vegetation of the Sagavanirktok surface resembled modern PST. It is likely that as effective moisture increased between 10,000 and 7000 cal BP, soil moisture increased on the Sagavanirktok surface, leading to higher vegetation cover, permafrost aggradation, increased soil acidity and anoxia, and slower decomposition. By 7000 cal BP Sagavanirktok vegetation was similar to modern DST. In contrast, the vegetation of the Itkillik II surface did not experience major vegetational changes between the early and middle Holocene. Because their coarse texture results in inherently low water-holding capacity, Itkillik II soils likely support relatively xeric tundra like PST regardless of climate. The critical role of substrate in the Holocene vegetational history of the central Arctic Foothills suggests that edaphic variability may impart strong spatial heterogeneity on the response of tundra ecosystems to future climate change.