Late Quaternary vegetation and lake level changes in central Alaska

Abstract

The threat of significant high-latitude global warming over the next 50 years requires that we assess the response of vegetation to climate change. One approach is to see how plants have reacted to past climate change. In this study high-resolution reconstructions of past vegetation and climate, based on pollen and lake level changes, provide useful insights into vegetation and climate change in central Alaska since 14,000 years ago. Climate changed substantially at about 12,000 years ago, between 11,000 and 10,000 years ago, and about 8,000 years ago. At 12,000 years ago, a significant transition is reflected by the appearance of shrub birch into a region that had been dominated by grass, sage, and sedge. The vegetation became denser; shrubs occupied the moister sites, and herbaceous taxa grew on well-drained, exposed ridges and slopes. Lake levels increased at this time, suggesting the climate became warmer and wetter than it had been previously. Between 11,000 and 10,000 years ago, the vegetation at some sites reverted to a grass and sage-rich flora, suggesting a return to drier and/or cooler conditions. This period of climate change has not been recognized before from pollen records in central Alaska. The timing of this vegetation shift suggests it is related to the Younger Dryas event, a world-wide episode of climatic deterioration. About 8,500 to 8,000 years ago, spruce appeared in the region, coincident with a significant lake level rise, suggesting that the spruce expansion was aided by wetter conditions, as well as warmer temperatures. In central Alaska, periods of past vegetation change are marked by shifts in moisture. Today, central Alaska receives very little rain, and in some areas the vegetation is moisture-limited, suggesting that during the past, changes in moisture could have had a strong effect on the vegetation. In terms of future global change, this study suggests that any shifts in moisture associated with the predicted temperature changes, especially towards drier conditions, will strongly affect the current vegetation distribution.