Variable responses to climate change: Drought, recovery, and vegetation shifts 1500 years ago

Background/Question/Methods

Understanding the relative sensitivity of different types of forests to past climate change provides important context for predicting future changes. We analyzed vegetation changes over the last 3500 years, a time when a long-term increase in moisture led to an increase in yellow birch (Betula alleganiensis) and eastern hemlock (Tsuga canadensis). We analyzed pollen from a 280 cm 3500 year sediment core from Eska Lake, Taylor County and compared our results with regional pollen data from a variety of soil types including sandy soils where hemlock does not grow. Pollen based vegetation history was also compared with vegetation-independent climate reconstructions to aid in a regional interpretation of forest changes in response to century scale changes in moisture availability. 

Results/Conclusions

Hemlock abundance at Eska Lake increased from 0-3% before 1500 years ago to >5% after.  A period of low moisture availability from 1800 to at least 1500 years ago is indicated by a decrease in birch and increase in pines, herbs and aquatics. Birch increases again as hemlock begins to increase 1500 years ago, but aquatics and herbs remain relatively high for another 200 years, perhaps indicating that hemlock began to increase as moisture availability increased but conditions were still drier than modern.  This century scale drought followed by increased moisture after 1500 years ago is consistent with vegetation independent estimates from other sites. A step-wise increase in hemlock abundance 1500 years ago occurs at several sites in the region, including Hemlock Lake, Lake Mary, and Wood Lake, consistent with a response to an increase in moisture availability. Other types of vegetation shifts occur in the region at the same time. Shifts from oak to pine dominated vegetation on sandy soils where hemlock does not grow may also be driven by increased moisture availability. Vegetation does not change at several sites suggesting that different types of forest respond very differently to the same climate changes.