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Harvard Forest Symposium Abstract 2005

• Title: Vegetation dynamics across southern New England during the middle Holocene
• Primary Author: Wyatt Oswald (Emerson College)
• Additional Authors: Edward Faison (Highstead, Inc.); David Foster (Harvard University)
• Abstract:

  Lake-sediment pollen records from sites throughout eastern North America are characterized by a dramatic drop in Tsuga pollen percentages at ~5400 years before present (BP). This event, known as the mid-Holocene Tsuga decline, has been attributed to a phytophagous pest outbreak (e.g., Bhiry, N. and Filion, L. 1996. Quaternary Research 45: 312-320), but recent studies suggest that the period from ~5400-3000 years BP was an interval of relatively dry climate (e.g., Shuman, B. et al. 2004. Ecology 85: 1297-1310). This evidence has led to the hypothesis that the Tsuga decline was caused (1) directly by the adverse effect of dry environmental conditions on Tsuga canadensis, or (2) indirectly via the exacerbation of insect activity by the change in climate. We have developed a series of new pollen records from southern New England and Long Island, New York, which provide insights into mid-Holocene environmental change and vegetation dynamics. Pollen records from western Connecticut and northwestern Massachusetts have a particularly strong Tsuga decline, suggesting that the abundance of Tsuga declined severely in the higher elevation areas of southern New England. For example, in the record from Little Pond in north-central Massachusetts, Tsuga pollen percentages drop from ~25% to <1% during the decline (Figure 1). However, at sites along the coast, where Tsuga presumably was rare or absent before 5400 years BP, pollen evidence shows that taxa other than Tsuga experienced substantial changes during the middle Holocene, suggesting that a major environmental change occurred at 5400 years BP. For example, in the records from Deep Pond on Cape Cod and No Bottom Pond on Nantucket, Quercus pollen percentages decrease and Fagus abundance increases during the interval from 5400-3000 years BP. In the record from Wildwood Lake, located on Long Island, that same interval has an increase in the abundance of Pediastrum algal cell nets, indicating lower lake levels and thus reduced effective moisture. Taken together, the paleoecological evidence from southern New England and Long Island supports the hypothesis that regional climate became drier ~5400 years BP, which in turn suggests that environmental change played a role in the decline of Tsuga during the middle Holocene.

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