Drought history and the effects of abrupt climate change on deciduous forest ecosystems: A case example from coastal Massachusetts

Background/Question/Methods   One of the most abrupt vegetation changes in North America during the Holocene is the Tsuga (hemlock) decline at ca. ~5.3 ka, which is associated with sharp decreases in hemlock pollen abundance from Michigan to Maine. The decline was initially attributed to a disease or insect outbreak because of 1) the range-wide abrupt decline, 2) the appearance of change in only one taxon, and 3) a lack of evidence for climate change or other drivers. New evidence of simultaneous increases in Fagus (beech) and decreases in Quercus (oak) populations in coastal Massachusetts, combined with evidence for low water levels and drought from Ontario to New Hampshire, indicate that two of the three initial observations about the hemlock decline are not accurate. However, the driver of the coastal vegetation changes, involving abrupt changes in multiple taxa, is poorly understood. Here, we investigate the potential that moisture-balance changes at ~5.3 ka differed between the coast and inland. To do so, we examine the water-level history of Deep Pond, a kettle lake situated on Cape Cod, where the oak-beech dynamics were first recognized. To examine the water-level history, geophysical surveys and sediment cores were collected along a transect across the pond, and were analyzed to evaluate past positions of the lake's shoreline.

Results/Conclusions   The data indicate a series of large sand layers at the lake margin, which are associated with near-shore unconformities, consistent with evidence for repeated low-water episodes found at other inland lakes. Radiocarbon dates support long-term drought as the cause of the sand layers because they represent multiple centuries or more of slow sedimentation rates rather than instantaneous high-deposition (e.g., flood or slump) events. Two large sand layers fall between dates of 5.1 and 3.6 ka, overlapping with the hemlock and oak declines, and the beech rise, within the uncertainties of the 5.1 ka date. The results help to confirm that repeated severe droughts may have played an important role in the hemlock decline, even though the transition from oak to beech could have indicated wet conditions along the coast. Coastal cooling associated with the droughts may instead explain the oak to beech transition, and would be consistent with low temperatures elsewhere around the North Atlantic as well as the synoptic climatology of recent severe droughts (e.g., AD 1965). These results are important for understanding ecological dynamics of multiple tree taxa in response to Holocene climate changes.