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Harvard Forest Symposium Abstract 2010

  • Title: Middle-Holocene dynamics of eastern hemlock in northern New England
  • Primary Author: Wyatt Oswald (Emerson College)
  • Additional Authors: David Foster (Harvard University)
  • Abstract:

    The abrupt, range-wide decline of eastern hemlock ~5500 calibrated radiocarbon years before present (cal yr BP) is perhaps the most-studied event in North American paleoecology. Little attention, however, has been given to an earlier hemlock decline, dated to ~6000 cal yr BP in two pollen records from southern Ontario, Canada (Fuller, 1998; Ecology 79: 2337-2351). To investigate whether this event occurred over a broader area of eastern North America, we analyzed the middle-Holocene interval of a lake-sediment record from Knob Hill Pond, located in northern Vermont, USA, an area of historically high hemlock abundance. A dramatic, short-lived drop in hemlock pollen abundance does occur at ~6000 cal yr BP in the Knob Hill Pond record, indicating that hemlock populations declined across the entire region. Paleoclimatic data from elsewhere in New England suggest that both of the middle-Holocene declines of hemlock may have been caused by the deleterious effects of pronounced droughts on this moisture-sensitive tree. Close examination of pollen data from other sites in New England reveals that the earlier decline of hemlock is present in several other records, although some aspects of the event appear to have varied geographically. While northern and higher-elevation sites exhibit a nearly full recovery of hemlock populations between the two declines, several records further to the south are characterized by a stair-step pattern of decline. At sites near its southern range limit, relatively warm conditions between ~6000 and 5500 cal yr BP were apparently not conducive to the reestablishment and survival of hemlock, and thus it was unable to rebound between the drought events.

  • Research Category: Historical and Retrospective Studies