Middle-Holocene dynamics of Tsuga canadensis (eastern hemlock) in northern New England, USA

The abrupt, range-wide decline of Tsuga canadensis similar to 5500 calibrated years before present (cal. yr BP) is one of the most-studied events in North American paleoecology. Little attention, however, has been given to an earlier Tsuga decline, dated to similar to 6000 cal. yr BP in southern Ontario, Canada. To investigate whether this event occurred elsewhere in 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 Tsuga abundance. A dramatic, short-lived drop in Tsuga pollen abundance does occur at similar to 6000 cal. yr BP in the Knob Hill Pond record, indicating that Tsuga populations declined in various parts of its range. We hypothesize that both middle-Holocene declines of Tsuga were caused by the deleterious effects of pronounced droughts on this moisture-sensitive tree. Close examination of pollen data from a transect of sites across New England reveals that the earlier decline of Tsuga is present in 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 Tsuga populations between the two declines, records further to the south are characterized by a stair-step pattern of progressive decline. At sites near its southern range limit, relatively warm conditions between similar to 6000 and 5500 cal. yr BP were apparently not conducive to the reestablishment and survival of Tsuga, and thus it was unable to recover between the drought events.