Late Holocene Stand-Scale Invasion by Hemlock (Tsuga canadensis) at Its Western Range Limit

The movement of plant range limits in the past has been clearly documented as a response to changing climate, but the pattern and timing of local invasions that result from these plant migrations are not well understood. Sedimentary evidence front Small forest hollows is used here to reconstruct the pattern of stand-scale invasion by eastern hemlock (Tsuga canadensis (L.) Carr.) at its current range limit in Wisconsin. Pollen records from lakes show a slowly increasing regional hemlock population over the past 2500 yr, as its range expanded from the cast in response to cooler, wetter climate. In contrast, hemlock pollen in each of the forest hollows increased abruptly within the past 500 yr, demonstrating stand-scale dynamics at a time when regional changes were small. Moreover, this abrupt rise in hemlock is part of a two-phase sequence of local invasion, First, stands were initially colonized as much as 2000 yr ago, but the size of these local populations remained low for as long as 1800 yr. This pattern suggests that scattered, small populations grew beyond the continuous range limit of hemlock, which was tens of kilometers to the cast. Second, these small hemlock populations expanded in size between 100 and 500 yr ago during a time of cooler, wetter climate, which was apparently limiting population growth. These results illustrate contrasting patterns of change between local and regional populations, and the importance of outlying colonies as sources of rapid population expansion when environmental conditions become favorable.