Abstract
Pollen diagrams from six sites on outwash plains of north-central Minnesota are interpreted as records of similar extralocal vegetation changes since c. 5000 BP. All of the study sites have undergone a shift from prairie to an aspen-oak community, followed by the jack pine-dominated vegetation that now occupies the sites. All of the species capable of directly invading prairie (aspen, bur oak, willow and some entomophilous shrubs) were probably established on the surrounding moraines prior to their invasion of the adjacent prairie-occupied outwash plains. Only short distances (<20 km) were involved in these local invasions, and the chronology of their establishment does not indicate a regional westward migration of these taxa in the late Holocene. Because climate varies little over the short distances separating the study sites, the asynchroneity of the dates for the establishment of aspen brush (Zone 3/Zone 4 boundary) and the following establishment of jack pine (Zone 2/Zone 3 boundary) indicates that locally varying conditions determined where and when these vegetational shifts occurred. Sloughs and wet swales are apparently prerequisite for aspen to invade prairie, and they probably appeared on the outwash plains at different times because locally varying hydrologic conditions determined the absolute increase in precipitation required for the water table to rise and flood depressions. The rate at which afforestation proceeded, culminating with the establishment of jack pine, appears to be related to local variability in the distribution of wet depressions as they serve as firebreaks. Where peat-filled channels and chains of lakes occur, afforestation occurred first and was relatively rapid. Where lakes and peatlands are scattered, afforestation occurred later and required more time. Where wet depressions are rare, some patches of prairie have persisted since the mid-Holocene.
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