A LANDSCAPE PERSPECTIVE ON LATE-HOLOCENE VEGETATION CHANGES ON GLACIAL OUTWASH DEPOSITS IN NORTHERN WISCONSIN

Vegetation communities change in response to climate changes, disturbance, and ecological succession, but little is known about the frequency of changes in particular communities, or about changes in the probability of switching between particular vegetation types over time. We addressed these issues by analyzing late Holocene pollen and charcoal records from small lakes on a sandy glacial outwash plain in northwestern Wisconsin. Eleven soil variables and the abundance of lakes and wetlands were used to classify the landscape within 5 km of each of 13 small lakes where pollen was analyzed at a median interval of 70 years and grouped into 16 community types with hierarchical cluster analysis. Transition matrices were calculated for shifts between each of the 16 community types and compared for groups of sites that shared similar soil and landscape attributes. Charcoal particles >125 microns in length were counted to identify local fire events and classified as charcoal signature types.

Transition matrices calculated using >500 transitions show that jack pine (Pinus banksiana) communities with strong fire feedbacks remained stable during several periods of climate change, including decadal or longer droughts and the increase in moisture at the beginning of the Little Ice Age ca. 700 cal yr BP. In contrast, species composition and disturbance regimes were more likely to change in oak (Quercus spp.) and white pine (Pinus strobus) communities. The relative abundance of pollen types in jack pine communities fluctuated more rapidly at 70-year intervals than other community types, but pollen assemblages remained within the range of variation of jack pine community types over thousands of years. We attribute this pattern of rapid fluctuations and long-term stability to the feedback between jack pine and stand-replacing fires. Communities with more oak and white pine had much larger and sometimes rapid state changes from oak- to mixed pine- to white pine-dominated forests. In some cases the state changes were coincident with relatively rapid climate changes at about 1500 and 700 years ago. Analysis of change in community types vs. smaller fluctuations in relative abundance at sites that differ in soil texture provides a framework for incorporation of long-term vegetation observations into landscape-scale vegetation models.