Quantifying trends and uncertainty in prehistoric forest composition

Forest ecosystems in eastern North America were in flux over the last
several thousand years, well before Euro-American land clearance and the
20th-century onset of anthropogenic climate change. However, the
magnitude and uncertainty of prehistoric vegetation change have been
difficult to quantify because of the multiple ecological, dispersal, and
sedimentary processes that govern the relationship between forest
composition and fossil pollen assemblages. Here we extend STEPPS, a
Bayesian hierarchical spatio-temporal pollen-vegetation model, to estimate
changes in forest composition in the upper Midwestern United States from
about 2000 to 200 years ago. Using this approach, we identify areas of
statistically and ecologically significant change. Between 2000 and 200
years ago, forest composition significantly changed across broad regions of
north-central Wisconsin and Minnesota. Rates of compositional change
varied spatially, and can be linked to previously reported events. The single
largest change is the infilling of Tsuga canadensis in northern Wisconsin
over the past 2000 years. Despite this range in-filling, the range limit of T.
canadensis was largely stable, with modest expansion westward. The
regional ecotone between temperate hardwood forests and northern mixed
hardwood/conifer forests shifted southwestward by 15-20 km in Minnesota
and Northwestern Wisconsin. Fraxinus, Ulmus, and other mesic hardwoods
expanded in the Big Woods region of southern Minnesota. However, some
areas showed no significant change, suggesting high complexity in the
spatiotemporal patterns of past forest dynamics. The increasing density of
paleoecological data networks and advances in statistical modeling
approaches now enables the confident detection of subtle but significant
changes in forest composition over the last 2000 years.