Abstract
Forest ecosystems in eastern North America have been in flux for 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 spatiotemporal pollen–vegetation model, to estimate changes in forest composition in the upper Midwestern United States from about 2,100 to 300 yr ago. Using this approach, we find evidence for large changes in the relative abundance of some species, and significant changes in community composition. However, these changes took place against a regional background of changes that were small in magnitude or not statistically significant, suggesting complexity in the spatiotemporal patterns of forest dynamics. The single largest change is the infilling of Tsuga canadensis in northern Wisconsin over the past 2,000 yr. Despite range infilling, 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. 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 2,000 yr.
Highlights
The presettlement data can be interpreted as a stable baseline and used to evaluate changes in the landscape caused by humans
As in Dawson et al (2016), we focus on 10 tree genera that include the most abundant taxa as well as several taxa that are less abundant but have been hypothesized in the literature to have undergone range shifts or compositional changes in the UMW over the recent millennia: Acer, Betula, Fagus, Fraxinus, Larix, Picea, Pinus, Quercus, Tsuga, and Ulmus
This assumption is standard in pollen–vegetation modeling and is a form of the broader assumptions of uniformitarianism in the geosciences: that the processes we observe today can be employed to gain insight into the unobservable and latent processes operating in the past
Summary
The presettlement data can be interpreted as a stable baseline and used to evaluate changes in the landscape caused by humans Such an evaluation is possible because the geographic distributions of major tree species. Networks of fossil pollen data provide empirical evidence about dynamics of plant populations and communities at timescales of decades to millennia. Fossil pollen data have provided rough estimates of range expansions and contractions (Woods and Davis 1989, Davis et al 1991, Parshall 2002), continental-scale changes in taxon distributions (Bernabo and Webb 1977, Williams et al 2004), and local- to landscape-scale community shifts (Grimm 1983, Umbanhowar et al 2006, Hotchkiss et al 2007, Jackson et al 2014) driven by long-term changes in climate and disturbance regimes. Despite a century of pollen analysis (Edwards et al 2017), obtaining robust statistical estimates of the rates and patterns of past forest change remains a major research frontier
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