Quantifying trends and uncertainty in prehistoric forest composition in the upper Midwestern United States

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.

Assessing the environmental and dispersal controls onFagus grandifoliadistributions in the Great Lakes region

“…These models include multiple sampling time points to understand the controls on species range dynamics over space and time (Schurr et al., ). Such data could also be augmented by palaeoecological data for earlier time periods (Bennett, ; Davis et al., ; Wang et al., ) and with work currently underway to build new pollen‐vegetation models to better quantify past changes in forest composition (Dawson et al., in revision). A next step forward is to more fully integrate the historical datasets analysed here with both contemporary data on tree distributions, genetic evidence, and palaeoecological observations data in order to understand the environmental and ecological controls on species distributions and dynamics.…”

Using paleo-archives to safeguard biodiversity under climate change

“…Studies of tree populations, for example, reveal the importance of non-stationary Holocene climate variability and its interactions with long-distance dispersal, local demographic processes, and species life-history traits(56)(57)(58)(59). Moreover, Holocene records show population expansions and declines are not necessarily accompanied by changes in geographic distribution(60,61), as is often assumed in conservation assessments(62). For example, a rapid population increase of eastern hemlock (Tsuga canadensis) near its western range margin was not accompanied by geographic expansion(60), while its geographic distribution held steady during a dramatic range-wide population decline(61).…”

“…Moreover, Holocene records show population expansions and declines are not necessarily accompanied by changes in geographic distribution(60,61), as is often assumed in conservation assessments(62). For example, a rapid population increase of eastern hemlock (Tsuga canadensis) near its western range margin was not accompanied by geographic expansion(60), while its geographic distribution held steady during a dramatic range-wide population decline(61). The latter example represents a rapid ecosystem transformation, whereby a dominant conifer (hemlock) was replaced by deciduous trees and pines, forming forests with different structural and functional properties, in response to a contingent series of climatic and ecological processes operating at different temporal and spatial scales(61,63).…”

Comparison of settlement-era vegetation reconstructions for STEPPS and REVEALS pollen–vegetation models in the northeastern United States

“…Such reconstructions enable the quantitative study of the biotic and abiotic drivers of forest dynamics at centennial to multimillennial time scales (Schoonmaker and Foster, 1991). As ecological forecasting emerges as a distinct field of inquiry (Clark et al, 2001; Dietze, 2017; Dietze et al, 2018), long-term vegetation reconstructions, accompanied by well-grounded estimates of uncertainty, offer empirical constraints on terrestrial ecosystem models of vegetation dynamics and the biogeochemical and biogeophysical interactions between the atmosphere and terrestrial biosphere (Bonan, 2008; Dawson et al, 2019).…”