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

Abstract

Reconstructions of prehistoric vegetation composition help establish natural baselines, variability, and trajectories of forest dynamics before and during the emergence of intensive anthropogenic land use. Pollen–vegetation models (PVMs) enable such reconstructions from fossil pollen assemblages using process-based representations of taxon-specific pollen production and dispersal. However, several PVMs and variants now exist, and the sensitivity of vegetation inferences to PVM selection, variant, and calibration domain is poorly understood. Here, we compare the reconstructions, parameter estimates, and structure of a Bayesian hierarchical PVM, STEPPS, both to observations and to REVEALS, a widely used PVM, for the pre–Euro-American settlement-era vegetation in the northeastern United States (NEUS). We also compare NEUS-based STEPPS parameter estimates to those for the upper midwestern United States (UMW). Both PVMs predict the observed macroscale patterns of vegetation composition in the NEUS; however, reconstructions of minor taxa are less accurate and predictions for some taxa differ between PVMs. These differences can be attributed to intermodel differences in structure and parameter estimates. Estimates of pollen productivity from STEPPS broadly agree with estimates produced for use in REVEALS, while comparison between pollen dispersal parameter estimates shows no significant relationship. STEPPS parameter estimates are similar between the UMW and NEUS, suggesting that STEPPS parameter estimates are transferable between floristically similar regions and scales.

Forest responses to last‐millennium hydroclimate variability are governed by spatial variations in ecosystem sensitivity

“…Pollen‐based inferences are based on statistical pollen‐vegetation models (PVMs) called STEPPS and ReFAB, and represent fractional vegetation composition and total woody biomass, respectively, for 12 tree genera that are common elements of upper Midwest forests. STEPPS is a Bayesian hierarchical spatio‐temporal model that infers fractional forest composition from networks of fossil pollen records (Paciorek and McLachlan, 2009; Dawson et al ., 2016, 2019b; Trachsel et al ., 2020b). STEPPS employs a process‐based representation of pollen dispersal and production, with taxon‐specific parameterisations.…”

“…Using this framework, STEPPS: (1) explicitly characterises uncertainty in data and processes, with posterior distributions of process parameters and state variables such as forest composition, and (2) borrows information across space and time, allowing for spatially comprehensive estimates of composition. For both the upper Midwestern USA (UMW; Minnesota, Wisconsin, Michigan) (Dawson et al 2019a) and the northeastern USA (NEUS) (Trachsel et al 2020a,b), STEPPS has been used to estimate centennially resolved forest composition for the late Holocene (250 B.C. to 1750 A.D) at a 24 km grid; here we use the results from 850 to 1750 AD.…”

More than one way to kill a spruce forest: The role of fire and climate in the late‐glacial termination of spruce woodlands across the southern Great Lakes

“…Macroscopic charcoal particles are not transported far from the source fire before they are deposited in lakes via slopewash or airborne fall‐out (Whitlock & Larsen, 2001). Thus, the pollen records represent larger source areas than the charcoal records, with source radii on the order of several hundred kilometres (Dawson et al., 2016; Trachsel et al., 2020) for Appleman, Silver and Bonnett Lakes (17–41 ha lake area) and tens of kilometres for Stotzel‐Leis and Triangle Lake Bog (0.5–1.1 ha). Macroscopic charcoal is used to calculate fire frequency, which is interpreted to represent local fire history (within 1–3 km of each lake), and background charcoal, which typically integrates over a broader domain (>10 km) (Higuera et al., 2010).…”

“…Macroscopic charcoal particles are not transported far from the source fire before they are deposited in lakes via slopewash or airborne fall-out (Whitlock & Larsen, 2001). Thus, the pollen records represent larger source areas than the charcoal records, with source radii on the order of several hundred kilometres (Dawson et al, 2016;Trachsel et al, 2020) for Appleman, Silver…”

The forests of the midwestern United States at Euro-American settlement: Spatial and physical structure based on contemporaneous survey data

“…Historical vegetation surveys from this time period, collected during the time of land surveys and allotment, provide critical context for understanding terrestrial ecosystems, the carbon cycle, and vegetation-atmosphere feedbacks [2,8,9]. These datasets allow researchers to define 'baseline' conditions for purposes of conservation planning [10], to understand ecosystem processes at decadal and centennial scales [11], to track how vegetation changes with changing climate [12,13], to understand changes in ecosystems after widespread land use change [2,14], and to calibrate paleoecological data [15,16]. The presettlement composition and vegetation types for the Midwestern states from Ohio to Minnesota have previously been comprehensively mapped, for example [17][18][19][20][21][22].…”