An approach to modelling the impact of prehistoric farming on Holocene landscape phosphorus dynamics

The lake sediment phosphorus (P) record at Hatch Mere (Cheshire, UK) is investigated to assess the role of human activity in modifying Holocene landscape P export dynamics, and to develop an approach to incorporating this effect into a pre-existing long-term, large-scale landscape model of natural P export. Analysis of the lake sediment record shows that the catchment P yield is low and constant prior to ca. 6000 BP, but then increases up to the present day. This increase occurs in steps that coincide with the Neolithic, Bronze Age and Medieval periods, and the first half of the 20th century, consistent with an anthropogenic cause. Such an interpretation is supported by close correlation of the P export flux with the estimated regional population density. We demonstrate that the effect of human activity on landscape P dynamics can be incorporated into the existing P export model by scaling the soil secondary P leakage rate coefficient to population density. The findings of this study suggest that lake sediment P accumulation rate data may provide a novel method for estimating prehistoric local population density. Additionally, we show that the pre-Neolithic landscape P export was low, and we reject the widely held view that Hatch Mere is naturally eutrophic.</p

Novel and Lost Forests in the Upper Midwestern United States, from New Estimates of Settlement-Era Composition, Stem Density, and Biomass

<div><p>Background</p><p>EuroAmerican land-use and its legacies have transformed forest structure and composition across the United States (US). More accurate reconstructions of historical states are critical to understanding the processes governing past, current, and future forest dynamics. Here we present new gridded (8x8km) reconstructions of pre-settlement (1800s) forest composition and structure from the upper Midwestern US (Minnesota, Wisconsin, and most of Michigan), using 19th Century Public Land Survey System (PLSS), with estimates of relative composition, above-ground biomass, stem density, and basal area for 28 tree types. This mapping is more robust than past efforts, using spatially varying correction factors to accommodate sampling design, azimuthal censoring, and biases in tree selection.</p><p>Changes in Forest Structure</p><p>We compare pre-settlement to modern forests using US Forest Service Forest Inventory and Analysis (FIA) data to show the prevalence of lost forests (pre-settlement forests with no current analog), and novel forests (modern forests with no past analogs). Differences between pre-settlement and modern forests are spatially structured owing to differences in land-use impacts and accompanying ecological responses. Modern forests are more homogeneous, and ecotonal gradients are more diffuse today than in the past. Novel forest assemblages represent 28% of all FIA cells, and 28% of pre-settlement forests no longer exist in a modern context. Lost forests include tamarack forests in northeastern Minnesota, hemlock and cedar dominated forests in north-central Wisconsin and along the Upper Peninsula of Michigan, and elm, oak, basswood and ironwood forests along the forest-prairie boundary in south central Minnesota and eastern Wisconsin. Novel FIA forest assemblages are distributed evenly across the region, but novelty shows a strong relationship to spatial distance from remnant forests in the upper Midwest, with novelty predicted at between 20 to 60km from remnants, depending on historical forest type. The spatial relationships between remnant and novel forests, shifts in ecotone structure and the loss of historic forest types point to significant challenges for land managers if landscape restoration is a priority. The spatial signals of novelty and ecological change also point to potential challenges in using modern spatial distributions of species and communities and their relationship to underlying geophysical and climatic attributes in understanding potential responses to changing climate. The signal of human settlement on modern forests is broad, spatially varying and acts to homogenize modern forests relative to their historic counterparts, with significant implications for future management.</p></div