Ecosystem development and response to climatic change: A comparative study of forest-lake ecosystems on different substrates

Analysis of sediment chemistry, feldspar morphology, and fossil pollen from two lakes in the upper Midwest reveals long-term differences between two terrestrial ecosystems—one that developed on sandy outwash and the other on till. These differences are expressed in the trajectories of their development, their connection to aquatic ecosystems, and their response to climatic change. The geochemical record from a lake on sandy outwash provides strong evidence for soil development, showing base cation leaching, acidification, and podzolization in the catchment soils during the Holocene. In contrast, a site 10 km away on till has a strongly developed soil and a geochemical record that implies rapid podzolization but contains only a subtle record of changes in weathering intensity over the Holocene; nearly all weathering products have remained in the soil rather than moving to the lake. Analysis of feldspars in soils and sediment at the sites using scanning electron microscopy and energy dispersive spectroscopy supports the geochemical conclusion that weathering was slower at the outwash site than at the till site. The difference in soil water-holding capacity at the two sites has had a strong influence on the vegetation throughout the Holocene. Vegetation at the outwash site has had incomplete forest cover for much of its history, while the till site developed a closed-canopy forest earlier and has supported species with greater water requirements. Early and middle Holocene changes in soil and vegetation are synchronous within but not across sites, suggesting local interactions between soil and vegetation during ecosystem development. Weathering increased at both sites in the late Holocene, likely because of a change to a wetter climate. However, only the till site shows a corresponding vegetation change, indicating that large changes in soil properties can occur while species composition remains stable. This study suggests that the dynamic interaction between soil and vegetation may have been more important in ecosystem development than the direct effect of climate for much of the Holocene.