The influence of substrate on vegetation history and ecosystem development

Comparison of vegetation, soil, and climatic history at two sites differing in soil texture and thus soil water-holding capacity reveals long-term differences in terrestrial ecosystem development. Vegetation history reconstructed using pollen analysis and modern surface samples reveals that the site on till has hosted species with higher moisture requirements than those at the outwash site since the early Holocene. The outwash site had incomplete forest cover for much of the Holocene in contrast to the till site, which likely had a closed-canopy forest for most of its history. Comparison of vegetation and soil records reveals a correspondence between the timing of changes in vegetation and soil at both sites for most of the Holocene and provides evidence for greater weathering rates under coniferous than deciduous forest or savanna. Soil and vegetation changes are synchronous within, but not across, sites 13000-3000 yr BP, suggesting that local interactions between soil and vegetation may have been important in determining the timing of ecosystem change. In contrast, there was synchronous change in soil and vegetation across sites similar to3000 yr BP, when hemlock became established at the till site and weathering intensified at both sites. This synchrony suggests a regional driver of ecosystem change, most likely the climatic change to higher moisture availability that occurred in the late Holocene. Thus, both climate and interactions between soil and vegetation have influenced the timing of ecosystem change, and the relative importance of their influence has varied over time.