Landscape context and long-term tree influences shape the dynamics of forest-meadow ecotones in mountain ecosystems

Abstract

Forest-meadow ecotones are prominent and dynamic features of mountain ecosystems. Understanding how vegetation changes are shaped by long-term interactions with trees and are mediated by the physical environment is critical to predicting future trends in biological diversity across these landscapes. We examined 26 yr of vegetation change (1983–2009) across 20 forest-meadow ecotones spanning a range of landforms/hydrologies and elevations (montane and subalpine) in the Three Sisters Biosphere Reserve, Oregon (USA). We quantified changes in tree structure (cover, density, and basal area) and in the abundance and diversity of ground-layer vegetation based on species' habitat associations and growth forms. To explore the contributions of tree structure, landscape context, and initial vegetation to changes in ecotonal communities, we used a combination of NMDS, PCA, and multiple regression. Despite a long history (50–100 yr) of tree invasion, ecotones were still dominated by meadow species in 1983. Ecotones exhibited significant but varying patterns of change over the study period while adjacent forest and meadow habitats remained stable. Despite a significant increase in summer temperature, we found little evidence of a direct influence of climate on ecotonal changes. Declines in total richness, and in the cover and richness of meadow species, were greater where soil moisture was seasonally limiting (montane mesic slopes and subalpine early snowmelt sites). Forest species showed much greater increases in montane than subalpine ecotones; limited colonization of the latter reflects the depauperate nature of subalpine forest understories in this region. Vegetation changes were related to initial tree structure but not to changes in structure over the study period. Past tree invasions, a legacy of both climate variation and disturbance history, continue to exert strong influences on ecotonal ground-layer communities. However, the consequences for local diversity vary across the landscape. Quantifying the nature of this variation through long-term observations is a critical step toward predicting future changes in the biological diversity of these and other mountain ecosystems.