Vegetation responses to conifer encroachment in a western Cascade meadow: a chronosequence approach

Abstract

Conifer invasion of mountain meadows is pervasive in western North America, but its consequences for plant composition and diversity have not been studied. We used a chronosequence approach to reconstruct temporal patterns of vegetation change in response to ~200 years of conifer encroachment of a dry, montane meadow in the western Cascade Range, Oregon. Tree age distributions, forest structure, light, and vegetation composition were determined for three hundred and fifty-six 10 m × 10 m samples representing a gradient from open meadow to old forest (>90 years). Nonmetric multidimensional scaling revealed strong turnover in species composition from open meadow to old forest. Ordination axes were highly correlated with light availability and density/basal area of encroaching grand fir, Abies grandis (Dougl. ex D. Don) Lindl., and lodgepole pine, Pinus contorta Dougl. ex Loud. Cover of meadow species declined steeply with establishment of A. grandis ; richness declined more gradually and extirpations of meadow species were rare. Forest herbs colonized within two decades of tree establishment and within 60–80 years dominated the understory flora. In contrast with meadow species, changes in richness and cover of forest herbs were weakly related to changes in forest structure. Rapid replacement of meadow by forest species and long-term effects of trees on soils may limit potential for restoration of these unique habitats.