Intraspecific trait variation and the differential decline of meadow species during conifer encroachment

Conifer encroachment has reduced the extent and habitat quality of mountain meadows throughout western North America. Past studies in the Cascade Range of Oregon reveal surprising variation in the pace at which meadow species are lost to encroachment. We hypothesized that this variation relates to intraspecific variability in plant functional traits that are adaptive as light levels decline during the transition from open meadow to closed forest. For 13 meadow species with varying sensitivities to encroachment, we compared how the rate of decline in abundance relates to intraspecific variation in three morphological traits (specific leaf area, shoot/root ratio, and shoot height) and to clonal ability (no, limited, or strong potential for vegetative spread). For each species, we computed an index of sensitivity to encroachment, ISE, the coefficient of variation in cover across the light gradient. For each trait of each species, we computed an index of variation, the linear slope of the relationship between trait values and available light. For most traits, the correlation between ISE and trait variation (or clonality) was weak. Although specific leaf area increased in the shade for all species, the magnitude of increase did not correlate with ISE. Only variability in leaf area was positively correlated with ISE, increasing in the shade for less sensitive species and decreasing for more sensitive species. Responses to encroachment may reflect differences in species' physiological rather than morphological variability, or species may be responding to changes in resources other than, or in addition to, light.