The role of intraspecific functional trait variation in determining the differential decline of meadow species following conifer encroachment

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In the Oregon Cascade Range, conifer encroachment has reduced the extent of mountain meadows by as much as 50% since the mid-1940s. A previous study of encroachment-driven species decline in the Oregon Cascades demonstrated that meadow species differ in their sensitivities to tree influence: some species show rapid declines soon after tree establishment and others persist in the understory for many decades. Here, we explore whether this variability among species can be explained by intraspecific variation in the expression of morphological traits that relate to the capture of light, the resource assumed to be most limiting in the understory. Traits include specific leaf area (SLA; leaf area;mass), allocation of biomass to above- vs. below-ground structures, maximum shoot height, and clonality. We hypothesized that sensitivity to encroachment is greater in non-clonal species and those that show less intraspecific variation in trait expression across the light gradient. From a large set of meadow species present at Bunchgrass Ridge, OR, we chose 13 (10 forbs and three grasses) that varied in their rates of decline across a chronosequence of invasion states. For each species we estimated cover and light availability across the encroachment gradient, modeled the relationship between cover and light, and computed sensitivity to encroachment (SEI) as the coefficient of variation (CV) of predicted cover across a range of light values. We then measured the functional traits listed above for 15-17 mature individuals of each species (N = 216) in locations representing the range of light environments. For each species, trait variability was expressed as the slope of the linear trait-light relationship. Clonality was treated as an ordinal variable with classes representing increasing potential for lateral spread: non-clonal, limited clonality, and strong clonality. To test our hypothesis, we computed for each trait, the correlation between SEI and trait variability among species. Clonality and intraspecific trait variability explained little variation in SEI. Although SLA increased in the shade for all meadow species (an adaptive response to shade) the magnitude of response (slope of SLA-light relationship) did not correlate with sensitivity to encroachment (r = 0.24, p = 0.46). Analyses of the components of SLA revealed a significant correlation with SEI for leaf area, but not leaf mass. Leaf area increased in the shade for less sensitive species, but declined in the shade for more sensitive species. The adaptive significance of this pattern is not clear given that leaf area and plant cover (upon which SEI is based) likely co-vary. Sensitivity to conifer encroachment may relate more to variation in the physiological traits of species (e.g., adjustment of photosynthetic systems) or in the ability of species to respond to changes in resources or limiting factors other than light.