The Role of Intraspecific Functional Trait Variation in the Differential Decline of Meadow Species Following Conifer Encroachment

In the Oregon Cascade Range, conifer encroachment has reduced the extent of mountain
meadows by as much as 50% since the mid-1940s. Although encroachment results in a general
decline of meadow species abundance and diversity, species differ in their sensitivities to
encroachment: some show rapid declines whereas others persist in the understory for many
decades. Here, we explore whether this variability can be explained by intraspecific variation in
morphological traits associated with the capture of light, the resource assumed to be most
limiting in the understory. These include specific leaf area (SLA; ratio of leaf area to 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
in those that show less variation in trait expression across the light gradient. From a larger set of
meadow species at Bunchgrass Ridge, Oregon, we chose 13 (10 forbs and three grasses) that
varied in their rates of decline across a chronosequence of encroachment states. For each species
we estimated cover and light availability at multiple locations representing the encroachment
gradient (open meadow to >100-year-old forest). We modeled the relationship between cover
and light and computed an index of sensitivity to encroachment, SEI, as the coefficient of variation (CV) of predicted cover across the range of light values. We then measured the
morphological traits of 15-17 mature individuals of each species in locations representing the
range of light environments. For each species, variation in a trait (trait variability) was expressed
by the linear slope of the relationship with light. Clonality was treated as an ordinal variable with
one of three value, 0 (non-clonal), 0.5 (limited clonality), or 1 (strongly clonal). To test the
hypothesized relationship between sensitivity to encroachment and trait variability, we computed
for each trait, the correlation between SEI and trait variability (n = 13).
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.