Abstract: Experimental evidence suggests that light competition is size-asymmetric, with taller plants expected to capture more incoming light while also contributing to the competitive exclusion of shorter plants. However, shorter plants, which often represent the most shaded individuals within natural vegetation, remain abundant and ubiquitous across community datasets, and are able to reach reproduction under heavy shading. Here, we considered the possibility that the size-advantage predicted in light competition by experimental studies may not be realized in natural vegetation, perhaps indicating some benefit to shade adaptation in reducing the potential for competitive exclusion or other impacts associated with high light stress (e.g., cell damage). Although herbaceous plants are considered shade-tolerant within forests, it is commonly assumed that species in herbaceous vegetation (e.g., grasslands, old fields) are shade-avoidant. However, this assumption lacks robust evidence, and shade conditions can be comparable between these vegetation types. The goal of this work was to evaluate the incidence of shade tolerance adaptation, and associated traits (e.g., large leaf area aids in light capture, and low chlorophyll a:b ratio indicates greater exploitation of the light spectrum), for resident species within naturally occurring perennial, herbaceous vegetation. Specifically, we tested the hypothesis that short maximum height is predictive of trait values consistent with shade tolerance (i.e., “shade tolerance potential”). To assess a broad scope of plant performance, we measured 17 morphological (e.g., leaf area, stomatal density) and physiological (e.g., leaf chlorophyll and nitrogen contents) traits relevant to shade tolerance from 10 individuals for each of 81 herbaceous species within a natural old field community. As traits consistent with shade tolerance are considered continuous rather than absolute measures (i.e., relatively “higher” or “lower” than other plants), we fitted linear regressions for each trait with maximum height, light availability, and soil moisture as predictor variables. Neutral trait correlations would have indicated a lack of shade tolerance potential within herbaceous vegetation; however, contrary to this common assumption, we found that 12 traits formed correlations consistent with shade tolerance, and that light availability was the best predictor variable for these trends. In conclusion, our results provide evidence of shade tolerance potential within natural herbaceous vegetation, and although short maximum height was not predictive of shade tolerance, herbaceous plants of various heights may still benefit from shade adaptation.
