Smaller species experience mild adversity under shading in an old-field plant community

Background: Plant competition experiments commonly suggest that larger
species have an advantage, primarily in terms of light acquisition.
However, within crowded natural vegetation, where competition evidently
impacts fitness, most resident species are relatively small. It remains
unclear, therefore, whether the size-advantage observed in controlled
experiments is normally realized in habitats where competition is most
intense. Methods: We characterized the light environment and tested for
evidence of a size-advantage in competition for light in an old-field
plant community composed of herbaceous perennial species. We investigated
whether larger species contributed to reduced light penetration (i.e.,
greater shading), and examined the impact of shade on smaller species by
testing whether the abundance and richness of smaller species were lower
in plots with less light penetration. Results: Light penetration in plots
ranged from 0.3-72.4%. Plots with a greater mean species height had
significantly lower light penetration. Plots with lower light penetration
had significantly lower small species abundance and richness. However, the
impact of shade on small species abundance and richness was relatively
small (R2 values between 8% and 15%) and depended on how we defined “small
species”. Significant effects were more common when analyses focused on
small plant species that reached reproduction; focusing on only flowering
plants can clarify patterns. Synthesis: Our results confirm that light
penetration in herbaceous vegetation can be comparable to levels seen in
forests, that plots with taller species cast more shade, and that smaller
species are less abundant and diverse in plots where light penetration is
low. However, variation in mean plot height explained less than 10% of
variation in light penetration, and light penetration explained between
5-15% of variation in small species abundance and richness. Coupled with
the fact that reproductive small species were present even within the most
heavily shaded plots, our results suggest that any advantage in light
competition by large species is limited. One explanation is that at least
some small species in these communities are shade tolerant. Shade
tolerance in predominantly herbaceous communities, particularly among
small plant species, requires further research.