Smaller species experience mild adversity under shading in an old-field
plant community
Abstract
Plant competition experiments commonly suggest that larger species have
an advantage, especially in 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
realized in habitats under intensive competition. We 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 their abundance and richness were lower in plots with
less light penetration. Light penetration in plots ranged from
0.3-72.4%. Plots with 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 individuals that reached reproduction; focusing
on only flowering plants can clarify patterns. 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 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 some small species in these communities are shade
tolerant.