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


Balfour, Kelly et al. (2023), Smaller species experience mild adversity under shading in an old-field plant community, Dryad, Dataset,


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.


See manuscript for full details.

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Natural Sciences and Engineering Research Council of Canada