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. 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 perennial herbaceous 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% to 72.4%. Significant effects were more common when analyses focused on small plants that reached reproduction (i.e., flowering rooted units); focusing on only flowering plants (i.e., excluding nonflowering rooted units) can clarify community patterns. Plots with a greater mean species height had significantly lower light penetration, and plots with lower light penetration had significantly lower flowering abundance and richness of small species. However, the impact of shade on the flowering abundance and richness of small species was relatively small (R2 values between 8% and 15%) and depended on how we defined “small species.” 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 flowering 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 and 15% of variation in the flowering abundance and richness of small species. Coupled with the fact that flowering 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.
We conducted this experiment from June to August 2006 within a 67.5 × 50 m old-field (known locally as the “Cemetery Field”) located at the Queen's University Biological Station, north of Kingston, Ontario, Canada (44° 31′ 17.9″ N, 76° 23′ 07.9″ W), and consisting primarily of perennial herbaceous species. For about 50 years preceding this work, this field was harvested for hay annually (excluding 2005 and 2006) but has not otherwise been disturbed. In July 2005, the field was divided into 486 plots, each 1 × 1 m, separated by 1 m laneways to minimize the impact of edge effects and disturbance during data collection (Piggott, 2007). Of these 486 pre-existing plots, 49 were randomly chosen for data collection in this study.