Intraspecifically aggregated seed arrival increases plant diversity in a low diversity grassland
Intraspecific aggregation is a commonly observed pattern in plant communities that may increase plant diversity by reducing the rate of competitive exclusion. Although recent experiments report some support for this idea during community assembly, it is unclear if the potential positive effect of intraspecific aggregation influences new species colonizing an area with strong competitors already established. I tested this idea by sowing native forbs in either aggregated or uniform spatial arrangements at three densities, which spanned three orders of magnitude, into a low diversity grassland in south-central Kansas, USA. Seventy, 3x4 m plots were laid out in a grid across the grassland with each sowing*density treatment replicated ten times with an additional ten plots included as unsown controls. The sown plots were divided into forty-eight, 0.5x0.5 m patches that received fifteen species either sown in monospecific patches resulting in intraspecifically aggregated plots or all species sown into each patch creating plots with a uniform spatial pattern. Within each density treatment, seed density was held constant at the plot scale but partitioned to create either uniform or aggregated spatial seeding patterns.
After six growing seasons, species richness was higher in aggregated than uniformly sown plots, but only at the highest seeding rates. Colonization of individual species varied between the uniform and aggregated sowing treatments. For the three most abundant sown species: 1) Desmanthus illinoensis (Illinois bundle-flower) had higher abundance in the uniform compared to aggregated treatments, 2) Amorpha canescens (lead plant) abundance was substantially higher in aggregated than uniform treatments, while 3) Lespedeza capitata (round-head bush clover) was insensitive to the spatial sowing pattern at all sowing densities. These results suggest that aggregated seed arrival can confer an advantage during colonization even when strong competitors (resident species) are already present. However, the results also suggest that intraspecific aggregation reduces competition among sown species or generates a self-facilitative effect. For example, A. canescens had low, but equivalent colonization at all sowing densities when seeds of all species were mixed (uniform plots), while colonization increased substantially with sowing density when seeds were intraspecifically aggregated.