Seed aggregation increases plant diversity in grassland communities

Background/Question/Methods

Intraspecific spatial aggregation is a common pattern found in plant communities that may contribute to species coexistence if spatial clustering causes intraspecific competition to exceed interspecific competition.  One potential source of interspecific aggregation is seed dispersal, which is typically non-random, but it is unclear to what extent aggregated seed distributions affect plant diversity in real communities.  I tested whether aggregated seed dispersal alters community structure in comparison to uniform spatial seeding patterns and whether these effects vary with sowing density.  Sixty, 3x4 m plots were arrayed in a low-diversity grassland located in south-central Kansas.  Each plot was divided into forty-eight, 0.5x0.5m patches.  For the uniform seeding treatment, 15 species were added to all patches at equal seeding rates.  In the aggregated seeding treatment, each species was randomly assigned to three patches while the cumulative seeding rates for each species were held constant at the plot scale so that only the spatial seeding pattern differed between the two treatments.  The spatial seeding treatments were fully crossed with three seed density treatments that varied over two orders of magnitude.  After two growing seasons, plant density was quantified for all added species in 15 randomly selected patches from each plot.  

Results/Conclusions

Species evenness of the colonizing species was higher under aggregated than uniform seeding patterns.  Although species richness was unaffected by the spatial seeding pattern within a subset of patches within plots, estimated species richness at the plot scale was higher under aggregated than uniform sowing.  Effects of seed aggregation on colonizer diversity were only apparent at moderate to high seed densities, yet all the sowing densities in the experiment were below the maximum seed production reported for grasslands in the region.  As sowing density increased, seed mass became an increasingly effective predictor of relative abundances for colonizing species, but only under uniform sowing patterns supporting the idea that aggregated dispersal may buffer weaker (smaller seeded) species from competition during colonization.  This experiment shows that aggregated patterns of seed dispersal patterns can increase plant diversity and suggests that previous seed dispersal experiments, which utilize uniform seed sowing, may underestimate the potential effect of dispersal on plant community structure.