Increased environmental heterogeneity is expected to enhance species diversity within communities and this pattern is confirmed by numerous observational studies. For plants, this spatial heterogeneity is likely driven by differences in soil substrate and by changes in soil conditions that occur as different species colonize and alter soils. Currently, few field experiments have attempted to tease apart the relative importance of these two sources of heterogeneity to plant community structure. Using a field experiment, we tested how the spatial pattern of seed arrival (aggregated versus uniform seed mixtures) and soil heterogeneity influence species richness in grassland plant communities in south-central Kansas. We established 96, 4x4.8 m plots each divided into 120 large-scale (0.4x0.4 m) or 480 small-scale (0.2x0.2 m) patches. We then excavated, mixed, and redistributed soil within each plot to create either homogenous or heterogeneous soil conditions. Finally, seeds from 40 species were then sown with either one species per soil patch, to create aggregated species distributions within plots, or were sown uniformly across plots.
Results/Conclusions
After three growing seasons, soil heterogeneity had no effect on species richness or evenness at the plot scale; however, plots with aggregated sowing had higher overall richness compared to uniform seeding. This difference was driven, not only by the establishment of the 40 sown species, but also by an increase in unsown species that colonized from the seedbank or dispersal from outside the plots. This effect was also stronger in plots comprised of large rather than small patches. In addition to richness, species identity and abundance was also influenced by aggregated sowing patterns and patch scale, but not soil heterogeneity. These results suggest that seed arrival may be more important than soil heterogeneity for controlling local plant diversity and community structure at least during the first three years of community assembly.