The relative experimental effects of soil heterogeneity, aggregated seed arrival, and patch size on plant community assembly

Esben Kjaer and Gregory Houseman, Biological Sciences, Wichita State University, Wichita, KS, Bryan L. Foster, Department of Ecology and Evolutionary Biology and Kansas Biological Survey, University of Kansas, Lawrence, KS, Lauri Laanisto, Institute of Agricultural and Environmental Sciences, Estonian University of Life Sciences, Tartu, Estonia, Antonio J. Golubski, Biological Sciences, Delaware State University, Dover, DE

Background/Question/Methods
Species diversity is expected to increase with environmental heterogeneity, and this pattern has been confirmed by many different 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 and the invasibility of the community. Additionally, the effects of spatial heterogeneity on diversity may be scale-dependent with smaller habitat patches generally being more diverse than large patches. Currently, few field experiments have attempted to tease apart the relative importance of these different sources of heterogeneity to plant community diversity. Using a field experiment, we tested how the spatial pattern of seed arrival (aggregated versus uniform seed mixtures), soil heterogeneity, and patch size 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 four growing seasons, species richness was influenced by interactive effects of seed arrival, patch size, and soil heterogeneity. Species richness was higher when seed arrival of each species was spatially aggregated rather than in uniform mixtures particularly when the patch scale was large. However, in spatially uniform seed arrival plots, richness was higher when soils were homogeneous compared with heterogeneous soils. These differences were primarily driven by the arrival of unsown species from the surrounding plant communities or the seed bank, which were more weedy than those utilized in the seeding treatments. These results suggest that the spatial structure of seed arrival and size of the seed patches may have stronger effects on species richness than soil heterogeneity. Additionally, the spatial structure of seed arrival may have important effects on community assembly.