Initial endogenous spatial structure alters plant establishment and community composition of tallgrass prairie restorations
A common outcome of tallgrass prairie restoration is that they fail to reestablish pre-disturbance plant diversity. Existing theoretical work and empirical studies suggest that endogenous spatial structure in plant communities can allow coexistence of species that would otherwise undergo competitive exclusion. One goal of our study is to determine if spatially explicit planting strategies can be used to enhance the establishment and diversity of plant species in prairie restorations. To do this, we established a long-term experiment with replicate restoration plots sown to species in equal abundances of seed. Experimental manipulations varied only the initial spatial distribution of species across patches within each restoration plot. Sowing treatments ranged from no spatial structure/uniform (where seed mix is homogenized; the most common method) to highly structured monospecific species aggregates; where each species is initially segregated from all other species. Additionally, three treatments were included that represent intermediate levels of spatial structure. These three treatments aggregate four-species mixtures into local patches based on functional group classifications to vary the amount of functional complementarity in patches of interacting species and determine how the interactions between major groups of prairie plants affects establishment and diversity.
In the third year of the experiment we found no treatment effects on species richness (S), Shannon diversity (H’) or evenness (H’/lnS) at the scale of whole restoration plots, nor did we find effects on the proportional representation of each plant functional group. However, there were strong treatment effects on species composition and total plant cover. In the uniformly planted treatment, three out of the four plant functional groups have significantly higher cover than the highly aggregated treatment resulting in significantly higher overall cover (p< 0.05, p<0.05). Additionally, spatial treatments significantly affect plant community composition. The uniformly planted treatment strongly favors the abundance of grasses whereas aggregated treatments discouraged the establishment of grasses. This suggests that spatially aggregating prairie species can alter their relative abundances in restorations in favor of prairie forbs. Significant differences in cover values and relative species abundances suggest that initial endogenous spatial structure can alter species establishment and community structure.