Clonal species reduce the biomass and species richness of non-clonal species in an experimental manipulation of resource heterogeneity: Data from 4 years of a field study

Background/Question/Methods

Spatial heterogeneity in soil resources is thought to promote the coexistence of plant species, with a critical factor being the scale of resource heterogeneity relative to plant size.  We hypothesized that clonal species, by integrating across patches, may experience patch sizes differently than non-clonal species and thus may reduce the effectiveness of small-scale heterogeneity to promote coexistence.  To examine the potential interaction between clonal species and resource heterogeneity, we manipulated the heterogeneity of soil fertility at two spatial scales, along with the presence/absence of clonal species, in a field experiment in a Michigan prairie savannah.  The experiment was initiated in 2004 when 34 native non-clonal species were seeded onto bare soil in four fertilizer treatments (control, uniform, small patch, large patch).  Propagules of clonal species were then added to half the plots and excluded from the other half, to produce communities with varying growth form diversity.  Slow release NPK fertilizer was added to the fertilized plots at four levels and two spatial scales in all four years, and we measured community response in above-ground biomass and species richness.

Results/Conclusions Clonal species biomass increased strongly in response to fertilization and reduced the biomass of non-clonal species in all treatments.  Clonal biomass responded more distinctly to the gradient in patch fertility when patch sizes were larger, as would be expected due to integration.  Non-clonal species biomass increased in fertilized plots only when clonal species were absent.  Moreover, the biomass response of non-clonals was less than that of clonal species.  By the fourth year, the pattern of non-clonal species richness to the fertilizer treatments varied between community types.  The presence of clonal species had a significant, negative effect on non-clonal species richness in all treatments and non-clonal species richness was lowest in the small patchy and uniformly fertilized treatments.  Soil nitrogen levels increased with fertilization and differed between the heterogeneity treatments, but the response in soil N was more complex than predicted based on simple nutrient translocation by clonals. Thus, the mechanism by which clonal species reduce the richness of non-clonal species may involve multiple factors.  Clonal species are common components of many plant communities and understanding their response to changes in fertility will be important for determining the role of resource heterogeneity in promoting coexistence.