The influence of soil heterogeneity on plant species richness

Abstract

Environmental heterogeneity is often cited as one of the driving mechanisms behind community species composition and diversity. However, its contribution to species composition in plant communities remains unclear because few experiments demonstrate a causal link between environmental heterogeneity and plant diversity. This is particularly true for soil manipulations despite that fact that soil is expected to provide the key resources necessary for plant growth. Here, I utilize a unique manipulation of the soil profile to create communities with spatially heterogeneous and homogeneous soil arrangements and examine the influence of soil heterogeneity on community structure, through species composition and flowering patterns, during community assembly. I employed an annual census of the assembling communities and recorded the identity and density of all species within the patches. After two years, I found that species richness was significantly higher in heterogeneous than in homogeneous plots. In the heterogeneous plots, thirteen species had higher greater establishment rates in a specific patch type representing one of the three soil strata. However, no species had greater association with the mixed stratum, comprising the homogeneous plots, than one of the heterogeneous strata. This pattern of species sorting between soil types suggests that the increased richness in heterogeneous plots is due to the increased variety of soil types comprising those communities. Alternatively, species richness in homogeneous plots, where species did not sort to distinct soils, was strongly associated with total plant density. This experiment is one of the first to provide clear, experimental evidence that fine-scale soil heterogeneity increases species richness through species sorting during community assembly.