1. In relatively fertile ecosystems, such as the tallgrass prairie, local species diversity is largely controlled by the competitive effects of community dominants. Despite the relative importance of soil resources in shaping competitive outcomes, we have a limited understanding of the ways by which plants partition below-ground space and resources while competing, and thus, how these interactions feedback to affect local diversity.
2. We experimentally tested whether potential rooting depth affected plant species diversity and composition by seeding 36 tallgrass prairie species into replicated, bare-ground plots in which soil depth was manipulated to produce shallow- (20 cm), medium- (42 cm) and deep-soil treatments all within one soil type. Because root architecture and foraging strategies differ among species, we hypothesized that soil depth alone could affect plant richness, diversity and community composition.
3. After 3 years, richness (S) significantly increased with soil depth (P < 0.0001), but there was no significant change in species diversity (P > 0.1) or composition (multi-response permutation procedure, P > 0.2). The lack of a depth effect upon diversity resulted from the opposing effect of increasing soil depth enhancing S, but decreasing evenness.
4. Species presence among depth treatments was strongly nested, with species found in shallow soils reflecting a subset of the species found in the medium-depth treatment, and the species found within the medium-depth treatment reflecting a subset of those found in the deepest soils.
5. All depth treatments contained the same dominant grasses, thus differences in S resulted from the nested loss of forbs. Conversely, increasing soil depth added sets of new species, but the specific identity of the species present appeared interchangeable among replicates of a given depth.
6. Synthesis. Our results provide the first field-based experimental evidence that altering soil depth alters species occurrence and diversity in predictable ways in seeded tallgrass prairie. Our results have important theoretical implications for understanding the processes promoting plant co-occurrence in grasslands, and generate testable hypotheses concerning the conditions under which root-niche partitioning is probably important for maintaining local richness in grasslands. Future work is needed to elucidate the generality and mechanistic basis of our results.