No effect of varying soil resource heterogeneity on plant species richness in a low fertility grassland

Abstract

Summary1 Coexistence theory predicts that greater heterogeneity of resources or other fitnessconstraining environmental factors will promote species diversity, yet this classic mechanism of coexistence has rarely been tested in manipulative field experiments. 2 Here we present results from the fourth year of a long-term experiment designed to test the heterogeneity-diversity hypothesis in a low productivity grassland where we directly manipulated the spatial heterogeneity of soil nutrients. In addition to unfertilized controls, we created heterogeneous and homogeneous nutrient enrichment treatments by applying fertilizer to field plots in a patchy or uniform manner. Fertilization treatments were crossed with seed addition and cover reduction treatments, to allow examination of the effects of recruitment limitation and competition from established vegetation on diversity responses to nutrient heterogeneity. 3 In contrast to predictions, we found that spatially heterogeneous soil nutrient supply did not promote increased species richness or greater species sorting relative to a uniform supply of soil nutrients. Instead, both nutrient enrichment treatments depressed plant species richness (especially at small spatial scales) and, depending on cover reduction and on spatial location in the field, increased the abundance of two spreading clonal species. 4 Our results do not support the heterogeneity-diversity hypothesis, and suggest that the response of clonal species to nutrient enrichment may constrain how plant diversity responds to resource heterogeneity. Future tests of the heterogeneity-diversity hypothesis should explicitly consider the scale of resource heterogeneity and foraging capability of the plant species in the community.

Vascular plant species richness on wetland remnants is determined by both area and habitat heterogeneity

“…In this work, not only single soil resource heterogeneity variables but also the integrated soil fertility heterogeneity showed non-significant correlations with species richness (Table 3). Similar results have also been reported by other ecologists (Stevens and Carson 2002;Wijesinghe et al 2005;Reynolds et al 2007). Reynolds et al (2007) attributed the absence of a significant correlation to the presence of clonal species, which dominated in the vegetation and decreased species richness by restraining other species' growth.…”

“…Therefore, soil resource heterogeneity was considered as a strong driver of plant species coexistence and community diversity (Tilman 1988). Currently, studies on the effect of soil resource heterogeneity on plant species have reported conflicting results: some studies showed a positive effect (e.g., Davies et al 2005;Marchand and Houle 2006;Zhou et al 2008), and others found no significant effect (e.g., Stevens and Carson 2002;Wijesinghe et al 2005;Reynolds et al 2007) or even negative effect (Kleb and Wilson 1997). These inconsistent patterns could be the result of different processes operating at different spatial scales (Davies et al 2005).…”

“…The coefficient of variation (CV) is usually employed as a measure of heterogeneity (e.g., Marchand and Houle 2006;Reynolds et al 2007;Zhou et al 2008). Using all quadrats' values in each site, the coefficient of variation (CV) for water condition and soil quality factors was calculated, and we used them to denote the water heterogeneity and soil resource heterogeneity respectively (see Table 1).…”

“…Variation in soil nutrients might be an especially important type of heterogeneity in plant communities, and different plant species are expected to sort into patches of differing soil fertility (Reynolds et al 2007). Therefore, soil resource heterogeneity was considered as a strong driver of plant species coexistence and community diversity (Tilman 1988).…”

Spatial variation in seed limitation of plant species richness and population sizes in floodplain tallgrass prairie

“…Spatial heterogeneity in resource availability is a critical mechanism of species coexistence in conceptual models of plant community structure (Tilman 1988, Tilman andPacala 1993), yet resource heterogeneity's importance in explaining variation in community composition in nature rarely has been evaluated (but see Stevens and Carson 2002, Bakker et al 2003, Reynolds et al 2007). In theory, spatial heterogeneity in resource levels can enhance plant species richness by providing opportunities for niche differentiation, thereby allowing a higher proportion of colonizing species to avoid competitive exclusion.…”