Unimodal relationship between three-dimensional soil heterogeneity and plant species diversity in experimental mesocosms

Abstract

Soil heterogeneity is a primary mechanism explaining plant species diversity. Yet, controlled experiments yield inconsistent soil heterogeneity-diversity (SHD) relationships, ranging from positive, neutral to negative. Here we investigated the SHD relationship by experimentally alternating nutrient-rich and nutrient-poor substrate in three dimensions, creating four levels of soil configurational heterogeneity (cell sizes 0, 12, 24 and 48 cm). Across each mesocosm, a mixture of species with high and low nitrogen requirements was evenly sown. Contrary to earlier experimental findings, this approach yielded a unimodal SHD relationship, peaking at cell size 12 cm. This pattern originated mainly from increased plant diversity of species with high nitrogen requirement. Diversity increases with configurational heterogeneity were not due to greater variation in light niches, and diversity decreases were not due to success of fast growing species. Strikingly, plant density increased monotonically with increasing configurational heterogeneity, indicating that not only more species but also more individuals could coexist. This study provides experimental evidence for unimodal SHD curves in plant communities, which has hitherto only been predicted by models. Our results carry a striking similarity with other unimodal response patterns of plant species diversity, notably in diversity–disturbance and diversity–productivity relationships.