Perennial plant diversity matters to hydrologic functioning of semi-arid ecosystems

Despite the growing body of knowledge on relationships between diversity and ecosystem functioning, the role of diversity in the hydrologic functioning of ecosystems remains unknown. Morphological and physiological traits of plant species can have a substantial effect on water and sediment fluxes. Therefore, biodiversity and functional diversity can be expected to affect the plant community abilities to conserve water and soil in the system. However, these hypothesized relationships rarely have been explored. In this study, we examine the relationships between perennial plant diversity (species richness and functional diversity) and runoff and sediment yield in semiarid Mediterranean landscapes. According to the life-form and morphological traits that could affect rainfall partitioning, infiltration, and overland flow, the perennial plant species were grouped into five functional types: tussock grasses, sod-forming short grasses, chamaephytes, subshrubs and shrubs. We measured runoff and sediment yield in nine experimental field plots over a 4-year period. While perennial plant cover was similar between plots, vegetation species composition, functional diversity and plant density varied greatly. Perennial plant density was highly and positively correlated with plant functional diversity and species richness. We found an inverse linear relationship between plant functional diversity and runoff and sediment yields. This negative effect may be mediated by the spatial pattern of vegetation, which also drives runoff and erosion processes. Our results highlight the importance of diversity and spatial pattern as linked attributes of perennial plant communities for ecosystem hydrologic functioning.