Linking root traits and competitive success in grassland species

Competition is an important force shaping plant communities. Here we test the hypothesis that high overall root length density and selective root placement in nutrient patches, as two alternative strategies, confer competitive advantage in species mixtures. We performed a full-factorial pairwise competition experiment with eight grassland species in soil with homogeneously distributed nutrients, or with nutrients concentrated in a single patch. We measured species-specific relative growth rate, root length density, selective root placement, and ion uptake rates of all species in monocultures and in mixtures. Grasses showed higher specific root length overall and forbs a higher selective root placement in the nutrient patch. However, relative growth rate and root length density were more strongly related to competitive ability (measured as relative yield per plant), with little distinction between grasses and forbs. Our results suggest that short-term competitive success was related to fast growth and high root densities, irrespective of nutrient heterogeneity. Developing a large root mass quickly may overwhelm the importance of other traits in the establishment phase of plants, although these other traits may prove to be important in the long run.