Shoot, but not root, competition reduces community diversity in experimental mesocosms

1. We used a mesocosm experiment to determine the how both the intensity and relative importance of root and shoot competition, and their interactions, influence the structure of plant communities. Experimental communities of nine grassland plant species were planted under different levels of soil fertility. Root, shoot, and total competition intensity and relative importance, along with plant community structure (evenness) were measured in each mesocosm. Structural equation modelling was used to separate the direct and indirect influences of root, shoot and total competition on species evenness. 2. Increasing shoot competition intensity and importance reduced plant community evenness, while increases in root competition intensity and importance had no direct effects on evenness. Root competition intensity and importance indirectly reduced plant community evenness because increased root competition intensity increased shoot competition intensity. 3. We propose that the separate influences of shoot competition and root-shoot competition interactions on community structure can be integrated through a new state variable: the total size-asymmetry of competition. We propose that there is a direct relationship between the overall size-asymmetry of competition and the influence of competition on plant community structure. 4. Synthesis. This study provides the first experimental evidence that, even though there are no direct links between the intensity or relative importance of root competition and plant community structure, root competition may indirectly structure communities through root-shoot competition interactions. Integrating root and shoot competition as competitive size-asymmetry can explain why intense root competition can be unimportant for community structure, while relatively less intense shoot competition and root-shoot competition interactions can have strong effects on plant community composition and diversity.