Self-thinning and tree competition in savannas

This paper examines the feasibility of applying self�thinning concepts to savannas and how competition with herbaceous vegetation may modify self�thinning patterns among woody plants in these ecosystems. Competition among woody plants has seldom been invoked as a major explanation for the persistence of herbaceous vegetation in mixed tree⿿grass ecosystems. On the contrary, the primary resourcebased explanations for tree⿿grass coexistence are based on tree⿿grass competition (niche�separation) that assumes that trees are inferior competitors unless deeper rooting depths provide them exclusive access to water. Alternative nonresource�based hypotheses postulate that trees are the better competitors, but that tree populations are suppressed by mortality related to fire, herbivores, and other disturbances. If self�thinning of woody plants can be detected in savannas, stronger evidence for resource�limitation and competitive interactions among woody plants would suggest that the primary models of savannas need to be adjusted. We present data from savanna sites in South Africa to suggest that self�thinning among woody plants can be detected in low�disturbance situations, while also showing signs that juvenile trees, more so than adults, are suppressed when growing with herbaceous vegetation in these ecosystems. This finding we suggest is evidence for size�asymmetric competition in savannas.