TREE-GRASS INTERACTIONS IN SAVANNAS

Savannas occur where trees and grasses interact to create a biome that is neither grassland nor forest. Woody and gramineous plants interact by many mechanisms, some negative (competition) and some positive (fa cilitation). The strength and sign of the interaction Varies in both t ime and space, allowing a rich array of possible outcomes but no unive rsal predictive model. Simple models of coexistence of trees and grass es, based on separation in rooting depth, are theoretically and experi mentally inadequate. Explanation of the widely observed increase in tr ee biomass following introduction of commercial ranching into savannas requires inclusion of interactions among browsers, grazers, and fires , and their effects on tree recruitment. Prediction of the consequence s of manipulating tree biomass through clearing further requires an un derstanding of how trees modify light, water, and nutrient environment s of grasses. Understanding the nature of coexistence between trees an d grass, which under other circumstances are mutually exclusive or une qual partners, yields theoretical insights and has practical implicati ons.