The adaptive traits described above enable individual plant survival in seasonally variable climates, but what environmental factors operate at a landscape or regional scale that determine savanna structure.? Evidence suggests that four key environmental factors are responsible: (1) plant available moisture (PAM); (2) plant available nutrients (PANs); (3) fire regime; and (4) herbivory. Herbivores include vertebrates and invertebrates and consist of both browsers consuming woody biomass and grazers consuming grasses and herbs. The overarching determinants of savanna physiognomy (relative abundance of the tree and grass layer) are climate and soil type (PAM and PAN), which determines the potential growth and survival of trees and grasses at a given site. Growth potential is moderated by disturbance agents, fire, herbivory, and stochastic events (such as cyclones). These factors act in concert to influence both competitive interactions and facilitation of tree and grass growth and determine savanna structure, floristics, and productivity (Figure 3). The interaction of these factors is poorly understood and their variation in space and time makes experimental testing and isolation of any single determinant difficult. Spatial heterogeneity of vegetation due to local site histories (determined by antecedent rainfall, fire history, and herbivore numbers) and an inability to quantify these factors exacerbates this difficulty.
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