This article examines the ecological features of one of most important tropical ecosystems, the savannas. Savannas feature the coexistence of both trees and herbaceous plants and are distinct from grasslands (absence of woody plants) and closed forests (tree dominant). Savanna ecosystems occur in over 20 countries, largely in the seasonal tropics. Much of the world's livestock occurs in savanna, underlining their social and economic importance. Approximately 20% of the world's land surface is covered with savanna vegetation, which produces almost 30% of global net primary production (NPP). With tree and herbaceous components, savanna biodiversity is high, often higher than associated dry deciduous forests. Globally, tenure of savanna lands incorporates pastoral, private use, indigenous and national parks, with the disparate management aims of grazing, mining, tourism, subsistence livelihoods, and conservation. Given their size, savannas affect global carbon, nutrient and water cycles, and, with their frequent fires, significantly influence atmospheric chemistry. Savanna ecosystems have existed for millions of years in many regions, although paradoxically, many ecologists regard savannas as an ecologically unstable mixture of trees and grasses. Savanna boundaries are dynamic in space and time and their occurrence and structure are determined by a combination of environmental factors, such as available water, nutrients, the frequency of disturbances (e.g., fire and herbivory), and stochastic weather events. This range of factors results in significant structural variation and providing an overarching and strict definition of what constitutes a savanna has been problematic. This article provides a commonly used definition, describes savanna distribution, and examines factors that influence their structure and function. Understanding the determinants of savanna functioning, resilience and stability are vital ingredients for improved management. Management of savannas is especially important, as they are under increasing development pressure, especially in tropical regions, and threats to their long-term sustainability are examined.

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