When is a ‘forest’ a savanna, and why does it matter?

Abstract

Savannas are defined based on vegetation structure, the central concept being a discontinuous tree cover in a continuous grass understorey. However, at the highrainfall end of the tropical savanna biome, where heavily wooded mesic savannas begin to structurally resemble forests, or where tropical forests are degraded such that they open out to structurally resemble savannas, vegetation structure alone may be inadequate to distinguish mesic savanna from forest. Additional knowledge of the functional differences between these ecosystems which contrast sharply in their evolutionary and ecological history is required. Specifically, we suggest that tropical mesic savannas are predominantly mixed tree-C4 grass systems defined by fire tolerance and shade intolerance of their species, while forests, from which C4 grasses are largely absent, have species that are mostly fire intolerant and shade tolerant. Using this framework, we identify a suite of morphological, physiological and life-history traits that are likely to differ between tropical mesic savanna and forest species. We suggest that these traits can be used to distinguish between these ecosystems and thereby aid their appropriate management and conservation. We also suggest that many areas in South Asia classified as tropical dry forests, but characterized by fire-resistant tree species in a C4 grass-dominated understorey, would be better classified as mesic savannas requiring fire and light to maintain the unique mix of species that characterize them.

The deforestation story: testing for anthropogenic origins of Africa's flammable grassy biomes

“…The problem of defining forest is a general one in tropical landscapes [54][55][56]. Ratnam et al [57] provided a functional definition for distinguishing forest from 'savanna' based not on the trees but on the presence of a highly flammable, shade-intolerant C4 grassy understory in savannas and its absence in forests. It is the presence of the grasses that is informative as to the feedbacks that maintain the system and not merely the biomass and basal area of trees.…”

Local-Scale Mapping of Biomass in Tropical Lowland Pine Savannas Using ALOS PALSAR

“…The woody component of savannas can be variable [2]; however, many woody savannas can be characterized as forests according to the FAO definition [3]. The woody component is of major significance for storing biomass [4,5], supporting biodiversity [6] and sustaining the local hydrological cycle [7].…”

Modelling Biome Shifts in the Indian Subcontinent under Scenarios of Future Climate Change

“…The 'tropical and subtropical grasslands, savannas and shrublands' biome, as classified by Olson et al 49 , currently only comprises the tall grass systems of the Gangetic plain at the Himalayan foothills. However, other savanna and mixed tree-grass formations (ecoregions) also occur within the more arid biomes in the country 69,70 . These systems harbour unique biodiversity and support critical populations of several open habitat species [71][72][73] , but are also amongst the most threatened habitats as a result of land-use change and tree-planting activities 74,75 .…”