Many shades of green: the dynamic tropical forest–savannah transition zones

Abstract

The forest-savannah transition is the most widespread ecotone in tropical areas, separating two of the most productive terrestrial ecosystems. Here, we review current understanding of the factors that shape this transition, and how it may change under various drivers of local or global change. At broadest scales, the location of the transition is shaped by water availability, mediated strongly at local scales by fire regimes, herbivory pressure and spatial variation in soil properties. The frequently dynamic nature of this transition suggests that forest and savannah can exist as alternative stable states, maintained and separated by fire-grass feedbacks and tree shade-fire suppression feedback. However, this theory is still contested and the relative contributions of the main biotic and abiotic drivers and their interactions are yet not fully understood. These drivers interplay with a wide range of ecological processes and attributes at the global, continental, regional and local scales. The evolutionary history of the biotic and abiotic drivers and processes plays an important role in the current distributions of these transitions as well as in their species composition and ecosystem functioning. This ecotone can be sensitive to shifts in climate and other driving factors, but is also potentially stabilized by negative feedback processes. There is abundant evidence that these transitions are shifting under contemporary global and local changes, but the direction of shift varies according to region. However, it still remains uncertain how these transitions will respond to rapid and multi-faceted ongoing current changes, and how increasing human influence will interact with these shifts.This article is part of the themed issue 'Tropical grassy biomes: linking ecology, human use and conservation'.

Aridity and hominin environments

“…To understand the significance of aridity in shaping hominin environments in eastern Africa, we further consider the relationship between climate and ecology in modern African ecosystems. Vegetation in Africa is shaped by complex interactions between multiple abiotic (e.g., rainfall amount and seasonality, fire, atmospheric pCO 2 ) and biotic (e.g., herbivory) factors, and the relative importance of these factors is contingent on the ecological history of each area (52)(53)(54). Although woody cover is constrained by aridity (55), vegetation does not respond in a direct or continuous manner to changes in annual rainfall, and each biome (e.g., forest, savanna, grassland) is distributed over a wide rainfall range (1,000-3,000 mm/y) (52,56,57).…”

“…S6). The possibility of a smaller long-term increase in aridity, undetected owing to uncertainty in WD estimates, cannot be discounted, but would not necessarily have been a major environmental driver, given that ecological feedback in African biomes inhibits vegetation responses to climate change (52)(53)(54). Thus, the cause of the major long-term expansion of C 4 biomass within Turkana and elsewhere remains unclear, but may be related to climatic and ecological dynamics that are unrelated to annual WD and need not be equivalent across basins or regions (1,23,25).…”

Inserting Tropical Dry Forests Into the Discussion on Biome Transitions in the Tropics

“…Climatic factors, such as mean annual precipitation (MAP) and its seasonality are of obvious importance, but edaphic factors, and disturbance via fire, humans and herbivores also play key roles. Recent large-scale studies across the tropics have focused on transitions between forest and savanna (Hirota et al, 2011;Staver et al, 2011;Oliveras and Malhi, 2016;Xu et al, 2016;Langan et al, 2017). While there is value to simplifying vegetation concepts in the tropics, we believe the simplification used by these authors in defining "forest" goes perhaps one step too far.…”

Tropical grassy biomes: linking ecology, human use and conservation

“…Finally, the presence of numerous, well-documented, feedbacks structuring TGBs where the species composition can influence the strength and direction of effects (figure 1; for example, tree cover-fire; fire-grazing; grazing-browsing), combined with the importance of historical contingencies means that multiple states influencing both the limits and structure of TGBs are highly likely. Oliveras & Malhi [55] examine the shades of green in our understanding of the processes structuring the limits of TGBs highlighting how biotic and abiotic processes operate at different scales and that the nature of vegetation dynamics is context-dependent.…”