Savannas are globally important ecosystems of great significance to human economies. In these biomes, which are characterized by the co-dominance of trees and grasses, woody cover is a chief determinant of ecosystem properties. The availability of resources (water, nutrients) and disturbance regimes (fire, herbivory) are thought to be important in regulating woody cover, but perceptions differ on which of these are the primary drivers of savanna structure. Here we show, using data from 854 sites across Africa, that maximum woody cover in savannas receiving a mean annual precipitation (MAP) of less than approximately 650 mm is constrained by, and increases linearly with, MAP. These arid and semi-arid savannas may be considered 'stable' systems in which water constrains woody cover and permits grasses to coexist, while fire, herbivory and soil properties interact to reduce woody cover below the MAP-controlled upper bound. Above a MAP of approximately 650 mm, savannas are 'unstable' systems in which MAP is sufficient for woody canopy closure, and disturbances (fire, herbivory) are required for the coexistence of trees and grass. These results provide insights into the nature of African savannas and suggest that future changes in precipitation may considerably affect their distribution and dynamics.
Existing statistical methods for spatial pattern analysis now permit precise analyses of patterns and, given the appropriate interpretation keys, the formulation of testable hypotheses about the underlying processes. From a comprehensive analysis of the spatial pattern of a plant population and its environment, we infer scenarios of links between demographic processes, plant spatial patterns, and environmental heterogeneity. The palm tree Borassus aethiopum has easily identifiable demographic stages, a root foraging strategy enabling adults to reach distant nutrient sources, and a marked senescence starting with the onset of reproduction. We analyzed map data for palm individuals (in three different life history stages plus two sexes for adults) and for nutrient‐rich patches (clumps of other tree species and termite mounds) in three vegetation types (facies) of a humid savanna of West Africa (Lamto, Ivory Coast). Spatial analyses were based on Diggle’s nearest neighbor functions F and G and on Ripley’s K function. The main results were as follows: (1) juveniles and seedlings are aggregated, while adults have a random pattern or are more loosely aggregated; (2) all stages except female adults are spatially associated with nutrient‐rich patches, but association distances increase with stage in the life cycle; and (3) seedlings are associated with female adults, whereas the association of juveniles at longer distances is not clear‐cut. We propose from these results a parsimonious scenario linking spatial pattern and mortality pattern during the life cycle. The initial pattern of seedlings (close to maternal trees) results from low dispersal distance. Later stages (older seedlings and juveniles) are mostly restricted to nutrient‐rich patches through nutrient shortage away from these patches (environment‐induced mortality) and form dense clumps of immature palms. Competition on nutrient‐rich patches then favors the few juveniles that manage to survive farther from these patches (density‐dependent mortality). Finally, the last surviving juvenile of a clump suddenly experiences almost no competition with conspecifics, due to the long distance between clumps of juveniles, and owing to its root‐foraging ability, it can now recruit to the adult stage, subject only to senescence. Pattern variations among savanna facies are consistent with this scenario.
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