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.
Experiments in Serengeti National Park, Tanzania, provide direct evidence that large, free-ranging mammalian grazers accelerate nutrient cycling in a natural ecosystem in a way that enhances their own carrying capacity. Both nitrogen and sodium were at considerably higher plant-available levels in soils of highly grazed sites than in soils of nearby areas where animal density is sparse. Fencing that uncoupled grazers and soils indicated that the animals promote nitrogen availability on soils of inherently similar fertility and select sites of higher sodium availability as well as enhancing that availability.
Does grazing by large wild mammals, an intense form of aboveground herbivory, influence belowground productivity? The vast majority of literature data concentrate on short‐term pot studies and indicate that clipping consistently retards root growth. Field studies are few and contradictory, but tend to indicate that grazing has little effect on grassland belowground production. We sampled root‐soil cores at 0–10 and 10–20 cm increments, at 11 locations across the Serengeti ecosystem, on 10 dates over an annual cycle, sampling monthly during the rainy and early dry seasons and every 2 mo during peak dry season. Fenced and unfenced plots were replicated (n = 2 or 3) at each location. Live roots, identified visually by brightness and texture, were sorted, washed, dried, and weighed. In addition, profiles were sampled at 10‐cm increments to 50 cm in fenced and unfenced plots in short, mid‐height, and tall grasslands, representing a gradient of grazing, during the month of peak root biomass. Exclosures erected 22–25 yr previously were similarly sampled in short and tall grasslands to a 30‐cm depth. Root biomass reached a pronounced minimum in mid‐wet season (February) and a decided maximum at the beginning of the dry season (June). Net productivity, based on maximum–minimum biomass, ranged from 100 to 600 g·m−2·yr−1 to a 20‐cm depth, with minima ranging from 40 to 150 g/m2 and maxima from 230 to 700 g/m2, according to location. There was no evidence that grazing reduced root productivity over the annual cycle. Vertical biomass profiles at peak standing crop were similar for short, mid‐height, and tall grasslands, with root biomass dropping sharply with depth, except for short grasslands on soils that, atypically, lack a hardpan. In those grasslands, shallow root biomass was lower than in other grasslands, but biomass at depth was distinctly greater. For long‐term protected grasslands, root biomasses at peak were identical in short grasslands, whether fenced or unfenced, but fenced tall grasslands had a lower root biomass than grazed plots. We conclude that intense herbivory does not inhibit root biomass or belowground productivity of Serengeti grasslands over either the short or the long term.
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