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.

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Small-scale fertilization experiments have shown that soil nutrients limit plant productivity in many semiarid grasslands and savannas, but linkages among nutrients, grasses, and grazers are rarely studied in an ecosystem context. We used hectare-scale heterogeneity in soil nutrients created by cattle management practices within a geologically homogeneous savanna to examine relationships among soil nitrogen and phosphorus, aboveground net primary production (ANPP), grass nutrient content, and a mixed community of native and domestic herbivores on central Kenyan rangeland. Increasing soil N and P content was consistently associated with increasing plant productivity and rainfall use efficiency in wet, dry, and drought years. A fertilization experiment and analyses of grass N:P ratios across sites indicated that N is the primary limiting nutrient on nutrient-rich glades, whereas N and P co-limit productivity on nutrient-poor bushland sites. Variation in ANPP among patches within the landscape was linearly correlated with consumption rates of large herbivores. Grazing pressure was consistently high (Ͼ60% of ANPP) at all but one site in a dry year (1999), and was greater in nutrient-rich glades (73 Ϯ 4% of ANPP) than in nutrientpoor bushland sites (43 Ϯ 7% of ANPP) in a wet year (2001). Grasses of nutrient-rich sites contained sufficient P concentrations to meet requirements for pregnant and lactating ungulates, whereas grasses in nutrient-poor swards were P deficient. Even though native and domestic herbivores selectively used and intensively grazed nutrient-rich sites, productivity on these sites remained high throughout the study. Analyses of nitrogen budgets for nutrientrich and nutrient-poor sites showed that large herbivores themselves caused a net N input to the former and a net N loss from the latter. Thus, large herbivores not only respond to heterogeneity in soil and plant nutrients across the landscape, but also play a role in maintaining the N-enriched status of highly productive and intensively grazed sites.

Summary1. The effect of livestock on African rangelands has been a major focus of recent research, but little attention has been paid to the way livestock affects the distribution and availability of soil nutrients. In East African savannas, overnight containment of livestock in thorn-scrub corrals or 'bomas' concentrates large quantities of nutrients into small areas, potentially altering the landscape distribution of nitrogen (N) and phosphorus (P) in soils and plants. 2. This study was designed to (i) measure the density, turnover rates and soil nutrient concentrations of abandoned cattle bomas on nutrient-poor rangeland in central Kenya; (ii) determine whether long-term glades dominated by Cynodon plectostachyus are derived from abandoned bomas; and (iii) evaluate the effect of cattle bomas on the landscape-level distribution of N and P. 3. In the study area, glades (> 39 years old) averaged 0·71 ha in size and occurred at a density of 0·71 km. Abandoned bomas (1-39 years since abandonment) averaged 0·39 ha and occurred at a density of 1·21 km − 2 . During 1961-2000, no glades reverted to bushland vegetation, while 53 bomas were abandoned. 4. All characteristics of soils measured across a boma-glade chronosequence indicated glades were indeed derived from abandoned bomas. Soil N, P and organic matter quality in the surface (0-15 cm) layer were similar for glades and 30-39-year-old bomas, but were significantly enriched relative to surrounding bushland. In contrast, at 40-65 cm depth beneath bomas, glades and bushland, soil N was similar. The texture of surface soils from bomas, glades and bushland was similar, indicating glades were not derived from a unique parent material. 5. Leaves of C. plectostachyus from 12-24-year bomas and long-term glades were enriched in P, calcium (Ca) and N relative to leaves of Cynodon dactylon from nearby bushland sites. In particular, P in boma and glade grass was above recommended levels for growing and lactating livestock, while P content of bushland grass was lower than recommended levels. 6. Cattle management via bomas exerts a greater effect on the distribution of P relative to N within the landscape. For cattle grazing an area of 20-25 km 2 boma, an estimated 0·24-0·30 g N m − 2 year − 1 is removed from the rangeland and deposited into bomas.Within 1·5 years of boma abandonment, 70% of this N is already lost from the manure and upper soil layer. Permanent N loss does occur via leaching, but the majority is probably volatilized and redistributed in rainfall. N deposition in rainfall (0·43 g N m) is more than sufficient to offset losses due to cattle grazing and deposition in bomas. In contrast, P deposited in bomas is more tightly retained, creating small P-enriched 'hotspots' while causing a permanent loss of the order of 0·021-0·026 g P m − 2 year − 1 from the surrounding bushland landscape. 7. Synthesis and applications. Results indicate that abandoned bomas persist as nutrientenriched patches for at least four decades. Rangeland managers should recognize that the plac...

Abstract1. Herbivores, edaphic features and fire are primary factors regulating the balance between woody and herbaceous vegetation in savannas. Many observational studies have evaluated the potential effects of browsing herbivores on woody plant dynamics in African savannas, but few experimental studies have compared the dynamics of African savannas with and without browsers. 2. A replicated herbivore exclusion experiment was used to assess the role that native ungulates play in regulating woody plant dynamics on commercial rangeland in central Kenya, where the indigenous fauna have been allowed to coexist with cattle. 3. Exclusion of native browsing ungulates for just 3 years showed that they have dramatic effects at every scale from individual twig growth rates to overall rates of woody biomass accumulation in the ecosystem. 4. At the scale of individual Acacia twigs, browsers significantly reduced leaf density, leaf biomass and growth rates of twigs < 50 cm above-ground (within the foraging height of dik-diks Madoqua kirkii ), but browsers had no effects on twig leaf density or leaf biomass at a height of 1·0-1·25 m above-ground. 5. Reductions in the growth rate of twigs within the foraging height of dik-diks was associated with a 6-fold reduction in the rate at which shrubs < 0·5-m tall were recruited into the 0·5-1·5 m height class. This reduced recruitment combined with measured rates of shrub mortality in larger height classes shows that browsers reduced the rate of increase in shrub density nearly to zero (7·1 ± 10·2 shrubs ha − 1 year − 1 ) compared to the rapid rate of increase in the absence of browsers (136·9 ± 13·6 shrubs ha − 1 year − 1 ). 6. Damage to shrub canopies by elephants Loxodonta africana caused large, significant reductions in cover of A. mellifera and Grewia tenax , but lesser reductions in cover of A. etbaica . For Acacias , elephant damage was focused on shrubs > 2·5 m tall, such that Acacias in intermediate height classes (0·5-2·5 m) experienced minimal browser impacts. Elephants influenced shrubland dynamics by altering shrub height-class distributions, shifting species composition from broad-leaved Grewia tenax to fine-leaved Acacia species, and suppressing woody biomass accumulation; but elephants had little influence on changes in shrub density. 7. Synthesis and applications . Our results suggest that a community of native browsers that includes both small, selective species (e.g. dik-diks) and large, bulk-feeding species (elephants) can provide an important ecosystem service by suppressing shrub encroachment on commercial rangeland.

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The effects of grazing by white-tailed deer (Odocoileus virginianus) on populations of Trillium spp. were examined in remnant, old-growth patches of the highly fragmented Big Woods forest ecosystem in southeastern Minnesota. We conducted three separate studies involving an exclosure experiment, transplant experiments, and comparisons of Trillium populations among study sites. The highest grazing intensity was observed where deer occurred at high overwinter concentrations (‫ف‬ 25-35/km 2); significantly lower grazing intensities occurred at low overwinter density (‫ف‬ 5-10/km 2). Deer focused their grazing on large, reproductive plants; at sites with high deer density, Trillium population structure was skewed toward small plants, and deer consistently caused over 50% reduction in reproduction during the growing season. Protection of individual plants from deer for two growing seasons resulted in dramatically increased flowering rates and significantly greater leaf area compared to control plants. No significant impact of current-year herbivory on reproduction in the following year was detected. Nevertheless, flowering rates at one site with high overwinter deer densities for at least the past 5 years suggest that the cumulative effects of grazing over several years can reduce reproduction in subsequent years. Transplant experiments with Trillium grandiflorum also showed that deer had significant effects on growth and reproduction where deer occur at high density. Our results suggest that changes in landscape structure and local deer abundance have altered plant-deer relationships such that grazing can lead to the local extirpation of sensitive forbs such as Trillium spp. As a result, active, long-term management of deer at low densities appears necessary for the conservation and restoration of fragmented forest communities in eastern North America. Efectos del Venado Cola Blanca en Poblaciones de Trillium spp. en Bosques Decidous Fragmentados Deer Impacts on Trillium spp.