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Rainfall variability, including drought, consistently translates into variation in grass productivity in savanna ecosystems. These variations in grass availability can impact grazer populations, which can in turn shape grass responses to drought.However, despite clear evidence that grazers move freely in response to forage and water availability, drought ecologists have mostly failed to consider how these landscape‐scale processes influence vegetation responses to drought.Here, we ask how rainfall variability and drought, in interaction with population dynamics and movement of large grazers, shape grass layer responses in extant savanna landscapes in Kruger National Park in South Africa.We found that grass biomass was highly responsive to rainfall variability; notably, grass declines during drought were especially severe—even more severe than previously documented elsewhere.Grazing probably contributed, exacerbating drought effects in droughted regions and spatially extending them into non‐droughted refugia, where grass biomass also declined. Moreover, the existence of these refugia potentially prevented a grazer population crash.Synthesis. Our current focus on the physiological effects of drought is probably insufficient for understanding vegetation responses to drought. A renewed focus on landscape processes, including animal movement, will be critical to predicting savanna responses to increasingly frequent extreme events. [ABSTRACT FROM AUTHOR]

Changes in savanna tree species composition, both within landscapes and across climatic gradients, suggest that species differ in their ability to utilize resources and cope with grass competition. Linking trait variation among species to their relative performance under resource limitation and competition treatments could provide mechanistic understanding of species' turnover across environmental gradients in savannas. We investigated how tree species differ in response to competition from grasses and nutrient supply, and whether these responses can be related to plant traits. As humid savannas are often associated with low fertility, species of humid savannas may grow more efficiently under nutrient suppression than species from semi‐arid savannas. In heterogeneous grass swards, fast‐growing, resource‐wasteful species may be found in low grass biomass patches and slow‐growing, conservative species in high biomass patches.We compared seedlings of 40 tree species sampled from humid and semi‐arid savannas of three continents (Africa, Australia, South America) with and without grass competition and with high and low nutrient supply. We measured traits related to soil resource capture, light capture, and growth, and tested whether these traits were related to performance under the four treatments.All tree species were suppressed by grass competition and most by nutrient limitation. Only species from humid savannas in Australia grew better under nutrient limitation than their semi‐arid counterparts. Deciduous species from semi‐arid climates were more negatively impacted by grass competition than deciduous species from humid climates whereas the reverse pattern was observed among evergreen species. Faster growing species were more severely suppressed by grass competition and low nutrients than slower growing species. Traits associated with soil resource capture and seed mass supported growth under grass competition and nutrient limitation, but the traits differed by continent and by leaf habit.Synthesis. We found limited evidence that tree seedlings from humid savannas grow better under nutrient limitation than species of semi‐arid savannas. We confirmed that fast‐growing species may be advantaged in low grass biomass patches and conservative species in high biomass patches. Traits that improved performance under grass competition differed by continent, which may relate to differences in leaf habit and constraints on seed size. [ABSTRACT FROM AUTHOR]

Savanna ecosystems span a diverse range of climates, edaphic conditions, and disturbance regimes, the complexity of which has stimulated long‐standing interest in the mechanisms that maintain tree–grass coexistence. One hypothesis suggests that tree establishment is strongly limited by one or several demographic bottlenecks at early stages of the tree life cycle. A major impediment to testing this hypothesis is the lack of data on the relative strengths of different bottlenecks across key environmental gradients.To identify demographic bottlenecks that limit early tree establishment (0–18 months), we conducted a series of transplant experiments with two savanna trees species (Acacia robusta and Acacia tortilis) across a natural rainfall and soil fertility gradient in the Serengeti ecosystem, Tanzania. We tested the interactive effects of precipitation, herbivory, seed scarification, grass competition, water limitation, and tree species identity on two key life stages: germination and early seedling survival (0–2 months), and juvenile seedling survival (2–18 months).Germination and early seedling survival increased as a function of rainfall, in the absence of herbivores and when seeds were scarified. Juvenile seedling survival, in contrast, decreased with rainfall but increased in the absence of herbivores. Grass removal had the single strongest (positive) effect on juvenile seedling survival of any treatment. Soil moisture monitoring and grass‐addition treatments revealed that grasses negatively affected seedlings in ways that were not necessarily linked to soil moisture.A demographic model combining all effects across early life stages showed that the strength of grass competition on juvenile seedling survival was the key factor limiting early tree establishment. While rainfall had an unexpected opposing effect on the two life stages, the net effect of mean annual precipitation on early tree establishment was positive.Synthesis. Successful tree establishment in Serengeti is maximized by a seemingly unlikely sequence of events: (a) scarification of seeds by browsers, (b) heavy rainfall to promote germination, (c) intensive grazing (but absence of browsers), and (d) dry conditions during juvenile seedling growth (>2 months) to reduce competition with grasses. By considering a wide suite of conditions and their interactions, our experimental results are relevant to ongoing debates about savanna vegetation dynamics and structural shifts in tree:grass ratios. Successful tree establishment in Serengeti is maximized by a seemingly unlikely sequence of events: (a) scarification of seeds by browsers, (b) heavy rainfall to promote germination, (c) intensive grazing (but absence of browsers), and (d) dry conditions during juvenile seedling growth (>2 months) to reduce competition with grasses. By considering a wide suite of conditions and their interactions, results from our transplant experiments are relevant to ongoing debates about savanna vegetation dynamics and structural shifts in tree:grass ratios. [ABSTRACT FROM AUTHOR]

Territorial or resting behaviour of large herbivores can cause strong local deposits of dung, in different places than where they graze. Additionally, dung beetles and other macrodetritivores can subsequently affect local nutrient budgets through post-depositional re-dispersion of dung and accompanying nutrients. Such horizontal displacement of nutrients by animals has strong implications for savanna ecosystem functioning, but remains poorly studied as it is notoriously difficult to accurately map these flows and incredibly time-consuming., In an African savanna, with alternating patches of lawn, bunch grasses and trees/shrubs, we undertook such effort and studied nutrient aggregation and redistribution by different large herbivore functional groups and dung beetles for a full growing season. We used movable cages to quantify herbivore consumption rates and measured nutrient return through biweekly dung counts. Furthermore, we estimated the offtake of nitrogen (N) and phosphorus (P) by the dominant megagrazer (white rhinoceros) to middens (dung deposition hotspots). Last, we experimentally measured the removal amount and movement paths of telocoprid dung beetles to quantify their nutrient redistribution effects., Our estimates suggest white rhinoceros to cause a large export of nutrients from grazing areas towards middens resulting in negative nutrient budgets for both lawn and bunch grassland types. Mesograzers (50-600 kg) realized a net nitrogen input towards high forage quality lawn vegetation at the expense of lower quality bunch grasslands. Browsers caused a net flow from trees/shrubs towards grassland patches., Interestingly, while the magnitude of our estimated flows of N consumption and return by large herbivores were rather similar, the P returns were about half of what has been consumed. This is in agreement with ecological stoichiometry theory that predicts that large herbivores should recycle more N than P, due to their relatively high P demand. Furthermore, dung-rolling beetles had a small, but significant, directed movement from lawn to bunch grassland vegetation., Synthesis. We conclude that within-ecosystem nutrient redistributions by animals are important and approximately of the same order of magnitude as regional atmospheric nutrient in and outputs (e.g. fire emissions, atmospheric N deposition, biological N fixation), and hence are important for understanding savanna ecosystem functioning. [ABSTRACT FROM AUTHOR]

Elephants and fire are individually well-known disturbance agents within savanna ecosystems, but their interactive role in governing tree-cover dynamics and savanna-forest biome boundaries remains unresolved. Of central importance are the mechanisms by which elephants vs. fire affect tree biomass and cover, and how - over long time periods - both factors interact with rainfall and soils to govern tree biomass and carbon dynamics., Here, we evaluated the response of woody vegetation to 56 years of fire manipulation in South Africa's Kruger National Park, with three fire regimes (annual, triennial and unburned) replicated across a productivity gradient and subject to two periods of contrasting elephant abundances (generated by the cessation of culling in 1994)., Higher fire frequencies had a negative effect on woody biomass in the low-elephant period, but this effect was weak to negligible in the high-elephant period as the difference among fire treatments diminished. Moreover, elephants removed increasing amounts of woody biomass as productivity increased across study sites, but fire did not. We infer that elephant-induced tree mortality could overcome increases in woody-plant productivity, while fire-induced mortality alone could not., Elephants caused woody-plant carbon to shift from a sink to a source; this effect was independent of fire treatment, with highest rates of net carbon removal in the wettest and most productive site., Synthesis. Our results reveal a context-dependent interaction between fire and elephants as disturbance agents in savanna: the influence of fire on woody plants was sensitive to the abundance of elephants and diminished with increased plant productivity. In contrast, elephants were capable of shifting landscapes from relatively dense woodland to open savanna, even in unburned sites, and exerted strong impacts irrespective of site conditions and plant productivity. [ABSTRACT FROM AUTHOR]

Establishing trade-offs among traits and the degree to which they covary along environmental gradients has become a key focal point in the effort to develop community ecology into a predictive science. While there is evidence for these relationships across global data sets, they are often too broad in scale and do not consider the particularities of local to regional species pools. This decreases their usefulness for developing predictive models at scales relevant for conservation and management., We tested for trade-offs between traits and relationships with environmental gradients in trees and shrubs sampled across southern African savannas and explored evidence for acquisitive versus conservative resource-use strategies using a phylogenetically explicit approach., We found a distinct trade-off between two major poles of specialization indicative of acquisitive (high leaf nitrogen concentration, leaf phosphorus concentration, leaf N:P, specific leaf area and average leaf area) and conservative resource-use strategies (high leaf carbon to nitrogen ratios (C:N), tensile strength and leaf dry matter content). Although we found that trait variance and species occurrence were constrained by phylogeny, phylogenetically informed analyses did not contradict non-phylogenetic analyses, strengthening relationships in most cases., The high intrasite trait variability and weak relationships with soils and climate may in part be explained by the high levels of deciduousness and disturbance (i.e. fire and herbivory) inherent in African savannas., Synthesis. The relationships between traits and between traits and environmental gradients were far weaker than, and often contradictory to, broad-scale studies that compare these relationships across biomes and growth forms, cautioning against making generalizations about relationships at specific sites based on broad-scale analyses. [ABSTRACT FROM AUTHOR]

Despite widespread acknowledgement that large mammal herbivory can strongly affect vegetation structure in savanna, we still lack a theoretical and practical understanding of savanna dynamics in response to herbivory., Like fire, browsing may impose height-structured recruitment limitations on trees (i.e. a 'browse trap'), but the demographics of herbivore effects have rarely been considered explicitly. Evidence that cohorts of trees in savannas may establish during herbivore population crashes and persist long term in savanna landscapes is anecdotal., Here, we use an experimental approach in Hluhluwe iMfolozi Park in South Africa, examining the response of grass biomass and tree populations to 10 years of graduated herbivore exclusion, and their subsequent response when exclosures were removed., We found that grazer exclusion increased grass biomass and that, despite presumable increases in fire intensity and grass competition, herbivore - especially mesoherbivore, including impala and nyala - exclusion resulted in increases in tree size. After herbivore reintroduction, grazers reduced grass biomass over short time-scales, but tree release from browsing persisted, regardless of tree size., Synthesis. This work provides the first experimental evidence that release from browsing trumps grazer-grass-fire interactions to result in increases in tree size that persist even after browser reintroduction. Escape from the 'browse trap' may be incremental and not strictly episodic, but, over longer time-scales, reductions in browsing pressure may lead to tree establishment events in savanna that persist even during periods of intense browsing. Explicitly considering the temporal demographic effects of browsing will be the key for a much-needed evaluation of the potential global extent of herbivore impacts in savanna. [ABSTRACT FROM AUTHOR]

Large mammalian herbivores are well recognized to play important roles in regulating woody cover and biomass in savannas, but the extent to which browsing ungulates are capable of regulating woody populations in the absence of other disturbances such as fire is unclear. Moreover, the degree to which browser effects on savannas operate through effects on woody plant recruitment vs. mortality has rarely been examined., We conducted a 10-year, replicated herbivore exclusion experiment in a semi-arid savanna in East Africa (mean annual rainfall = 514 mm), where fires have been actively suppressed for decades. Browsers dramatically influenced recruitment, growth and mortality of all size classes of woody vegetation. A decade of herbivore exclusion resulted in a sevenfold increase in recruitment, a 2.5-fold decrease in mortality and a threefold increase in woody biomass inside exclosures, while biomass outside exclosures remained relatively unchanged., At the plant community level, extensive browsing of seedlings by small-bodied ungulates suppressed woody recruitment in this semi-arid system, generating a 'browsing trap' comparable to the 'fire trap' reported for mesic systems. Browsing by large- and medium-bodied ungulates reduced both growth and survival of individuals in larger size classes., At the plant species level, browser impacts were variable. Although browsers negatively influenced recruitment of all species, they had little to no impact on the mortality of some dominant species, resulting in a long-term, browser-driven shift in woody species composition that was largely mediated via their differential effects on plant mortality rates rather than recruitment., Synthesis. Our results demonstrate unequivocally that, even in the absence of fire, native browsing ungulates can exert dramatic 'top-down' controls in semi-arid savannas, influencing all aspects of woody plant demography. Besides suppressing woody plant recruitment, browsers can also have substantial cumulative long-term impacts on growth and mortality rates of woody plants, including adults, which can differ between species in ways that fundamentally alter the structure and function of woody vegetation. In semi-arid rangelands, intact communities of native browsing ungulates thus provide a critical ecosystem service by regulating woody cover, and their removal (or extinction) from these systems can lead to rapid woody encroachment. [ABSTRACT FROM AUTHOR]

A recent meta-analysis suggested that differences in rainfall are a cause of variation in tree-grass interactions in savannas, with trees facilitating growth of understorey grasses in low-rainfall areas, but competing with them under higher rainfall. We hypothesized that this effect of rainfall upon understorey productivity is modified by differences in the growth form of the woody plants (i.e. the height of the lower canopy) or by their capacity to fix nitrogen., We performed a meta-analysis of the effects of woody plants on understorey productivity, incorporating canopy height and N-fixation, and their interaction with rainfall., N-fixing woody plants enhanced understorey productivity, whereas non-fixers had a neutral or negative effect, depending on high or low canopy, respectively. We found a strong negative correlation between rainfall and the degree to which trees enhanced understorey productivity, but only for trees with a high canopy., Synthesis. The effect of woody plants on understorey productivity depends not only on rainfall, but also on their growth form and their capacity to fix N. Facilitation occurs mostly when woody plants ameliorate both water and nitrogen conditions. However, a low canopy suppresses understorey vegetation by competing for light, regardless of water and nutrient relations. [ABSTRACT FROM AUTHOR]

Fire and herbivory are the main disturbances shaping the structure of savannas. In these ecosystems, the key strategies by which woody plants escape fire are either early height growth (the lanky strategy) or early bark growth (the corky strategy). We hypothesize that the dominance of each strategy in different savannas depends on the prevailing disturbance regimes. Given the importance of herbivory in afrotropical savanna, we expect woody plants in these savannas to be taller and have thinner barks (the lanky strategy) than plants in neotropical savanna where fire tends to be more intense (the corky strategy)., We compiled data on bark thickness and stem height in relation to stem diameter for afrotropical and neotropical savanna woody species and tested for differences in the allometric relationship between these two savannas with a general linear mixed model ( GLMM)., Fire intensities were higher in neotropical than in afrotropical savannas. Afrotropical savanna plants were taller and had thinner barks for a given diameter than neotropical savanna plants - supporting our hypothesis that because of the contrasting disturbance regimes, the lanky strategy is more adaptive in afrotropical savannas, whereas the corky strategy is more adaptive in neotropical savannas., Synthesis. While the lanky strategy is more associated with heavily browsed and fuel-controlled savannas, the corky strategy is associated with lightly browsed savannas that experience more intense fires. Because the relative role of disturbances varies across the globe, we suggest that the height-bark-diameter scheme is a powerful framework for understanding the ecology of many savannas. [ABSTRACT FROM AUTHOR]

Fire is a process that shapes the structure and composition of vegetation in many regions. Species in these regions have presumably evolved life-history strategies that allow success in fire-prone environments., In this study, we examine the extent to which the ecological success of savanna trees is determined by traits that enhance the capacity to tolerate fire and/or traits indicative of an ecophysiological capacity for rapid growth. We define ecological success as the relative change in stem density over the course of a long-term (circa 40 year) fire experiment conducted in the Kruger National Park, South Africa., We first examine the extent to which differences in the capacity of trees to tolerate fire can be explained by allometries describing bark traits and tree size. We then examine whether these differences in fire tolerance traits can explain observed shifts in abundance., We show that species differ in their topkill responses (probability of above-ground mortality) and that these differences are explained in part by differences in bark moisture content and the allometry between height and diameter. Contrary to previous studies, we find no evidence that bark thickness is important in explaining susceptibility to topkill., Synthesis. Fire tolerance traits did explain a significant component of the variance in observed shifts in the abundance of tree species. However, traits related to the carbon economy of photosynthesis were also important. [ABSTRACT FROM AUTHOR]

1. Plant organ biomass partitioning has been hypothesized to be driven by resources, such that species from drier environments allocate more biomass to roots than species from wetter environments to access water at greater soil depths. In savanna systems, fire may select for greater allocation to root biomass, especially in humid environments where fire is more frequent. Therefore, species from drier environments may have been under selection pressure to reach deeper soil water more effectively than species from humid environments, through faster root extension, more efficient depth penetration, and faster plant growth rates to respond rapidly to variable rainfall events. 2. We compared biomass partitioning, root morphology traits [root extension rate, RER; specific taproot length (STRL)] and relative growth rate (RGR) of seedlings of 51 savanna tree species, sampled from three continents (Africa, Australia and South America) in a greenhouse experiment. We used phylogenetically corrected and uncorrected analyses to compare the traits of the groups. We conducted a permanova on the combined traits to establish whether species could be distinguished on the basis of their combined traits. 3. On average, species from humid environments allocated more biomass to roots and less to stems than species from semi-arid environments, consistent with the expectation that fire pressure selects for greater allocation to roots in humid environments. However, some species from humid environments had fast growth rates instead of high allocation to roots. Both RER and STRL were greater among species of semi-arid environments than among species of humid environments, and also differed between continents. Differences between strategies under each climate type appear to be associated with leaf habit. 4. Synthesis. Plant biomass partitioning has been selected by defoliation pressure and the effects of this selection pressure can supersede any selection in response to local water constraints. Root morphological adaptations, but not plant growth rate, of tree seedlings, have been selected in response to water deficits. [ABSTRACT FROM AUTHOR]

1. Callitris intratropica is a long-lived, obligate-seeding, fire-sensitive overstorey conifer that typically occurs in small groves (<0.1 ha) of much higher tree densities than the surrounding, eucalypt-dominated tropical savanna in northern Australia. We used C. intratropica groves of varying canopy cover to examine the role of feedbacks between fire and tree cover in the persistence of a fire-sensitive tree species and the maintenance of habitat heterogeneity in a highly flammable savanna. 2. We examined the population structure and floristic composition of C. intratropica groves and conducted controlled burns with Aboriginal landowners to test the prediction that groves of C. intratropica with closed canopies inhibit savanna fires by physically altering the fuel arrays below trees. We measured pre- and post-burn fuel availability, the probability of burning and fire intensity along transects (55-75 m) spanning entire C. intratropica groves and extending into the surrounding savanna matrix. 3. We found that closed-canopy groves of C. intratropica had higher densities of seedlings and saplings than open-canopy groves and supported a distinct plant community. Closed-canopy groves also had a lower probability of burning and less severe fires due to a lower availability of fine fuels than the surrounding savanna. 4. Synthesis. Our results suggest that the observed regeneration within closed-canopy C. intratropica groves within frequently burnt savanna reflects a vegetation-fire feedback. A significant, negative relationship between canopy cover and the probability of burning provides strong evidence that closed-canopy C. intratropica groves are capable of excluding low-intensity savanna fires, thereby enabling the persistence of patches of fire-sensitive forest or woodland amid open, highly flammable savanna vegetation. We present our findings as evidence that alternative stable state dynamics may play a role in determining savanna diversity and structure. [ABSTRACT FROM AUTHOR]

Summary 1. Termites and large herbivores represent important functional groups in savanna ecosystems. Termites affect vegetation far beyond their mounds. In addition, large herbivores feed selectively on termite mound vegetation or in the vicinity of mounds. Previous studies of savanna vegetation communities have focused on termites and large herbivores separately, although interaction effects may be predicted. 2. We studied the effects of large herbivores and large vegetated Macrotermes mounds on the herbaceous vegetation in Lake Mburo National Park in Uganda. We recorded herbaceous vegetation change over 3 years on savanna areas (with and without large herbivores) and on corresponding termite mounds (with and without large herbivores) in a randomized block design. 3. Termite mounds and savannas had significantly different plant communities, but large herbivore grazing exclusions did not result in significant shifts in plant communities during this study period. A canonical correspondence analysis separated species mainly along an axis from termitaria to savanna. Only a few species responded to grazing exclusion. Some erect species, such as Hyparrhenia filipendula and Themeda triandra, increased in cover, and creeping species, such as Cynodon dactylon, decreased, following the exclusion of grazers. Forbs dominated mound areas, while graminoids dominated the savanna areas. Fencing increased the cover of graminoids over time and led to gradual increase in the relative cover of graminoids compared with forbs. 4. Mound soil was higher in pH, calcium and magnesium and lower in sodium compared with adjacent savanna areas. Nitrogen and carbon soil content did not differ between the two habitats. Soil phosphorus increased following grazing exclusion. 5. Synthesis. This study shows that termites may exert a far more important effect on the herbaceous community than large herbivores in savanna areas, even if the biomass of large herbivores is relatively high. Thus, future studies on savanna vegetation ecology should focus increasingly on important insect groups in addition to the more conspicuous large mammal guild. [ABSTRACT FROM AUTHOR]

1. Through their effects on plant communities, herbivores can exert strong direct and indirect effects on savanna ecosystems and have the potential to create and maintain savanna landscape heterogeneity. Throughout much of sub-Saharan Africa, periodic creation and abandonment of livestock corrals leads to landscape mosaics of long-term ecosystem hotspots that attract both cattle and large ungulate wildlife. 2. The development and maintenance of vegetation in these types of hotspots may be controlled in part by herbivory. Cattle and wildlife may have different, potentially contrasting effects on plant succession and plant–plant interactions. We ask how cattle and wild herbivores affect the maintenance and vegetation development of corral-derived landscape heterogeneity (0.25–1.0 ha treeless ‘glades’) in Laikipia, Kenya, through their effects on long-term successional and short-term plant–plant dynamics. 3. We used the Kenya Long-term Exclosure Experiment to exclude from glades different combinations of cattle, large ungulate wildlife (i.e. zebras, gazelles and other antelopes), and mega-herbivore wildlife (i.e. giraffes and elephants). We first assessed long-term changes in cover of the dominant grass species, Cynodon plectostachyus and Pennisetum stramineum (the early- and late-dominant species, respectively). We then used a neighbour removal experiment to test the effects of different herbivores on competition and facilitation between the two glade grass species. 4. In the long-term experiment, we found that large ungulate wildlife reinforced landscape heterogeneity over time by helping maintain glades in their early C. plectostachyus-dominated form. Cattle and mega-herbivore wildlife, on the other hand, appeared to reduce the positive effects through forage preference for C. plectostachyus. 5. In the neighbour removal experiment, we found that each grass species benefited from facilitation when it was the preferred forage for the dominant grazer. Facilitation of C. plectostachyus by P. stramineum was strongest when cattle co-occurred with wildlife, whereas facilitation of P. stramineum by C. plectostachyus was strongest when cattle were absent. 6. Synthesis. Our results demonstrate that different combinations of cattle and wildlife have different effects, largely via contrasting forage preferences, on the persistence of landscape heterogeneity in this savanna landscape. More generally, we provide evidence for contrasting effects of cattle and wildlife on short-term plant interactions (facilitation) and successional processes within the herbaceous plant community. [ABSTRACT FROM AUTHOR]

1. A substantial portion of the crop of fruiting trees falls beneath parent plants as a result of dispersal failure. Such diaspores are considered as waste because the likelihood of plant recruitment is usually very small close to parent trees. However, many animals may rescue fallen diaspores and provide them with another chance of dispersal and establishment. 2. We investigated the effectiveness of two broad types of seed dispersal vectors for the regeneration of Xylopia aromatica in the Brazilian cerrado savanna: birds that remove diaspores from plant canopies and ants that harvest diaspores on the ground under the parent plant (as rescuers) or from bird feces (as secondary dispersers). 3. Birds removed a mean of 32% of the crop from plant canopies, but removal was independent of crop size. A large part of the crop (mean of 25%) landed beneath parent plants or was dropped after manipulation by vertebrate frugivores as viable diaspores. Ants from at least five genera removed most fallen diaspores (up to 83%) within 24 h. Ants influenced the fate of a large amount of the crop, and for some trees ants removed as many diaspores as birds. 4. Large ants rescued some diaspores to distances beyond the parent plant crown, but birds may remove diaspores 40-fold farther. However, seedlings of X. aromatica were only found close to nests of large ants, probably due to diaspore rescuing and/or directed secondary dispersal by certain ant groups following primary dispersal by birds. Although an unknown percentage of seeds was lost to granivorous ants, diaspore removal by ants potentially enhances the likelihood of plant recruitment due to distance-related benefits and directed dispersal to ant nests, while birds play a premier role in long-distance seed dispersal and metapopulation dynamics. 5. Synthesis. Birds and ants provide complementary seed dispersal at different spatial scales to X. aromatica. Since ants remove most fallen diaspores beneath parent plants, the use of diaspore removal rates from plant canopy as a surrogate of plant fitness may be misleading. By acting as secondary dispersers, ants may also provide a fine-tuned dispersal following long-distance dispersal by birds (i.e. diplochory). [ABSTRACT FROM AUTHOR]

1. Savanna ecosystems – defined by the coexistence of trees and grasses – cover more than one-fifth the world’s land surface and harbour most of the world’s rangelands, livestock and large mammal diversity. Savanna trees can have a variety of effects on grasses, with consequences for the wild and domestic herbivores that depend on them. 2. Studies of these effects have focused on two different spatial scales. At the scale of individual trees, many studies have shown net positive effects of trees on sub-canopy grass nutrient concentrations and biomass. At the landscape scale, other studies have shown negative effects of high tree densities on grass productivity. These disparate results have led to different conclusions about the effects of trees on forage quality and ungulate nutrition in savannas. 3. We integrate these approaches by examining the effects of trees on grasses at both spatial scales and across a range of landscape-scale tree densities. We quantified grass biomass, species composition and nutrient concentrations in these different contexts in an Acacia drepanolobium savanna in Laikipia, Kenya. 4. Individual trees had positive effects on grass biomass, most likely because trees enrich soil nitrogen. Grass leaf phosphorus in sub-canopy areas, however, was depressed. The effects of individual trees could explain the effects of increasing landscape-scale tree cover for the biomass of only two of the four dominant grass species. 5. The negative effects of trees on grass and soil phosphorus, combined with depressed grass productivity in areas of high tree cover, suggest that ungulate nutrition may be compromised in areas with many trees. 6. Synthesis. We conclude that few, isolated trees may have positive local effects on savanna grasses and forage, but in areas of high tree density the negative landscape-scale effects of trees are likely to outweigh these positive effects. In savannas and other patchy landscapes, attempts to predict the consequences of changes in patch abundances for ecosystem services (e.g. rangeland productivity and carbon sequestration) will depend on our understanding of the extent to which local, patch-scale dynamics do or do not predict landscape-scale dynamics. [ABSTRACT FROM AUTHOR]

1 We identified species with low re-colonization potential, which could he used as indicators ot recovery ot species-rich pine savannas, by comparing the ground-cover flora ot a 64-year-old slash pine plantation (recovery site) with that ot a nearby natural longleat pine savanna (reference site). We also determined life-history traits that were useful predictors of recolonization potential. 2 The high floristic overlap in species between reference and recovery sites and similar species richness at scales ≥ 10 m suggests that substantial vegetation recovery occurred over the 65-year period however, for areas < 10 m the lower species packing in the recovery sites indicates that coexistence ot a high number ot species at small scales is dependent on local dispersal and establishment, and may take much longer to achieve. 3 The absence, or near absence, of some species from the recovery site, even after 65 years, suggests that some species may he particularly vulnerable to disturbance and may re-establish infrequently, if ever. Several dispersal distance-restricted species were identified that require active reintroduction. While no particular guild of species was a strong indicator of recovery in this study, we identified a group of species that assess the absence of or the degree of recovery from, prior soil disturbance 4 Local dispersal appears to he an important factor structuring species richness patterns in pine savannas Limitations ot dispersal distance in some species, particularly those with gravity and ant-dispersal mechanisms, represent an obstacle to passive restoration that can only he overcome either by introduction of propagules in the restoration process or by allowing for longer periods of recruitment. 5 This study demonstrates a method for identifying a suite of species that may he unsuccessful at recolonization. The method would he applicable to numerous degraded ecosystems, particularly similar species-rich savannas, grasslands and forests. [ABSTRACT FROM AUTHOR]

Journal of Ecology (2003) 91, 999–1007 [ABSTRACT FROM AUTHOR]

Studies the variable structure and mass of vegetation, nutrient pools and responses to fire along a common vegetation gradient from grassland to open evergreen woodland in the Brazilian Cerrado. Biomass of the fuel loads; Nutrients which were consistently lost in the highest quantities during fire.

Studies the effects of tree demography, fire-induced mortality and seed dispersal on the spatial spread of single tree species in a humid savanna in Lamto, Ivory Coast. Use of a spatially explicit cellular automation model; Role of fire in enhancing tree aggregation.

1. Maracá, Brazil, is a large riverine island which is at the boundary between lowland evergreen rain forest and savanna. The island is now uninhabited but there is evidence that much of its forest (including the plots described in this paper) is secondary, dating from before 1880. 2. The mean annual rainfall is about 2300mm and there is a dry season from October to March. 3. Six replicate plots of 0.25 ha each were set up in the rain forest. Three of these were later felled, as part of a long-term study on forest regeneration, and three left undisturbed. 4. Soils were described from a profile pit and for surface samples (0-10cm) in the three undisturbed plots. The soils were a Grossarenic Plinthic Paleudult (USDA). They were acid, had exceptionally low concentrations of total phosphorus, low concentrations of exchangeable cations but a fairly high base saturation, and a sandy texture. There was no surface root mat, no surface accumulation of organic matter, and no sign of podzolization. 5. All trees and lianas ≥10cm d.b.h. in the forest plots were enumerated, and structural features of the trees (including fine-root biomass) and smaller plants were quantified. The plots had a mean tree basal area of 23.8m[Sup2] and the emergents were up to 40m tall. 6. The mean tree species richness for the plots was forty per 0.25 ha and overall the Moraccae were the most important family in terms of basal area. 7. Analyses of leaves of 38 tree species showed wide interspecific variations but, in general. leaf nutrient concentrations were moderate or high. 8. It is concluded that although the Maracá forest occurs on very nutrient-poor soils it shows neither physiognomic nor foliar chemical features which are held to be characteristic of rain forests on such soils. Moreover, even though the soils are dry and sandy, the Maracá forest shows no features of heath forests. [ABSTRACT FROM AUTHOR]

1. The Maracã rain forest occurs on sandy soils which have very low concentrations of nutrients, but it has none of the structural features which are often held to be associated with forests on nutrient-poor soils. 2. The present paper reports tests of three hypotheses which were proposed to account for these unexpected features of the Maracá forest: (a) that the forest is unproductive and has a low nutrient demand: (b) that the trees are particularly efficient at withdrawing foliar nutrients before leaf abscission: and (c) that decomposition is rapid and the nutrients are quickly recycled. 3. Small litterfall (leaves, wood ≤2cm in diameter, reproductive parts. and trash) was measured for 12 months in three replicate 0-25-ha plots for quantity. seasonal distribution and concentrations of nitrogen. phosphorus. potassium, calcium and magnesium. 4. The mean total small litterfall was relatively high with a mean value for the three plots of 9.28tha[Sup-1] year[Sup-1] (range 885-9-52). 5. The nitrogen concentrations in the Maracá small litterfall were at the lower end of the range reported for other tropical forests whilst the concentrations of phosphorus. potassium, calcium and magnesium were relatively high. 6. Small-litter layer mass was measured in the plots on four occasions over a year. It showed changes with season and the mean values for total small-litter for the three plots were least (4.00tha[Sup-1]) in the wet season and highest (5.14tha[Sup-1]) in the dry season. 7. The concentrations of nitrogen, magnesium and potassium were lower in the small-litter layer than those of the litterfall. whilst phosphorus and calcium con- centrations were similar. 8. Mean large-wood (≥2cm in diameter) litter (including standing dead trees) had a mean value of 5-80tha[Sup-1](range 354-7.37) for the three plots. 9. Decomposition quotient (kL) values were relatively high with mean value of 2.94 for the leaf fraction and 2.01 for total small litterfall. [ABSTRACT FROM AUTHOR]

Studies the dynamics of the tree layer of the Eragrostis pallens-Burkea africana savanna. General successional sequence; Detrimental effect of porcupines and fire on the development of a mature, closed-canopy woodland; Maintenance of Eragrostis pallens-Burkea africana savanna by porcupines and fire as a mosaic of grassland and small woodland patches undergoing cyclical successions.

(1) Root samples of five grass species, distributed over four soil types, were collected from a savanna region in southern Kenya. (2) All samples, including those from seasonally flooded sites, had vesiculararbuscular mycorrhizal infection. (3) In the main collection in November, near the start of the wet season, infection was lowest on a freely drained soil low in organic matter. This appears to have been due to a decline in mycorrhizas since the end of the previous wet season. (4) The speed of development of an effective mycorrhizal system early in the wet season may be important for savanna grasses. [ABSTRACT FROM AUTHOR]

(1) Semi-arid savannas wherever they occur have generally been overgrazed and encroached on by bush. A model is developed which accounts for the growth of woody vegetation and of grasses, and analyses the competition between them for available soft water. (2) The model is based on Walter's two layer hypothesis. Woody vegetation and grasses compete for water m the surface layers of the soil, but woody vegetation has exclusive access to a source of water relatively deep underground. Where there is only a small biomass of grass the soil surface tends to become impermeable and m these conditions, the model shows that two different steady states may develop with a lot of woody vegetation alone, or with a relatively large biomass of grass and rather little woody vegetation. (3) The results are discussed in terms of the concept of resilience. The continued existence of both stable states under ranching conditions seems to depend on periodic heavy or over- grazing which allows for the maintenance of unpalatable or unstable grass species which thus set a minimum to grass biomass—a minimum which cannot be reduced by herbivores. (4) Comparison of the dynamics of various savanna and other natural systems leads to the conclusion that the resilience of the systems decreases as their stability (usually induced) increases. [ABSTRACT FROM AUTHOR]

The article presents a detailed phytosociological analysis of the savanna vegetation of the Venezuelan Llanos. The analysis aims to acquire a precise knowledge of the internal variability of vegetation and of the environmental factors involved and to test different methods of vegetation analysis in order to select that most appropriate for this type of vegetation. The association-analysis method was used and the Biological Station of Los Llanos was chosen for this study because it is a protected stand of vegetation exclusively used for research purposes. A brief outline of the entire inland plain region of Venezuela and its main vegetation and climatic characteristics as well as the significance of the particular type of savanna chosen for the analysis is presented.

The article focuses on the study of the species composition, structure, and environment of the vegetation in Mole Game Reserve in Ghana. Mole Game Reserve is a natural, and developed reserve in Ghana. The study conducted observations on the reserve's vegetation, soil, and microclimate during the dry end seasons. The study found two dominant trees, which are the Burkea africana, and the Terminalia avicennioides. Also conducted was the excavation of tree roots, from which all fourteen species were found sprouting.

The article reports on the seasonal changes in the herb stratum in the Olokemeji Forest Reserve savanna site. It mentions that growth in dry weight and height occurs most during the wet season, and fire and day-length initiate the flowering of many plant species. The herb stratum reaches a recorded maximum dry weight of 6.8 t/ha, and the index of the leaf area was 6.0. Through principal component analysis, the sample dates were ordinated showing a seasonal trend and difference between years. The result of a normal association-analysis showed that few of the thirteen common groups distinguished remain constant throughout the year indicating that a sample position may be allocated to different groups at different times of the year.

The article reports on the classification of plant communities by the method of normal association-analysis. The savanna vegetation of a 2146 morgen farm was objectively sub-divided into plant communities. The objective of the article was to assess its practical value for classifying vegetation on an ecological basis under South African conditions. Certain physical and chemical properties of the soil of ten major plant communities were investigated. Convincing evidence was obtained which indicated that the communities were largely coincident with underlying differences in the soil, in respect of the distribution of some of their constituent species.

The article presents an environmental analysis of the savanna vegetation in Northern Nigeria. The study indicates that soil parent material is not a major factor in the arboreal vegetation of an area. It reveals that the main factors are the water availability in the soil and the degree of human interference. The factors control the general ordination of both noda and groups, soil water and human interference. Moreover, the general case differences in soil parent material do not necessarily imply differences in the arboreal vegetation.

The article presents the study regarding the vegetation of the different types of soil parent materials recognized in the Middle Gongola region. The seven parent materials are analysed separately and a total of 26 distinct noda emerge. In 14 cases, noda remains separate and each forms the basis of a group,while in three cases, two noda join through mutually associated plots to bi-nodal groups, and in two cases three noda combine to form three nodal groups. It states that in general, the groups are topographical or physiognomic units or, both which are usually recognizable on aerial photographs can be used as mapping units.

The article investigates the ripple-like vegetation patterns near Gummi, Nigeria. The patterns, which unlike previous patterns are not clearly related to gross soil differences developed under a desert regime, can be observed to the north of the Zamfara river, between Gummi and Bukwium. It was found out that the ripple-like pattern consisted of almost pure Combretum nigricans. Compared to stripes and arcs known in Somalia and Sudan, the ripple-like patterns in the Savanna woodland of Combretum occur in sandy soils with an approximately 750mm annual rainfall.

Summary 1 Savanna ecosystems are characterized by the codominance of two different life forms: grasses and trees. An operational understanding of how these two different life forms coexist is essential for understanding savanna function and for predicting its response to future environmental change. 2 The existing model, which proposes that grasses and trees coexist by a separation of rooting niches, is not supported by recent empirical investigations. Our aim was to define an alternative mechanism of grass–tree coexistence in savanna ecosystems. The model we have built concentrates on life history–disturbance interactions between grasses and trees. 3 The model demonstrates coexistence for a wide range of environmental conditions, and exhibits long periods of slow decline in adult tree numbers interspersed with relatively infrequent recruitment events. Recruitment is controlled by rainfall, which limits seedling establishment, and fire, which prevents recruitment into adult size classes. Decline in adult tree numbers is the result of continuing, but low levels, of adult mortality. Both aspects of the dynamics are consistent with an established non-equilibrium mechanism of coexistence (the storage effect). 4 A sensitivity analysis indicated that data on tree resprouting ability, stem growth rates and the relationship between seedling establishment and wet season drought are essential for predicting both the range of conditions for which coexistence is possible and the response of savanna ecosystems to environmental change. 5 Our analysis suggests that understanding grass–tree interactions in savanna requires consideration of the long-term effects of life history–disturbance interactions on demography, rather than the fine-scale effects of resource competition on physiological performance. [ABSTRACT FROM AUTHOR]