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Grasses fuel most fires on Earth and strongly influence local fire behaviour through traits that determine how flammable they are. Therefore, grass communities that differ in their species and trait compositions give rise to significant spatial variation in savanna fire regimes across the world, which cannot be otherwise explained. Likewise, fire regimes are continuously modified by alterations to savanna grass community traits, through species introductions and climatic changes. However, current representation of grassy fuels in global fire models misses important variation and therefore limits predictive power. The inclusion of grass trait diversity in models, using remotely sensed trait proxies, for example, will greatly improve our ability to understand and project savanna fires and their roles in the Earth system. Most fires on Earth occur in savannas, but vary considerably in their extent, frequency, and intensity. These patterns cannot fully be explained by climate or land management. An important 'missing-piece' in explaining the variation in savanna fire regimes is the diversity of grasses that fuel fires and whose traits strongly influence fire behaviour. The major differences in savanna fire behaviour across continents may lie in the contrasting species and trait assemblages in savanna grass communities. Fire regimes change significantly when savanna grass communities or traits are altered by the introduction of invasive species and climatic changes. Improved representation of grass trait diversity in fire models will be essential for reliable forecasts of the behaviour and impacts of the majority of fires on Earth. [ABSTRACT FROM AUTHOR]

Newly germinated seedlings are vulnerable to biomass removal but usually have at least 6 months to grow before they are exposed to dry‐season fires, a major disturbance in savannas. In contrast, plants are exposed to browsers from the time they germinate, making browsing potentially a very powerful bottleneck for establishing seedlings.Here we assess the resilience of seedlings of 10 savanna tree species to topkill during the first 6 months of growth. Newly germinated seeds from four dominant African genera from across the rainfall gradient were planted in a common garden experiment at the Wits Rural Facility and clipped at 1 cm when they were ~2, 3, 4 and 5 months old. Survival, growth and key plant traits were monitored for the following 2.5 years.Seedlings from environments with high herbivory pressure survived topkill at a younger age than those from low‐herbivore environments, and more palatable genera had higher herbivore tolerance. Most individuals that survived were able to recover lost biomass within 12 months, but the clipping treatment affected root mass fraction and branching patterns.Synthesis. The impact of early browsing as a demographic bottleneck can be predicted by integrating information on the probability of being browsed and the probability of surviving a browse event. Establishment limitation through early browsing is an under‐recognized constraint on savanna tree species distributions. [ABSTRACT FROM AUTHOR]

In savannas, ruminant herbivores can have divergent impacts on tree recruitment and subsequent woody cover. Whereas heavy grazing by cattle results in woody thickening, intensive grazing by wildlife instead tends to be associated with lower woody cover.To disentangle why woody cover is low in areas heavily grazed by wildlife, we investigated (a) whether grazing lawns attract indigenous mammalian browsers, and if a preference for short‐grass habitat decreases with browser body mass as predator susceptibility decreases; and (b) whether browser attraction to grazing lawns translates into the suppression of woody plants, including seedlings and saplings, thus maintaining the openness of heavily grazed short‐grass areas.In Kruger National Park, South Africa, we contrasted browser abundance (using dung counts) on grazing lawns and on low‐herbivory sites characterised by tall grass. Additionally, a herbivore exclosure experiment was set up to investigate the combined impact of browser removal and grass height habitat type on seedling survival and sapling growth of a dominant woody plant species. Finally, in Hluhluwe‐iMfolozi Park (HiP), we examined the long‐term (10 years) impact of browser removal on the growth rates of a range of woody species, monitored across ten sites along a gradient of herbivory ranging from grazing lawn to tall grass.Steenbok and impala selected short‐ over tall grass as preferred browsing sites, while elephant preferred tall grass. Browser abundance on short grass decreased with browser body mass, indicating that predator avoidance might be a key factor driving mesoherbivores to utilise grazing lawns.Seedling survival was lowest on grazing lawns when browsers were present, with mortality occurring in two out of every three seedlings. Similarly, sapling growth was lowest on grazing lawns, although browser removal had no significant effect. Evidence for increased browser impact on grazing lawns was clearest from our long‐term herbivore exclosure experiment in HiP, which demonstrated that browsers strongly modify the growth rates of woody plants in short‐grass habitats.Synthesis. These results provide support for the hypothesis that browsers, particularly browsing mesoherbivores and mixed feeders, are attracted to short‐grass habitats, and that they help maintain grazing lawn openness by suppressing seedling survival and woody plant growth where grass is kept short by grazers. [ABSTRACT FROM AUTHOR]

Savannas are known as ecosystems with tree cover below climate-defined equilibrium values. However, a predictive framework for understanding constraints on tree cover is lacking. We present (a) a spatially, extensive analysis of tree cover and fire distribution in sub-Saharan Africa, and (b) a model, based on empirical results, demonstrating that savanna and forest may be alternative stable states in parts of Africa, with implications for understanding savanna distributions. Tree cover does not increase continuously with rainfall, but rather is constrained to low (<50%, "savanna") or high tree cover (>75%, "forest"). Intermediate tree cover rarely occurs. Fire, which prevents trees from establishing, differentiates high and low tree cover, especially in areas with rainfall between 1000 mm and 2000 mm. Fire is less important at low rainfall (<1000 mm), where rainfall limits tree cover, and at high rainfall (>2000 mm), where fire is rare. This pattern suggests that complex' interactions between climate and disturbance produce emergent alternative states in tree cover. The relationship between tree cover and fire was incorporated into a dynamic model including grass, savanna tree saplings, and savanna trees: Only recruitment from sapling to adult tree varied depending on the amount of grass in the system. Based on our empirical analysis and previous work, fires spread only at tree cover of 40% or less, producing a sigmoidal fire probability distribution as a function of grass cover and therefore a sigmoidal sapling to tree recruitment function. This model demonstrates that, given relatively conservative and empirically supported assumptions about the establishment of trees in savannas, alternative stable states for the same set of environmental conditions (i.e., model parameters) are possible via a fire feedback mechanism. Integrating alternative stable state dynamics into models of biome distributions could improve our ability to predict changes in biome distributions and in carbon storage under climate and global change scenarios. [ABSTRACT FROM AUTHOR]

In order to investigate how environmental factors other than light availability affect tree architecture, differences in branching architecture and allometry were analysed in populations of Acacia karroo Hein. from three different environments in South Africa: forests, savannas and arid-shrublands. Factors such as fire and herbivory have a large effect on tree life history in certain environments and are likely to have selected for trees that have different architectures from those of forest trees, whose major limitation is light assimilation. Significant differences were found in stem architecture and branching architecture between trees in each environment. Compared with forest trees, trees in savannas had an elongated growth form with small canopy and leaf areas, and tall, thin, unbranched trunks. Trees in arid areas showed opposite trends with wider canopies, and increased lateral branching. Savanna trees had significantly smaller spines than trees in other environments, and both forest and savanna trees showed delayed reproduction. These differences are probably related to a trade-off between an architecture geared towards rapid height-gain and one promoting lateral spread, and can be explained with reference to the different selective pressures in each environment. In forests, vertical and horizontal growth are both important. However, in savannas there is a great pressure for rapid vertical growth to escape fires, while in arid areas a defensive, lateral growth form is selected for. Savanna trees and arid karoo trees have evolved architectures that are more extreme vertically and laterally than the range of architectures displayed in a forest community. [ABSTRACT FROM AUTHOR]

Theoretically, fire-tree cover feedbacks can maintain savanna and forest as alternative stable states. However, the global extent of fire-driven discontinuities in tree cover is unknown, especially accounting for seasonality and soils. We use tree cover, climate, fire, and soils data sets to show that tree cover is globally discontinuous. Climate influences tree cover globally but, at intermediate rainfall (1000 to 2500 millimeters) with mild seasonality (less than 7 months), tree cover is bimodal, and only fire differentiates between savanna and forest. These may be alternative states over large areas, including parts of Amazonia and the Congo. Changes in biome distributions, whether at the cost of savanna (due to fragmentation) or forest (due to climate), will be neither smooth nor easily reversible. [ABSTRACT FROM AUTHOR]

Fire and herbivory are competitive processes in grassy ecosystems as they both consume above‐ground biomass. Despite this interaction between fire and grazing, herbivore and fire management plans in conservation areas are usually perceived, and hence managed, independently. This study aims to demonstrate the importance of acknowledging and integrating the interdependence between herbivore and fire management.We compare average proportional area burnt during years when large‐scale culling kept herbivore biomass within a narrow range within the Kruger National Park, South Africa, against subsequent years of no culling when herbivore densities increased to effectively double the biomass. Furthermore, to improve our mechanistic understanding, we utilize natural gradients in rainfall (north to south) and grazing (distance from rivers) to test a conceptual model of how herbivory influences fire occurrence under different rainfall regimes.Fires were more prevalent during the culling period than thereafter, and the fire suppression effect of increasing grazing biomass was most pronounced in areas closer to rivers (50% reduction within 1 km of rivers).The "grazer effect" (i.e. difference between % area burnt between low and high grazer biomass years) seems to be more pronounced in arid landscapes than in semiarid landscapes, which in turn is more pronounced than in the mesic landscapes.Despite elephants being mixed feeders, elephant density influences fire occurrence more than the density of buffaloes or hippopotami.Synthesis and applications: Herbivore and fire management actions should be integrated, as events that influence herbivore densities go on to influence fire occurrence in subsequent years. In areas with 400–600 mm MAP it is particularly important because changes in herbivore numbers—due to drought, predation, disease, or human interventions—can result in switches between fire‐ and herbivore‐dominated systems, with cascading consequences for biodiversity and ecosystem function. Furthermore, grazing gradients (in this case distance to closest perennial river) are critical as it allows both fire and herbivore‐dominated systems to occur within the same system along the gradient. This increases management possibilities, as management actions that induce grazing gradients can have positive implications for maintaining biodiversity in protected areas. Herbivore and fire management actions should be integrated, as events that influence herbivore densities go on to influence fire occurrence in subsequent years. In areas with 400–600 mm MAP it is particularly important because changes in herbivore numbers—due to drought, predation, disease, or human interventions—can result in switches between fire‐ and herbivore‐dominated systems, with cascading consequences for biodiversity and ecosystem function. Furthermore, grazing gradients (in this case distance to closest perennial river) are critical as it allows both fire and herbivore‐dominated systems to occur within the same system along the gradient. This increases management possibilities, as management actions that induce grazing gradients can have positive implications for maintaining biodiversity in protected areas. [ABSTRACT FROM AUTHOR]

Humans can alter fire dynamics in grassland systems by changing fire frequency, fire seasonality and fuel conditions. These changes have effects on vegetation structure and recovery, species composition, and ecosystem function. Understanding how human management can affect fire regimes is vital to detect potential changes in the resilience of plant communities, and to predict vegetation responses to human interventions. We evaluated the fire regimes of two recently protected areas in Madagascar (Ibity and Itremo NPA) and one in Brazil (Serra do Cipó NP) before and after livestock exclusion and fire suppression policies. We compare the pre- and post-management fire history in these areas and analyze differences in terms of total annual burned area, density of ignitions, burn scar size distribution, fire return period and seasonal fire distribution. More than 90% of total park areas were burned at least once during the studied period, for all parks. We observed a significant reduction in the number of ignitions for Ibity NPA and Serra do Cipó NP after livestock exclusion and active fire suppression, but no significant change in total burned area for each protected area. We also observed a seasonal shift in burning, with fires happening later in the fire season (October–November) after management intervention. However, the protected areas in Madagascar had shorter fire return intervals (3.23 and 1.82 years) than those in Brazil (7.91 years). Our results demonstrate that fire exclusion is unattainable, and probably unwarranted in tropical grassland conservation areas, but show how human intervention in fire and vegetation patterns can alter various aspects of the fire regimes. This information can help with formulating realistic and effective fire management policies in these valuable conservation areas. [ABSTRACT FROM AUTHOR]

Savannas are the only deciduous system where new leaf flush pre-empts the onset of suitable conditions for growth, a phenological phenomenon known as early-greening. Limited understanding of the frequency and drivers of the occurrence of early-greening in southern African savanna trees exists. We aimed to estimate the frequency of early-greening events across southern Africa and investigated potential environmental drivers of green-up. We selected and compared seven broad-leaved woodland sites where Burkea africana was a dominant species using remotely sensed data along a latitudinal gradient from South Africa to Zambia. Normalized difference vegetation index ( NDVI) values were extracted from the Moderate Resolution Imaging Spectroradiometer ( MODIS) satellite imagery at each site from January 2002 to June 2014. Using an austral year (July 1st-June 30th), early-greening was recorded if the green-up start date occurred prior to the onset date of seasonal rainfall. A latitudinal gradient of early-green-up was detected across southern Africa ( R2 = 0.74) with the two most northerly (Zambian) sites showing the earliest and most consistent green-up start dates (3 October ± 5.34 days). A strong latitudinal gradient was observed between the variability in the amount of rainfall in the first 6 months of green-up and the green-up start dates across southern Africa ( R2 = 0.92). Photoperiod appeared to play a role in areas where the onset of rainfall commenced late into the austral year. Mean maximum temperatures recorded 10 days prior to green-up start dates suggested a potential threshold of about 35°C, which could drive early-greening in the absence of rainfall. Correlations between the proportion of early-greening years and the above mentioned environmental factors indicated that rainfall variability had the strongest influence over the observed phenological gradient ( R2 = 0.96). Understanding early-greening in complex savanna systems is a vital step in furthering predictive phenological models under changing climatic conditions. [ABSTRACT FROM AUTHOR]

The article presents a research study regarding the limits of savanna on the continents of Africa, Australia, and South America. The effects of rainfall, seasonality, and soil fertility on the growth and distribution of savanna are examined, as well as the interaction of these factors with natural disturbances.

Hydraulic failure caused by severe drought contributes to aboveground dieback and whole‐plant death. The extent to which dieback or whole‐plant death can be predicted by plant hydraulic traits has rarely been tested among species with different leaf habits and/or growth forms. We investigated 19 hydraulic traits in 40 woody species in a tropical savanna and their potential correlations with drought response during an extreme drought event during the El Niño–Southern Oscillation in 2015. Plant hydraulic trait variation was partitioned substantially by leaf habit but not growth form along a trade‐off axis between traits that support drought tolerance versus avoidance. Semi‐deciduous species and shrubs had the highest branch dieback and top‐kill (complete aboveground death) among the leaf habits or growth forms. Dieback and top‐kill were well explained by combining hydraulic traits with leaf habit and growth form, suggesting integrating life history traits with hydraulic traits will yield better predictions. [ABSTRACT FROM AUTHOR]

The need for accurate quantification of the amount of carbon stored in the environment has never been greater. Carbon sequestration has become a vital component of the battle against global climate change, and monitoring and quantifying this process are major challenges for policymakers. Plant allometric equations allow managers and scientists to quantify the biomass contained in a tree without cutting it down, and therefore can play a pivotal role in measuring carbon sequestration in forests and savannahs. These equations have been available since the beginning of the 20th century, but their usefulness depends on the ability to estimate the error associated with the equations - something which has received scant attention in the past. This paper provides a method based on the theory of linear regression and the lognormal distribution to derive confidence limits for estimates of biomass derived from plant allometric equations. Allometric equations for several southern African savannah species are provided, as well as the parameters and equations required to calculate the confidence intervals. This method was applied to data collected from a sampling campaign carried out in a savannah landscape at the Skukuza flux site, Kruger National Park, South Africa. Here the error was 10% of the total site biomass for the woody biomass and 2% for the leaf biomass. When the data were split into individual plots and used to estimate site biomass (as would occur in most sampling schemes) the error increased to 16% and 12% of the woody and leaf biomasses, respectively, as the sampling errors were added to the errors in the allometric equation. These methods can be used in any discipline that applies allometric equations, such as health sciences and animal physiology. [ABSTRACT FROM AUTHOR]

The article discusses the highlights of a workshop about bud banks and below-ground plant organs held in Sao Paulo, Brazil in May 2019. Topics discussed include the significant role of below-ground structures in regeneration strategies of plant communities following disturbances, the resilience of South African grasslands and savannas to disturbances, and the management and conservation of ecosystems.

Grazing lawn and flammable-tussock grass communities are contrasting resource pools for mammalian grazers in terms of forage quantity and quality. Drought events fundamentally alter forage availability within these communities and therefore should alter herbivore use with repercussions for the recovery and functioning of ecosystems after drought. During and after an intense El Niño drought (2014–2017) in Kruger National Park, South Africa, we addressed two questions: (1) how does herbivore use of different grass types change during a drought and (2) how do these changes affect grass productivity post-drought? We monitored grazer use of three different grass communities (lawn, tussock and burned-tussock) at a landscape scale and measured primary productivity monthly during and post-drought. For the first drought year, grazer numbers were highest on grazing lawn communities. This pattern continued into the second dry growing season, until herbivores finally left the study area. Both lawns and tussock grasslands recovered rapidly after the first good rainfall (productivity > 150 g m−2 per month). However, grazers did not return to feed on the same patches they had frequented pre-drought resulting in grazing lawn grasses self-shading and senescing. Longer droughts have the potential to decouple grazers and grazing lawns with negative impacts on lawn productivity and persistence that could drive the loss of lawns in savanna landscapes and impact mesoherbivore populations. It is clear from our results that grazer effects need to be incorporated into drought frameworks to understand the consequences of droughts for grassland function. [ABSTRACT FROM AUTHOR]

Aim: An emerging framework for tropical ecosystems states that fire activity is either "fuel build‐up limited" or "fuel moisture limited", that is, as you move up along rainfall gradients, the major control on fire occurrence switches from being the amount of fuel, to the moisture content of the fuel. Here we used remotely sensed datasets to assess whether interannual variability of burned area is better explained by annual rainfall totals driving fuel build‐up, or by dry season rainfall driving fuel moisture. Location: Pantropical savannas and grasslands. Time period: 2002–2016. Methods: We explored the response of annual burned area to interannual variability in rainfall. We compared several linear models to understand how fuel moisture and fuel build‐up effect (accumulated rainfall during 6 and 24 months prior to the end of the burning season, respectively) determine the interannual variability of burned area and explore if tree cover, dry season duration and human activity modified these relationships. Results: Fuel and moisture controls on fire occurrence in tropical savannas varied across continents. Only 24% of South American savannas were fuel build‐up limited against 61% of Australian savannas and 47% of African savannas. On average, South America switched from fuel limited to moisture limited at 500 mm/year, Africa at 800 mm/year and Australia at 1,000 mm/year of mean annual rainfall. Main conclusions: In 42% of tropical savannas (accounting for 41% of current area burned) increased drought and higher temperatures will not increase fire, but there are savannas, particularly in South America, that are likely to become more flammable with increasing temperatures. These findings highlight that we cannot transfer knowledge of fire responses to global change across ecosystems/regions—local solutions to local fire management issues are required, and different tropical savanna regions may show contrasting responses to the same drivers of global change. [ABSTRACT FROM AUTHOR]

Fire is a fundamental process in savannas and is widely used for management. Pyrodiversity, variation in local fire characteristics, has been proposed as a driver of biodiversity although empirical evidence is equivocal. Using a new measure of pyrodiversity (Hempson et al.), we undertook the first continent-wide assessment of how pyrodiversity affects biodiversity in protected areas across African savannas. The influence of pyrodiversity on bird and mammal species richness varied with rainfall: strongest support for a positive effect occurred in wet savannas (> 650 mm/year), where species richness increased by 27% for mammals and 40% for birds in the most pyrodiverse regions. Range-restricted birds were most increased by pyrodiversity, suggesting the diversity of fire regimes increases the availability of rare niches. Our findings are significant because they explain the conflicting results found in previous studies of savannas. We argue that managing savanna landscapes to increase pyrodiversity is especially important in wet savannas. [ABSTRACT FROM AUTHOR]

Grasses using the C4 photosynthetic pathway dominate frequently burned savannas, where the pathway is hypothesized to be adaptive. However, independent C4 lineages also sort among different fire environments. Adaptations to fire may thus depend on evolutionary history, which could be as important as the possession of the C4 photosynthetic pathway for life in these environments. Here, using a comparative pot experiment and controlled burn, we examined C3 and C4 grasses belonging to four lineages from the same regional flora, and asked the following questions: Do lineages differ in their responses to fire, are responses consistent between photosynthetic types, and are responses related to fire frequency in natural habitats? We found that in the C4 Andropogoneae lineage, frost killed a large proportion of aboveground biomass and produced a large dry fuel load, which meant that only a small fraction of the living tissue was lost in the fire. C3 species from the Paniceae and Danthonioideae lineages generated smaller fuel loads and lost more living biomass, while species from the C4 lineage Aristida generated the smallest fuel loads and lost the most living tissue. Regrowth after the fire was more rapid and complete in the C4 Andropogoneae and C3 Paniceae, but incomplete and slower in the C3 Danthonioideae and C4 Aristida. Rapid recovery was associated with high photosynthetic rates, high specific leaf area, delayed flowering, and frequent fires in natural habitats. Results demonstrated that phylogenetic lineage was more important than photosynthetic type in determining the fire response of these grasses and that fire responses were related to the frequency that natural habitats burned. [ABSTRACT FROM AUTHOR]

The article asserts that the plan of 130 nations at the 2021 United Nations Climate Change Conference (COP26) to halt global deforestation by 2030 needs to be expanded to include savannas, which cover an area of 20 million square kilometers and are regarded as more important carbon sinks than forests.

ABSTRACT Savannas are defined based on vegetation structure, the central concept being a discontinuous tree cover in a continuous grass understorey. However, at the high-rainfall end of the tropical savanna biome, where heavily wooded mesic savannas begin to structurally resemble forests, or where tropical forests are degraded such that they open out to structurally resemble savannas, vegetation structure alone may be inadequate to distinguish mesic savanna from forest. Additional knowledge of the functional differences between these ecosystems which contrast sharply in their evolutionary and ecological history is required. Specifically, we suggest that tropical mesic savannas are predominantly mixed tree-C4 grass systems defined by fire tolerance and shade intolerance of their species, while forests, from which C4 grasses are largely absent, have species that are mostly fire intolerant and shade tolerant. Using this framework, we identify a suite of morphological, physiological and life-history traits that are likely to differ between tropical mesic savanna and forest species. We suggest that these traits can be used to distinguish between these ecosystems and thereby aid their appropriate management and conservation. We also suggest that many areas in South Asia classified as tropical dry forests, but characterized by fire-resistant tree species in a C4 grass-dominated understorey, would be better classified as mesic savannas requiring fire and light to maintain the unique mix of species that characterize them. [ABSTRACT FROM AUTHOR]