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"Savannas"
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Abandoned pastures cannot spontaneously recover the attributes of old-growth savannas
1. Active restoration strategies have been recommended to recover Neotropical savannas in abandoned lands, but no studies have investigated the trajectories and speeds of spontaneous recovery for these systems. Research into the dynamics of degraded savannas is urgently needed to guide restoration decision making. 2. We analysed the dynamics of secondary savannas in the Brazilian Cerrado by sampling 29 abandoned pastures (time since abandonment ranging from 3 to 25 years) and applying the space-for-time substitution method. We modelled the temporal changes in plant community attributes and estimated the time (years) required for these attributes to match those of two reference ecosystems (three replicates each), old-growth savanna and a forest-type savanna, which had encroached following fire suppression (encroached savanna). We also analysed the plant community composition of the study sites. 3. Our models showed that tree canopy cover, richness and density rapidly increased with time since pasture abandonment, easily surpassing the values of the old-growth savanna (28 years) and reaching the values of encroached savanna 49 years after abandonment. The cover and richness of the ground layer increased at a much slower pace. Since the species in this layer, including the exotic grasses, are shade intolerant, they will be eliminated by canopy closure over time. 4. Up to 25 years after abandonment, secondary savannas continued to lack many (37%) old-growth savanna species, mostly from the ground layer (82% of grasses absent). This period was also not sufficient for the secondary savannas to become floristically similar to the encroached savannas, which are dominated by shade-tolerant tree species. 5. Synthesis and applications. Despite the reported high natural regeneration of Neotropical savanna vegetation, abandoned pastures will not spontaneously return to an old-growth savanna state. Protected from fire and lacking the native ground layer, the end state of secondary savannas will be a low-diversity forest. If restoration goals include the recovery of old-growth savanna biodiversity and structure, interventions are required to prevent woody encroachment and reintroduce native grasses, forbs and shrubs. However, if the desirable endpoint is a low-diversity forest, passive restoration (non-intervention) and fire protection are appropriate.
Journal Article
Savannas of our birth
This book tells the sweeping story of the role that East African savannas played in human evolution, how people, livestock, and wildlife interact in the region today, and how these relationships might shift as the climate warms, the world globalizes, and human populations grow. Our ancient human ancestors were nurtured by African savannas, which today support pastoral peoples and the last remnants of great Pleistocene herds of large mammals. Why has this wildlife thrived best where they live side-by-side with humans? Ecologist Robin S. Reid delves into the evidence to find that herding is often compatible with wildlife, and that pastoral land use sometimes enriches savanna landscapes and encourages biodiversity. Her balanced, scientific, and accessible examination of the current state of the relationships among the region's wildlife and people holds critical lessons for the future of conservation around the world.
eBook
Fire drives functional thresholds on the savanna-forest transition
In tropical landscapes, vegetation patches with contrasting tree densities are distributed as mosaics. However, the locations of patches and densities of trees within them cannot be predicted by climate models alone. It has been proposed that plant–fire feedbacks drive functional thresholds at a landscape scale, thereby maintaining open (savanna) and closed (forest) communities as two distinct stable states. However, there is little rigorous field evidence for this threshold model. Here we aim to provide support for such a model from a field perspective and to analyze the functional and phylogenetic consequences of fire in a Brazilian savanna landscape (Cerrado). We hypothesize that, in tropical landscapes, savanna and forest are two stable states maintained by plant–fire feedbacks. If so, their functional and diversity attributes should change abruptly along a community closure gradient. We set 98 plots along a gradient from open savanna to closed forest in the Brazilian Cerrado and tested for a threshold pattern in nine functional traits, five soil features, and seven diversity indicators. We then tested whether the threshold pattern was associated with different fire regimes. Most community attributes presented a threshold pattern on the savanna–forest transition with coinciding breakpoints. The thresholds separated two community states: (1) open environments with low‐diversity communities growing in poor soils and dominated by plants that are highly resistant to high‐intensity fires; and (2) closed environments with highly diverse plant communities growing in more fertile soils and dominated by shade‐tolerant species that efficiently prevent light from reaching the understory. In addition, each state was associated with contrasting fire regimes. Our results are consistent with the hypothesis that forests and savannas are two coexisting stable states with contrasting patterns of function and diversity that are regulated by fire–plant feedbacks; our results also shed light on the mechanism driving each state. Overall, our results support the idea that fire plays an important role in regulating the distribution of savanna and forest biomes in tropical landscapes.
Journal Article
Fire alters ecosystem carbon and nutrients but not plant nutrient stoichiometry or composition in tropical savanna
Fire and nutrients interact to influence the global distribution and dynamics of the savanna biome, but the results of these interactions are both complex and poorly known. A critical but unresolved question is whether short-term losses of carbon and nutrients caused by fire can trigger long-term and potentially compensatory responses in the nutrient stoichiometry of plants, or in the abundance of dinitrogen-fixing trees. There is disagreement in the literature about the potential role of fire on savanna nutrients, and, in turn, on plant stoichiometry and composition. A major limitation has been the lack of fire manipulations over time scales sufficiently long for these interactions to emerge. We use a 58-year, replicated, large-scale, fire manipulation experiment in Kruger National Park (South Africa) in savanna to quantify the effect of fire on (1) distributions of carbon, nitrogen, and phosphorus at the ecosystem scale; (2) carbon : nitrogen : phosphorus stoichiometry of above- and belowground tissues of plant species; and (3) abundance of plant functional groups including nitrogen fixers. Our results show dramatic effects of fire on the relative distribution of nutrients in soils, but that individual plant stoichiometry and plant community composition remained unexpectedly resilient. Moreover, measures of nutrients and carbon stable isotopes allowed us to discount the role of tree cover change in favor of the turnover of herbaceous biomass as the primary mechanism that mediates a transition from low to high soil carbon and nutrients in the absence of fire. We conclude that, in contrast to extra-tropical grasslands or closed-canopy forests, vegetation in the savanna biome may be uniquely adapted to nutrient losses caused by recurring fire.
Journal Article
Maize yield and nutrition during 4 years after biochar application to a Colombian savanna oxisol
The application of biochar (biomass-derived black carbon) to soil has been shown to improve crop yields, but the reasons for this are often not clearly demonstrated. Here, we studied the effect of a single application of 0, 8 and 20 t ha⁻¹ of biochar to a Colombian savanna Oxisol for 4 years (2003-2006), under a maize-soybean rotation. Soil sampling to 30 cm was carried out after maize harvest in all years but 2005, maize tissue samples were collected and crop biomass was measured at harvest. Maize grain yield did not significantly increase in the first year, but increases in the 20 t ha⁻¹ plots over the control were 28, 30 and 140% for 2004, 2005 and 2006, respectively. The availability of nutrients such as Ca and Mg was greater with biochar, and crop tissue analyses showed that Ca and Mg were limiting in this system. Soil pH increased, and exchangeable acidity showed a decreasing trend with biochar application. We attribute the greater crop yield and nutrient uptake primarily to the 77-320% greater available Ca and Mg in soil where biochar was applied.
Journal Article
Carbon accumulation and nitrogen pool recovery during transitions from savanna to forest in central Brazil
The expansion of tropical forest into savanna may potentially be a large carbon sink, but little is known about the patterns of carbon sequestration during transitional forest formation. Moreover, it is unclear how nutrient limitation, due to extended exposure to fire-driven nutrient losses, may constrain carbon accumulation. Here, we sampled plots that spanned a woody biomass gradient from savanna to transitional forest in response to differential fire protection in central Brazil. These plots were used to investigate how the process of transitional forest formation affects the size and distribution of carbon (C) and nitrogen (N) pools. This was paired with a detailed analysis of the nitrogen cycle to explore possible connections between carbon accumulation and nitrogen limitation. An analysis of carbon pools in the vegetation, upper soil, and litter shows that the transition from savanna to transitional forest can result in a fourfold increase in total carbon (from 43 to 179 Mg C/ha) with a doubling of carbon stocks in the litter and soil layers. Total nitrogen in the litter and soil layers increased with forest development in both the bulk (+68%) and plant-available (+150%) pools, with the most pronounced changes occurring in the upper layers. However, the analyses of nitrate concentrations, nitrate : ammonium ratios, plant stoichiometry of carbon and nitrogen, and soil and foliar nitrogen isotope ratios suggest that a conservative nitrogen cycle persists throughout forest development, indicating that nitrogen remains in low supply relative to demand. Furthermore, the lack of variation in underlying soil type (>20 cm depth) suggests that the biogeochemical trends across the gradient are driven by vegetation. Our results provide evidence for high carbon sequestration potential with forest encroachment on savanna, but nitrogen limitation may play a large and persistent role in governing carbon sequestration in savannas or other equally fire-disturbed tropical landscapes. In turn, the link between forest development and nitrogen pool recovery creates a framework for evaluating potential positive feedbacks on savanna-forest boundaries.
Journal Article
Woody cover in wet and dry African savannas after six decades of experimental fires
1. Fire is an integral process in savannas because it plays a crucial role in altering woody cover of this globally important biome. 2. In this study, we examine the long-term effects of varying fire frequencies over a 60-year time period in South Africa. We analyse the effects of fire exclusion and of experimental burns every 1, 2 and 3 years on woody cover, tree abundance and stem structure on a wet and dry savanna. 3. Increased fire frequency did not display a consistent effect on woody cover. The presence of fire, irrespective of frequency, was much more influential in lowering tree abundance in the wet savanna than the dry savanna. In the dry savanna, fire was more effective in greatly increasing coppicing in trees, when compared to the wet savannas. 4. Synthesis. The effects of fire on three measures of savanna woody vegetation differed between wet and dry experimental sites. We suggest that vegetation responses to fire are dependent on local conditions, which are likely influenced by rainfall. Therefore, we suggest that management strategies should take account of whether a savanna is a wet or dry system when implementing fire management regimes.
Journal Article
When is a 'forest' a savanna, and why does it matter?
Savannas are defined based on vegetation structure, the central concept being a discontinuous tree cover in a continuous grass understorey. However, at the highrainfall 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 C₁ grass-dominated understorey, would be better classified as mesic savannas requiring fire and light to maintain the unique mix of species that characterize them.
Journal Article