Fire and Herbivory Interactively Suppress the Survival and Growth of Trees in an African Semiarid Savanna (original) (raw)
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Ecology, 2021
Grassland and savanna ecosystems, important for both livelihoods and biodiversity conservation, are strongly affected by ecosystem drivers such as herbivory, fire, and drought. Interactions among fire, herbivores and vegetation produce complex feedbacks in these ecosystems, but these have rarely been studied in the context of fuel continuity and resultant fire heterogeneity. We carried out 36 controlled burns within replicated experimental plots that had allowed differential access by wild and domestic large herbivores since 1995 in a savanna ecosystem in Kenya. Half of these were reburns of plots burned 5 yr previously. We show here that the fine‐scale spatial heterogeneity of fire was greater in plots (1) previously burned, (2) accessible to large herbivores, and especially (3) these two in combination. An additional embedded experiment demonstrated that even small experimental burn‐free patches can have strong positive effects on tree saplings, which experienced less damage durin...
International Journal of Wildland Fire
Both wild and prescribed fire in savanna ecosystems influence habitat use by herbivores by creating or maintaining spatial and temporal heterogeneity in forage quality and vegetation cover. Yet little is known about how spatial scales influence long-term persistence of fire effects. We examined changes over a 6-year period in herbivore preference for experimentally burned patches that varied in spatial extent and grain. Avoidance for the burns by elephants and preference for the burns by impala and Grant’s gazelle decreased significantly. For the rest of the species (zebra, eland, oryx, hartebeest, warthog and hare), there were no significant changes in preference for the burns. Changes in preference for the burned areas depended on the spatial extent and grain of the burn, with intermediate-size (9-ha) burns and large (8-ha) patchy burns being more preferred 6–7 years after fire. Grain, but not the spatial extent of the burned area, influenced changes in grass height. Fire resulted...
Annals of Forest Science, 2011
Background The potential of forest to regenerate after harvesting is a key element for sustainability of the ecosystem. For semi-arid tropical savanna environments, managing resprouts after tree cutting is ideally suited because of the natural ability of many indigenous species to regenerate vegetatively. Regeneration in this ecosystem is, however, prone to many disturbance factors such as fire and grazing by livestock. Methods In this paper, we used a factorial experiment to examine the long-term effects of annual early burning and grazing on dominant species’ resprout biomass dynamics after selective cutting in the Sudanian savanna-woodlands of Burkina Faso, West Africa Results Burning decreased shoot mortality of Crossopteryx febrifuga while grazing increased that of Detarium microcarpum. Burning, in later measurement years, reduced resprouts’ size of Acacia macrostachya, C. febrifuga and D. microcarpum while an increased basal area was observed for Combretum glutinosum. There was no significant evidence of grazing hampering growth. Conclusions Moderate livestock grazing could be integrated in the forest management prescriptions in Burkina Faso for the sake of multi-purpose uses, while more attention should be paid to burning practices to lower fire severity, as complete fire exclusion is utopian in this savanna ecosystem.
Australian Journal of Botany, 2010
Although the demography of woody plants in savannas has long been shown to be due to many factors, there still is no consensus as to the relative importance of the top-down processes of fire and herbivory, nor on how fire and herbivory affect plant demography. We review the recent literature and suggest that further progress depends on the following: (i) a demographic framework with clear terminology and which focuses on recruitment, transitions and mortality, (ii) an understanding of mechanisms of how fire actually damages plants and how plants survive and out-grow this damage, mainly through height, bark thickness or diameter growth, (iii) an understanding of how losses in biomass due to herbivory may affect plant demography and, (iv) a consideration of interactions between fire and herbivory. Our synthesis suggests (i) strong recruitment limitation as well as some evidence of transition limitation by both fire and herbivory, (ii) that in some cases herbivory alone, notably by elephants and impala, can be more significant than fire alone, on woody plant population size, (iii) that fire and herbivory together are a lethal combination for woody plants and, (iv) that differences in strategies and responses of savanna plants to fire and herbivory are poorly explored.
EFFECTS OF FIRE AND HERBIVORY ON THE STABILITY OF SAVANNA ECOSYSTEMS
Ecology, 2003
Savanna ecosystems are characterized by the co-occurrence of trees and grasses. In this paper, we argue that the balance between trees and grasses is, to a large extent, determined by the indirect interactive effects of herbivory and fire. These effects are based on the positive feedback between fuel load (grass biomass) and fire intensity. An increase in the level of grazing leads to reduced fuel load, which makes fire less intense and, thus, less damaging to trees and, consequently, results in an increase in woody vegetation. The system then switches from a state with trees and grasses to a state with solely trees. Similarly, browsers may enhance the effect of fire on trees because they reduce woody biomass, thus indirectly stimulating grass growth. This consequent increase in fuel load results in more intense fire and increased decline of biomass. The system then switches from a state with solely trees to a state with trees and grasses. We maintain that the interaction between fire and herbivory provides a mechanistic explanation for observed discontinuous changes in woody and grass biomass. This is an alternative for the soil degradation mechanism, in which there is a positive feedback between the amount of grass biomass and the amount of water that infiltrates into the soil. The soil degradation mechanism predicts no discontinuous changes, such as bush encroachment, on sandy soils. Such changes, however, are frequently observed. Therefore, the interactive effects of fire and herbivory provide a more plausible explanation for the occurrence of discontinuous changes in savanna ecosystems.
Relative Impacts of Elephant and Fire on Large Trees in a Savanna Ecosystem
Ecosystems, 2011
Elephant and fire are considered to be among the most important agents that can modify the African savanna ecosystem. Although the synergistic relationship between these two key ecological drivers is well documented, it has proved much more difficult to establish the relative effects they have on savanna vegetation structure at a fine-scale over time.
Ecosphere, 2017
Herbivory and fire are important drivers of ecosystem processes within African rangelands. We explore whether mid-Holocene African savannas were dominated by herbivory as a means of cycling nutrients, and whether fire perhaps played a lesser role than today. Evidence from savanna ecology, paleoecology, and historical literature indicates higher herbivore densities in mid-Holocene and pre-colonial times compared to present. While fire may increase or decrease forage availability for herbivores, depending on the nutrient status of the environment, herbivory tends to decrease fire intensity and frequency by decreasing fuel loads. Given this competitive relationship between fire and herbivory and the higher herbivore densities of the past, we suggest that some fire-dominated present-day savannas are the product of anthropogenic alterations in herbivore and fire regimes, including the increasing use of fire as a tool for managing ecosystems. We discuss whether managing for an alternative stable state dominated by herbivory could stimulate ecosystem processes such as nutrient cycling and production, and whether this will achieve the same management objectives traditionally satisfied by fire. Management implications may include the adaptive manipulation of herbivore densities over time and space to maintain an appropriate carrying capacity for the rainfall and soil nutrient status of the area, occasional use of fire, and including a diversity of herbivore functional guilds.
N'Dri et al 2022, 2022
1. In mesic savannas worldwide, trees experience frequent fires, almost all set by humans. Management fires are set to reduce or enhance tree cover. Success depends greatly on responses of sub-adult trees to such fires. To date, the number of successive years that sub-adult trees can resprout nor the number of years that they must resist being top-killed by successive fires, nor the requisite height, have been reported. 2. In a 6-year experimental field study in Guinean savannas of West Africa, we monitored annually the heights and responses of 1,765 permanently tagged sub-adult trees under annual fires set in three different periods of the long dry season: early-dry season (EDS), mid-dry season (MDS) and late-dry season (LDS). Annual MDS fires are the common local management protocols of Guinean savannas, although EDS fires are common in some of the savannas. 3. Results showed that overall, the proportion of sub-adults that resisted being top-killed differed across fire seasons. Furthermore, resisting one fire gave a better chance of resisting the next. Only sub-adults that were able to resist direct damage for three successive EDS and MDS fires reached sufficient height to be recruited to the adult stage. Resistance height (avoiding topkill) was ∼1 m for EDS and ∼2 m for both MDS and LDS fires. Recruitment height (threshold for transition to adult stage) was ∼3 m for EDS and ∼ 3.3 m for MDS fires. No height was great enough for sub-adult trees to be recruited to adult stages in LDS fire. 4. Synthesis and applications. The results of this novel field study showed clearly that successive early- and mi-dry season fires can enhance tree density and that successive late-dry season fires alone reduce tree density in Guinean savannas due to the effects of successive fires on sub-adult trees. The results suggest that a planned regime of these seasons of fire could be used to maintain the desired tree density in Guinean savannas and may inform fire management in other mesic savannas where goals are to increase or decrease tree densities. It also provides relevant information for comparative studies on the mechanisms of recruitment of sub-adult trees to an adult stage in all mesic savannas, a process that ultimately determines savanna physiognomy.