TURNER REVIEW No. 19. Savanna woody plant dynamics: the role of fire and herbivory, separately and synergistically (original) (raw)
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Fire
There has been a long-standing interest in understanding how interactions between fire and herbivory influence woody vegetation dynamics in savanna ecosystems. However, controlled, replicated experiments examining how different fire regimes interact with different herbivore groups are rare. We tested the effects of single and repeated burns, crossed with six replicated herbivore treatments, on the mortality and growth of woody vegetation in the Kenya Long-term Exclosure Experiment plots located in a semi-arid savanna system in central Kenya. Burned plots experienced higher tree mortality overall, but differences between burns and non-burns were only significant in plots excluding all wild herbivores and in plots accessible to megaherbivores. Cattle ameliorated the negative effects of repeat burns on tree mortality, perhaps by suppressing fuel load accumulation. Across all herbivore treatments, trees experienced a significant reduction in height within the first two years after fire ...
Austral Ecology, 2011
Sclerocarya birrea ssp. caffra (marula), a typical savanna tree, is vulnerable to the effects of fire, herbivory and their combination. This paper investigated the relative importance of these agents of disturbance, at the level of the individual stem, by specifically focusing on the following questions: (i) What is the greatest cause of mortality in adult marula stems in conservation areas with both elephants and fire? (ii) Does fire interact with bark stripping to cause adult stem mortality and if so what is the dominant mechanism? (iii) At what stem diameter are marulas resistant to fire? Field surveys quantified the extent of damage in marula individuals in the Kruger National Park, South Africa, highlighting the high levels of extreme herbivory such as toppling (7%) and pollarding (8%), relative to bark stripping (only 6% with more than 50% of the circumference stripped). In addition to extreme herbivory, the progression from bark stripping through to invasion of the soft, exposed heartwood by wood borers, often facilitated by fire, through to toppling of the weakened stem after successive fires, appears to be the dominant mechanism by which fire interacts with herbivory to cause adult stem death. Bark stripping and fire manipulation experiments indicated that bark stripping failed to increase the vulnerability of stems to fire directly through transport tissue damage. However, the combination of bark stripping and fire reduced the ability of the stem to regrow bark, increasing the vulnerability of the exposed stem to boring insects and future fires. Fire manipulation experiments were used to identify the minimum stem diameter of resistance to fire. Marula resisted stem death when greater than 3.4 cm in basal diameter.This paper emphasizes the importance of both fire and herbivory in the development of woody plant population structure and by extension, the relative proportion of trees and grasses in savanna landscapes.
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.
The lanky and the corky: fire escape strategies in savanna woody species
Journal of Ecology
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). 2. 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). 3. 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 plantssupporting 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. 4. 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 heightbark-diameter scheme is a powerful framework for understanding the ecology of many savannas.
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.
Trees, grass, and fire in humid savannas-The importance of life history traits and spatial processes
We develop a model to investigate how trees can invade the grass stratum in humid savannas despite repeated fires. In the literature, it is clear that fire reduces tree canopy in savannas. However, fire alone may not be sufficient to prevent tree invasion because there are ecological mechanisms that hamper fire spread by undermining the continuity and density of the grass stratum, which is the means of fire propagation in savannas. Our model is spatially explicit and individual-based, and includes two important factors characterizing the interactions between fire, trees, and grass in savannas, viz. space and the strategies that trees use to cope with fire. The strategies that trees employ against fire emerge from life history traits. According to these strategies, we classify savanna trees into three categories: resprouters, which are able to resprout after their aboveground biomass is burned; resisters, which are able to resist fire due to a thick bark even in the juvenile stages; avoiders, which are very fire-vulnerable in the juvenile stages, but are able to grow fast in the absence of fire. Our results show that trees can invade the grass stratum and finally suppress fire spread because one of the following occurs: (1) trees may resprout and form a population that persists despite repeated effective fires; (2) trees may be fire-resistant; (3) if trees are fire-vulnerable they may cluster and grow in density until fire is prevented. Our results show that fire can be effective in preventing the initiation of the invasion process in the grass stratum. However, once the invasion process has begun, fire alone is not able to reverse this process because of the strategies employed by trees. Furthermore, when a high tree density is reached, grass density is insufficient to allow effective fire spread. From a management point of view, our results imply that fire must be coupled with other factors (browsing, mechanical thinning) to reduce tree density in encroached areas.
Which traits determine shifts in the abundance of tree species in a fire‐prone savanna?
2012
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. 2. 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. 3. 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. 4. 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. 5. 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.
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.
Effects of four decades of fire manipulation on woody vegetation structure in Savanna
Ecology, 2007
The amount of carbon stored in savannas represents a significant uncertainty in global carbon budgets, primarily because fire causes actual biomass to differ from potential biomass. We analyzed the structural response of woody plants to long-term experimental burning in savannas. The experiment uses a randomized block design to examine fire exclusion and the season and frequency of burn in 192 7-ha experimental plots located in four different savanna ecosystems. Although previous studies would lead us to expect tree density to respond to the fire regime, our results, obtained from four different savanna ecosystems, suggest that the density of woody individuals was unresponsive to fire. The relative dominance of small trees was, however, highly responsive to fire regime. The observed shift in the structure of tree populations has potentially large impacts on the carbon balance. However, the response of tree biomass to fire of the different savannas studied were different, making it d...
Juggling Carbon: Allocation Patterns of a Dominant Tree In a Fire-Prone Savanna
Oecologia, 2009
In frequently burnt mesic savannas, trees can get trapped into a cycle of surviving Wre-induced stem death (i.e. topkill) by resprouting, only to be topkilled again a year or two later. The ability of savanna saplings to resprout repeatedly after Wre is a key component of recent models of tree-grass coexistence in savannas. This study investigated the carbon allocation and biomass partitioning patterns that enable a dominant savanna tree, Acacia karroo, to survive frequent and repeated topkill. Root starch depletion and replenishment, foliage recovery and photosynthesis of burnt and unburnt plants were compared over the Wrst year after a burn. The concentration of starch in the roots of the burnt plants (0.08 § 0.01 g g ¡1) was half that of the unburnt plant (0.16 § 0.01 g g ¡1) at the end of the Wrst growing season after topkill. However, root starch reserves of the burnt plants were replenished over the dry season and matched that of unburnt plants within 1 year after topkill. The leaf area of resprouting plants recovered to match that of unburnt plants within 4-5 months after topkill. Shoot growth of resprouting plants was restricted to the Wrst few months of the wet season, whereas photosynthetic rates remained high into the dry season, allowing replenishment of root starch reserves. 14 C labeling showed that reserves were initially utilized for shoot growth after topkill. The rapid foliage recovery and the replenishment of reserves within a single year after topkill implies that A. karroo is well adapted to survive recurrent topkill and is poised to take advantage of unusually long Wre-free intervals to grow into adults. This paper provides some of the Wrst empirical evidence to explain how savanna trees in frequently burnt savannas are able to withstand frequent burning as juveniles and survive to become adults.