Structure, Long-Term Dynamics, and Demography of the Tree Community

Gignoux, Jacques; Menaut, Jean-Claude; Vuattoux, Roger

doi:10.1007/978-0-387-33857-6_18

Cited by 11 publications

(11 citation statements)

References 25 publications

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“…Depending on site‐specific characteristics (fire return interval, soil characteristics, browsing and grazing pressure), all values of tree cover, from zero to the climatic maximum, are achievable. This is supported by long‐term fire experiments (Gignoux et al ., 2006; Furley et al ., 2008) and models of local vegetation dynamics (e.g. Favier et al ., 2004; D'Odorico et al ., 2006).…”

Section: Introductionsupporting

confidence: 69%

Abrupt shifts in African savanna tree cover along a climatic gradient

Favier

Aleman

Brémond

et al. 2011

Global Ecology and Biogeography

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Aim.To describe patterns of tree cover in savannas over a climatic gradient and a range of spatial scales and test if there are identifiable climate-related mean structures, if tree cover always increases with water availability and if there is a continuous trend or a stepwise trend in tree cover. Location. Central Tropical Africa. Methods. We compared a new analysis of satellite tree cover data with botanical, phytogeographical and environmental data. Results. Along the climatic transect, six vegetation structures were distinguished according to their average tree cover, which can co-occur as mosaics. The resulting abrupt shifts in tree cover were not correlated to any shifts in either environmental variables or in tree species distributions. Main conclusions. A strong contrast appears between fine-scale variability in tree cover and coarse-scale structural states that are stable over several degrees of latitude. While climate parameters and species pools display a continuous evolution along the climatic gradient, these stable structural states have discontinuous transitions, resulting in regions containing mosaics of alternative stable states. Soils appear to have little effect inside the climatic stable state domains but a strong action on the location of the transitions. This indicates that savannas are patch dynamics systems, prone to feedbacks stabilizing their coarse-scale structure over wide ranges of environmental conditions. (Résumé d'auteur

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Section: Introductionsupporting

confidence: 69%

Abrupt shifts in African savanna tree cover along a climatic gradient

Favier

Aleman

Brémond

et al. 2011

Global Ecology and Biogeography

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“…At the wettest end of the climatic gradient where savannas can be found, fires are very frequent (they usually occur every year) and intense (Gignoux et al . 2006a; Russell‐Smith & Edwards 2006).…”

Section: Introductionmentioning

confidence: 99%

Establishment and early persistence of tree seedlings in an annually burned savanna

et al. 2009

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Summary 1.In severely disturbed habitats, the onset of resprouting as a persistence strategy might be problematic for tree species which do not accumulate sufficient reserves before the first disturbance event. This is due to the trade-off between the growth of reserves required to recover after disturbance and that of photosynthetic tissues. 2. In humid savannas, fire prevents trees from invading the whole landscape and nearby gallery forests have a completely different floristic composition. We test if the variations of survival during the first years of a young tree's life can explain the exclusion of forest species and the dominance pattern within savanna species. 3. Every six months for four years, we censused all seedlings and resprouts in 1 ha area of an annually burned savanna, to estimate their seasonal survival rates. We used capture-recapture statistical models to control for the probability of missing seedlings in the tall grass. 4. There were two main distinct patterns of survival among seedlings: 'fire-responding' species showed a 20-80% decrease in survival during the dry season, interpreted as mainly due to fire; 'drought-responding' species showed 20-80% variations in survival positively correlated to early-growing-season rainfall. 5. Yearly averaged survival probabilities of seedlings ranged between 0.10 and 0.63, reaching 0.850-0.996 for > 3-year-old resprouts of savanna species. Forest species showed no increase in survival with age. 6. A 4-year-survival-probability analysis showed that forest species were excluded from the savanna at the seedling stage. No parameter of the early survival curve related to the abundance of savanna species at the adult stage. 7. Synthesis . Savanna tree species follow two mutually exclusive main patterns of early survival probably related to fire and early-wet-season drought. The exclusion of forest species is consistent with a build up of reserves that is too slow due to the growth-resistance trade-off. We conclude from these findings that the use of resprouting as a persistence strategy is heavily constrained by disturbance frequency and imposes strong trade-offs on plant growth strategy.

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“…However, this relationship should also depend on tree distribution with lower values of α when trees are clumped and high values of α when they are randomly or regularly distributed. Although tree distribution can vary in different savanna ecosystems, it is often clustered (Barot et al, 1999; Gignoux et al, 2006). Similarly, we assume a positive relationship between γ and tree biomass T : the more trees, the more surface they occupy in the savanna.…”

Section: Methodsmentioning

confidence: 99%

Spatial heterogeneity of nitrification contributes to tree–grass coexistence in West African savannas

Konaré,

Koffi,

Boudsocq

et al. 2023

Journal of Ecology

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In savannas, the coexistence between trees and grasses is determined by complex mechanisms based on water partitioning and disturbances. But little is known about the contribution of other resources, such as soil nitrogen (N). In West African savannas, nitrification inhibition by grasses and nitrification stimulation by trees create spatial heterogeneity in nitrification fluxes and N stocks. Savanna trees can also extend part of their roots in the surrounding open area to absorb N. To investigate the role of the spatial heterogeneity of nitrification in tree–grass coexistence, we used a two‐patch model that simulates N dynamics between an open patch (without trees) and a tree clump patch (trees with grasses under their canopy). The open patch was characterized by a low nitrification rate, while the tree clump patch was characterized by a high nitrification rate. Both patches were connected through horizontal fluxes due to soil horizontal exploration by tree roots. We tested coexistence for different spatial tree distributions, as they are known to strongly influence savanna dynamics. Our results show that the spatial heterogeneity of nitrification induces spatial partitioning between ammonium (NH4+) and nitrate (NO3−) promoting tree–grass coexistence. As nitrification inhibition by grasses leads to high NH4+ availability in the open, the possibilities of coexistence are optimized when trees have different preferences in the open versus under their canopy. Thus, tree–grass coexistence is observed when grasses prefer NH4+, while trees prefer NH4+ in the open and NO3− under their canopy. Contrary to random tree distribution, tree clumping enhances tree–grass coexistence. Intraspecific aggregation strengthens the effect of spatial heterogeneity, which decreases interspecific competition and favours tree–grass coexistence. On the contrary, increasing the surface explored by tree roots in the open tends to increase tree–grass competition. This enhances the competitive ability of trees for N acquisition and consequently favours tree invasion. Synthesis. This study shows that this new coexistence mechanism based on mineral N partitioning into NH4+ and NO3− can be determinant in the functioning of West African humid savannas. This mechanism likely interacts with mechanisms based on disturbances, but such interactions should be studied using new models.

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Structure, Long-Term Dynamics, and Demography of the Tree Community

Cited by 11 publications

References 25 publications

Abrupt shifts in African savanna tree cover along a climatic gradient

Abrupt shifts in African savanna tree cover along a climatic gradient

Establishment and early persistence of tree seedlings in an annually burned savanna

Spatial heterogeneity of nitrification contributes to tree–grass coexistence in West African savannas

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