Distribution of tree species along a gallery forest–savanna gradient: patterns, overlaps and ecological thresholds

Azihou, Akomian Fortuné; Kakaï, Romain Lucas Glèlè; Bellefontaine, Ronald; Sinsin, Brice

doi:10.1017/s0266467412000727

Cited by 35 publications

(21 citation statements)

References 50 publications

(101 reference statements)

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“…), we observed no gallery forest effect (compare Azihou et al. ). Lithraea caustica seemed better able to retain soil moisture under its own canopy up to a week after rain, which might favor the germination of seeds requiring higher soil moisture to grow deeper root systems, or might be advantageous in particularly dry areas such as rain shadows or the northern limit of espinal distribution, if the increased shade is tolerated (compare Badano et al.…”

Section: Discussionsupporting

confidence: 49%

Acacia caven nurses endemic sclerophyllous trees along a successional pathway from silvopastoral savanna to forest

Root‐Bernstein

Valenzuela²,

Huerta³

et al. 2017

Ecosphere

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Citation: Root-Bernstein, M., R. Valenzuela, M. Huerta, J. Armesto, and F. Jaksic. 2017. Acacia caven nurses endemic sclerophyllous trees along a successional pathway from silvopastoral savanna to forest. Ecosphere 8(2):e01667. 10.1002/ecs2.1667Abstract. The successional pathways linking the Acacia caven-dominated savanna habitat "espinal" and the closed sclerophyllous forest of central Chile have long been debated. Previously, espinal was considered an invasive degradation of closed forest that tended toward desertification, could not be restored to forest, and had little ecological value. Recent GIS (Geographical Information System) research on land-use change has, however, detected apparent regeneration of sclerophyllous forest from espinal. This suggests that there is a successional path linking espinal and sclerophyllous forest. Here, we used surveys of transects in espinals and espinal-sclerophyllous forest transitions to ask whether (1) A. caven is an invasive species or a pioneer species, (2) forest regenerates by sclerophyllous trees "filling in" spaces between A. caven individuals, and then shading them out (plant-plant competition), or (3) forest regenerates by plant-plant facilitation between A. caven and other species, and (4) how current and historical management and condition affect these potential successional mechanisms. We find that A. caven establishes primarily in full sunlight and is unlikely to degrade forests via invasion. We also find, for the first time, evidence that A. caven is a nurse tree to several sclerophyllous forest tree-beneficiary species. Measurements of the under-canopy microhabitat of A. caven, compared to Lithraea caustica, another possible nurse species, suggest that it provides a balance between shade and soil moisture retention, making it a regeneration site not only for directed bird-dispersed seeds but also for undirected wind-dispersed ones. Conservation and restoration of espinals, especially in drier areas, could provide capacity for future dynamic successional pathways in central Chile.

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Section: Discussionsupporting

confidence: 49%

Acacia caven nurses endemic sclerophyllous trees along a successional pathway from silvopastoral savanna to forest

Root‐Bernstein

Valenzuela²,

Huerta³

et al. 2017

Ecosphere

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“…Fire has long been proposed as a key limit on the success of tree establishment in savannas (Scholes and Archer 1997, Bond 2008, Staver et al 2011, Murphy and Bowman 2012, where the vulnerability of tree seedlings to fire is expected to play a large role in maintaining diversity and codominance of trees and grasses in these ecosystems through nonequilibrium, demographic-bottleneck models (Higgins et al 2000, Sankaran et al 2004. Previous work in our study region has documented low seedling to sapling transitions in many gallery forest species, including K. senegalensis (Azihou et al 2013b), and has suggested that fire may play a key role in restricting these species to gallery forest in dry regions (Azihou et al 2013a). Consistent with these predictions, the extent of burning was a component of the final model for all response variables tested here except sapling survival, underscoring the importance of fire in driving seedling dynamics in this mesic savanna system.…”

Section: Facilitation Fire and Savanna Dynamicsmentioning

confidence: 79%

Defoliation by pastoralists affects savanna tree seedling dynamics by limiting the facilitative role of canopy cover

Bufford¹,

Gaoue²

2015

Ecological Applications

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Recurrent tree defoliation by pastoralists, akin to herbivory, can negatively affect plant reproduction and population dynamics. However, our understanding of the indirect role of defoliation in seedling recruitment and tree-grass dynamics in tropical savanna is limited. In West African savanna, Fulani pastoralists frequently defoliate several fodder tree species to feed livestock in the dry season. We investigated the direct and indirect effects of recurrent defoliation of African mahogany (Khaya senegalensis) by Fulani people on seedling (< 2 cm basal diameter) and sapling dynamics in West Africa using four years of demographic data on seedling and sapling density, growth, and survival, coupled with fruit production and microhabitat data over the same time period. Tree canopy cover facilitated seedlings but had negative effects on sapling growth possibly via intraspecific competition with adult plants. Interspecific competition with grasses strongly reduced seedling survival but had a weak effect on sapling growth. Fire reduced seedling survival and weakly reduced growth of seedlings and saplings, but did not affect sapling survival. These results indicate that the effect of fire on seedlings and saplings is distinct, a mechanism suitable for an episodic recruitment of seedlings into the sapling stage and consistent with predictions from the demographic bottleneck model. Defoliation affected seedling density and sapling growth through changes in canopy cover, but had no effect on seedling growth and sapling survival. In the moist region, sapling density was higher in sites with low-intensity defoliation, indicating that defoliation may strengthen the tree recruitment bottleneck. Our study suggests that large-scale defoliation can alter the facilitative role of nurse trees on seedling dynamics and tree-sapling competition. Given that tree defoliation by local people is a widespread activity throughout savanna-forest systems in West Africa, it has the potential to affect tree-grass coexistence. Incorporating the influence of large tree defoliation into existing models of savanna dynamics can further our understanding of tree-grass coexistence and improve management. A rotating harvest system, which allows seedlings to recruit episodically, or a patchwork harvest, which maintains some nursery trees in the mosaic, could help sustain seedling recruitment and minimize the indirect effects of harvest.

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“…The fact that the riparian woody density is lower in FA would result in much drier forests due to the increase in the surface exposure of the riparian area for soil moisture loss. This can increase the vulnerability and frequency of the RF to savanna fires (Azihou et al, 2013). Such fires can break the resilience of the riparian ecosystem to intensify climate change impacts to such a degree that species physiological tolerances can be exceeded and the rates of biophysical forest processes altered.…”

Section: Changes In Riparian Woody Species Density Basal Area and Simentioning

confidence: 99%

“…However, certain outcomes of climate change can be mitigated by maintaining intact riparian landscapes-including retaining soil moisture, regulation of localized fire, and provision of refuge and dispersal corridors for fauna (Goetze et al, 2006;Azihou et al, 2013). Ecologically, riparian forests (RF) are important as they protect farmlands from flooding, drying and sedimentation (Gray et al, 2014).…”

Section: Introductionmentioning

confidence: 99%

Threat of agricultural production on woody plant diversity in Tankwidi riparian buffer in the Sudanian Savanna of Ghana

Boakye¹,

Hyppolite²,

Barnes³

et al. 2015

Int. J. Biodivers. Conserv.

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Riparian forest buffers (RF) are integrative part of the savanna agricultural landscape. However, they are under threat of deforestation from agricultural intensification. To ascertain the impact of the deforestation, this study used remote sensing techniques and field inventorying to assess riparian woody plant diversity on farmland (FA) and forest reserve (FR) along Tankwidi rivercourse in the Sudanian savanna of Ghana. Post-classification analysis of Landsat images revealed a reduction in forest cover from 1986 (23%) to 2014 (7%) in the river basin. Ground survey of sixty randomly selected plots (500 m 2 per plot) equally divided between FA and FR along the river in a 50 m buffer zone showed a reduction in the number of woody species (diameter ≥ 5 cm) from FR (40) to FA (19). Anogeissus leiocarpus and Mitragyna inermis were the most abundant species in both FR and FA. Shannon-Wiener Index for species diversity reduced from FR (2.5±0.09) to FA (1.8±0.14). Within FR, there were more species (58%) in the lower diameter class (5 to 15 cm) than the higher diameter classes (15 to 50 cm) suggesting successful regeneration. The reverse was observed in FA where the individuals in the lower diameter class were fewer (26%) than the higher diameter classes. Reduction in species density from FR (355±21) to FA (146±11) will increase the surface exposure of the riparian area in farmland to heighten risks to climate disasters such as fires and flooding. Managing the risks will not be possible unless a conscious effort is made to educate farmers on the roles of RF, replanted to enhance diversity or riparian buffer excluded from farming for vegetation recovery.

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Distribution of tree species along a gallery forest–savanna gradient: patterns, overlaps and ecological thresholds

Cited by 35 publications

References 50 publications

Acacia caven nurses endemic sclerophyllous trees along a successional pathway from silvopastoral savanna to forest

Acacia caven nurses endemic sclerophyllous trees along a successional pathway from silvopastoral savanna to forest

Defoliation by pastoralists affects savanna tree seedling dynamics by limiting the facilitative role of canopy cover

Threat of agricultural production on woody plant diversity in Tankwidi riparian buffer in the Sudanian Savanna of Ghana

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