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

Azihou A.F., Glele Kakaï R.L., Bellefontaine R., Sinsin B.A.. 2013. Journal of Tropical Ecology, 29 (1) : p. 25-37.

DOI: 10.1017/S0266467412000727

Savannas intermingled with gallery forests are dynamic habitats typical in Africa. This study aims to determine if differences in species traits lead to non-overlapping distribution of gallery-forest and savanna species and abrupt transition between gallery forest and savanna. Tree species densities were measured in 375 plots of 1500 m2 covering a total sample area of 56.25 ha along forty 3-km transects located at right angles to a riverbed with gallery forest into surrounding savanna. Location, vegetation type, soil physical properties, erosion and fire occurrence were recorded as site factors. Data analysis included the quantification of co-occurrence patterns, threshold indicator taxa analysis and fuzzy set ordination. The gallery forest-savanna gradient predicted floristic composition of plots with a correlation of 0.595 but its accuracy was locally modified by the occurrence of fire and the physical properties of soil that coveredmore than 30% of the range of residuals. The distribution of gallery-forest and savanna tree species did not overlap. Along the gallery forest-savanna gradient, savanna species gradually increased in density while gallery-forest species showed a community threshold at 120 m from the river beyond the width of gallery forest. The forest species driving this trend should play an important role in the dynamics of gallery forest-savanna boundaries.

Mots-clés : arbre forestier; espèce; savane; distribution spatiale; espacement; inventaire forestier; écologie forestière; bénin; forêt galerie

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