Effects of abiotic and biotic factors on the regeneration of the threatened Afzelia africana Sm. ex Pers. (Fabaceae-Caesalpinioideae) in the Pendjari Biosphere Reserve (Benin, West Africa)
Afzelia africana (Fabaceae-Caesalpinioideae) is a woody spontaneous tree species found in forests and more in African savannahs. The species provides a very good quality of timber and rapidly increases woodworkers’ income, what threatnes the species by overexploitation in West Africa. All being equal, the species undergoes high pressure in unprotected area and were declared since two decades, endangered in Benin. Contrary with our expectation, A. africana early life stages’ transition to adult life stages is not guarantee even in protected area. If anthropogenic factors were indexed to be the main factors of the species decline in open country, what are the biotic and abiotic factors that threatenes the species in natural protected area? Knowledge of these key biotic and abiotic factors is fundamental for effective management and restoration actions and policies. The main objective of this study is to contribute to the threatened A. africana management and restoration in Benin. This research associated either existing literature in the focus field, population demographic structure, spatial pattern, seedling survival and growth trend to: (i) assess the heterospecific tree density and environmental factors that affect A. africana stands structure in the Pendjari Biosphere Reserve, (ii) establish the spatial patterns of A. africana conspecific and hetereospecific trees using a comparative life-stage based point pattern process analysis in the buffer and the core zones of the Pendjari Biosphere Reserve (iii) identify the biotic and abiotic factors affecting seedling-survival of the endangered A africana in the Pendjari Biosphere Reserve (iv) assess the seasonal variation effects in growth and foliage production of A. africana seedlings in the Pendjari Biosphere Reserve. Based on several calculated structural parameters, linear models were used to test for individual effects of abiotic and biotic factors on the species stands. We also tested for similarity of species composition among A. africana study stands using NMDS, ANOSIM and IVI. Findings showed that, the stands of A. africana exhibited in the Pendjari Biosphere Reserve a tree density structure with over-mature individuals due to a bottleneck at the juvenile stages. Whereas the heterospecific trees showed elsewhere positive correlation, their density was negatively related to A. africana mean diameter, basal area and Lorey height. Rock outcrop soils were convenable to A. africana basal area and regeneration development. The distribution pattern of A. africana with its conspecific and with its heterospecific trees was established using univariate and bivariate pair correlation g functions. Distance to the nearest neighbour was also used to assess tree-to-tree spacing. In the Pendjari Biosphere Reserve the spatial distribution pattern of A. africana is related to the life stages and the buffer and the core zones. Seedlings, saplings, small adults and large adults are aggregative in the buffer zone of Batia but randomly distributed in the core zone of Bali versus saplings, juveniles and very large adults which are randomly distributed in the Pendjari Biosphere Reserve. Seedlings are respectively well closer to very large adults, large adults and small adults versus saplings, juveniles and heterospecific trees which are randomly distributed with the all other life stages of A. africana in the buffer and the core zone. Seedlings survival probability and survival rate were calculated and used as response variables in a Generalized Linear Mixed Modeling (GLMM). A. africana seedling survival is at individual level negatively correlated with dry season but positively by the seedling intrinsic morphologic characters. At plot level, the seedling survival was adult conspecific-negative densitydependent and also undergo steeper soil slope negative effect. KruskalWallis test was performed to assign season change effects on seedlings growth parameters and allometric relationships were established and tested on these pair growth parameters. A. africana seedlings exhibited a none leaves production in dry season but a better growth trend when relationship is established between height-basal diameter and heightnumber of leaves versus a weak growth trend when relationship is established between basal diameter-number of leaves. These findings implications in management and restoration way, are widely discussed in this dissertation and future research avenues are suggested.
Keywords. Afzelia africana, management and restoration, abiotic and biotic factors, random/independence and aggregative/positive association pattern, adult-conspecific-negative-density-dependant, Benin.
Justin Akpovi ATANASSO , Cel (229) 66 44 98 56
Contact details: LABEF – Laboratoire de Biomathématique et d’Estimations Forestières, Faculty of Agronomic Sciences, University of Abomey-Calavi., 04 BP 1525 Cotonou, Benin Phone: (+229) 95 152 599 / (+229) 66 449 856, Email: firstname.lastname@example.org