Structure des peuplements ligneux dans les plantations d’Acacia senegal (L.) Willd dans la zone de Dahra (Ferlo, Sénégal)

Abstract

Woody stand structure in plantations of Acacia senegal (L.) Willd in Dahra area (Ferlo, Senegal). The present study gives an up-to-date inventory of natural woody stands in plantations of Acacia senegal (L.) Willd in Ferlo. It aims at determining the structure, specific composition and regeneration of woody stands in plantations of Acacia senegal. Four sites were retained for study : Acacia senegal plantations Ndodj 2001, Isra 1999, Boulal 2004 and Déali 2005, distributed along an increasing pluviometric gradient, with different topographic characteristics and protection status. Woody stands were characterized by dendrometric measurements. Woody plants belong to 12 species representing 10 genera and 7 families. The very high mortality rate observed on dunes would be linked to poor water availability. Regeneration appeared significant in hollows (Isra), especially concerning Boscia senegalensis (Pers.) Lam and Balanites aegyptiaca (L.) Del. It was also relatively important at Boulal where livestock is rather infrequent, as in Isra. On the other hand in Déali, regeneration was important at the top of the dunes with Calotropis procera Ait. and weak in hollows, areas frequented by the cattle. Compared to Ndodj, Boulal was characterized by a relatively high regeneration. This situation could be explained by the fact that Isra and Boulal are more protected than Déali which is much attended by the cattle. In Ndodj, we noted that plants are subject to attacks by termites what explains the high mortality rates observed in this site. Balanites aegyptiaca, Boscia senegalensis and Calotropis procera are at the origin of the increase of the number of individuals in the stand, whereas Acacia senegal shows a weak regeneration. This weak regeneration of Acacia senegal would be linked to the systematic gathering and collecting of fruits, abundantly used to feed domestic animals, but also at certain places to stripping and repetitive bleedings. As far as plant circumference and height are concerned, the observed distribution was unimodal for the shrub layer, except in hollows at Isra where it was bimodal in the shrubby as well as the arborescent layers. Field observations and obtained results show that floristic variability was more marked at Isra than at Déali (strong pressure of the cattle). The structure of the woody stand appeared strongly dependent on topography, moisture gradient and anthropic pressure.