Phytosociology of undergrowth vegetation and ecological impact of forest plantations on floristic diversity in southern and Central Benin
DJEGO Julien (2007)
PhD, University of Abomey-Calavi
Floristic, ecological and phytosociological studies of the undergrowth of exotic tree plantations and natural forests were undertaken in southern and central Benin. These studies dealt with the impacts of exotic species plantations on plant diversity, as well as on litter fall and its decomposition in the undergrowth.
Between 2000 and 2004, 252 phytosociological relevés, 537 species and corresponding measurement and analysis identified 21 environmental variables. Releves were done according to the Sigmatist method of Braun-Blanquet (1932).
Monte Carlo’s test was used to select following environmental variables as having a significant effect on undergrowth vegetation: pH, relative humidity, vegetation type, litter weight and soil type. The latter variable created a specific microclimate for some species, which developed and constituted a well-defined vegetation association. The development of a floristic identity for undergrowth vegetation was dependant on the interaction of some of the above variables.
Detrended Correspondence Analysis (DCA) and cluster analysis of relevés allowed the identification of 11 associations, distributed along a latitudinal, pedological and phytogeographical gradient.
From the viewpoint of adaptative forms and phytogeography, the phanerophytes were dominant in dense forests and exotic tree plantations; while in fallows, forbs were dominant. Natural, dense forests were predominated by Guineo-Congolese base-element species; those with a wide geographical dispersion dominated fallows and exotic tree plantations.
With respect to eco-sociological groups, dense forest species of the Strombosio-Parinarietea class dominated natural forests. Those of the secondary forests class dominated exotic tree plantations and those of the class of Soncho-Bidentetea pilosae and Ruderali-Manihotetea, were dominant in fallows.
Dissemination forms were dominated by the sarcochores. Through human activities, this tendency of predominance of the sarcochores has been modified to the benefit of anemochores (sclerochores and pogonochores) and even autochores (barochores). This human impact is not without influence on the biological diversity of the area.
The annual average of litter production varied from 6.2 to 9 tons/ha in natural forests and 4.3 tons/ha in exotic tree plantations. It can be adjusted to an unimodal distribution and varied significantly according to species richness of woody stratum (p < 0,05). Leaves constitute the largest fraction (57 to 91 %) of the litter fall. The loss of weight of the litter best fits a highly significant (P < 0.001) polynomial function with a R² close to 1. The mass of ground litter, the recovery and the species richness of the tree stratum, the temperature and the humidity are highly significant (p < 0.05) environmental variables for litter decomposition.
Ecological effects of exotic tree species of the study zone were noticeable on the ground and on the floristic composition of the undergrowth. These varied according to the species used for reforestation, the age of the plantation, tree density and station recovery. Tree plantations were poor in indigenous species and housed an undergrowth flora quite different from the native one surrounding the plantations. The dominance of long-range distribution species was an indicator of degradation and progressive loss of the local flora. Abundant litter, its slow decomposition, and the reduced light intensity in closed plantations, prevent development of undergrowth vegetation. Eucalyptus camaldulensis plantations influenced the flora diversity negatively through soil acidification. Conversely, Casuarina equisetifolia improved the pH of the ground and increased the cation exchange capacity. Casuarina equisetifolia alone or in association with Acacia auriculiformis increased floristic diversity of the undergrowth. Exotic trees impacted the plant diversity of the study zone negatively. Sustainable management of tree plantations should be further developed to insure the conservation of undergrowth resources.