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Biological and phytogeographical analysis of plant communities infested by Chromolaena odorata and Hyptis suaveolens in the Betecoucou region (Benin) Analyse biologique et phytogéographique des savanes colonisées par Chromolaena odorata et Hyptis suaveolens dans la région de Bétécoucou (Bénin)
Abstract
The paper presents phytosociological analyses of savannah vegetation infested by Chromolaena odorata and Hyptis suaveolens in the Betecoucou region of the Guinea-Congolia/Sudania regional transition zone of Benin. Some 79 phytosociological plots were analysed according to the Braun-Blanquet method. A total of 199 species of vascular plants have been recorded belonging to 53 families. Hemicryptophytes (4.8%) and geophytes (3.9%) are less abundant in the colonised sites than in the non-colonised ones. The colonised sites are dominated by species that are therophytes (50.8%) or chamaephytes (39.9%). Species that are widely distributed (45.9%), mainly pantropical (26.7%) and paleotropical (18.3%) in distribution, are also more frequent in infested than in non-infested sites. On the contrary, African pluriregional elements, e.g. Sudano-Zambezian species (44.4%) and afrotropical species (16.6%) are more frequent in non-infested than in infested sites. Among the widely distributed species, those that are ruderal or nitrophilous are the most frequent. © 2009 National Botanic Garden of Belgium Permission for use must always be obtained from the National Botanic Garden of Belgium.
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Six inselbergs of Benin have been studied in four localities. This study presents the results of 123 phytosociological "relevés". A total number of 285 plant species have been gathered belonging to 77 families, of which Poaceae (34 species) and Fabaceae (32 species) are the most representative. The 285 plant species identified have been classified in seven groups according to the vegetation types. The analysis of life-form spectra has revealed the abundance of therophytes (38.2% and 36.5%) and phanerophytes (39. 7% and 37.2%). The grasslands are dominated by annual species. The number of therophytes increases from south to north; this could be due to decrease of average annual precipitation. The phytogeographical spectrum of species have revealed the abundance of sudano-zambezian elements (21.4% and 23.9%) and African pluriregional elements (21.4% and 21.6%). No endemic species have been noted among the 285 species. The two phanerophytes most frequently encountered and restricted to the inselbergs are Hymenodictyon floribundum and Ficus abutilifolia. The most characteristical species of dry grasslands is Afrotrilepis pilosa. Inselberg vegetation is characterized by its homogeneousness due to harsh environmental conditions.
Woodland occurs in the Sudanian region of northern Benin where the tree stratum is dominated by Isoberlinia tomentosa or Monotes kerstingii at about 10°N. The objective of this study is to characterize floristic composition of herbaceous strata of woodland on the view point of life forms and species geographical distribution. Data were collected in relevé plots of 400 m2 that had permitted to distinguish three plant associations. Species richness varied from 48 to 67 in the herbaceous stratum compared to 17 or 18 tree species per 1000 m2 in tree layer. Therophyte species were the most abundant (34 to 44 %), but according to herbaceous strata structure defined by biovolume, hemicryptophyte species were the most abundant (77 to 94 %). According to species distribution the base-element abundance varied from one association to another. The abundance of the base-element was low in disturbed stations due to the impact, when abundance of large spreading species was higher. Phanerophyte species abundance was higher among the base-element species than therophyte and geophyte species. While considering widely distributed species, therophyte species abundance was the highest (54 to 63 %). This study shows that better of cattle grazing better understanding can be obtained of species richness and ecological patterns in Sudanian woodland using the abundance figures of respectively therophytic and phanerophytic species.
Invasiveness of two exotic species, Chromolaena odorata (Asteraceae) and Hyptis suaveolens (Lamiaceae), in relation with land use around Bétécoucou (Bénin). Very few studies have been conducted on the effects of farming systems on the invasive capacity of exotic species. This study fi{dotless}lls this gap by analysing the invasive capacity of two exotic species, Chromolaena odorata and Hyptis suaveolens, in farming systems in the Guineo-Congolian/Sudanian transition zone of Benin. The fi{dotless}ndings show that the annual seed yield of C. odorata (45 151 seeds/plant) is signifi{dotless}cantly higher than that of H. suaveolens (1 782 seeds/plant). On the other hand, seed germination rate of H. suaveolens (70%) was higher than that of C. odorata (30%). Seed production and population density of C. odorata were higher in fallows and tree-savannah than in deforested savannah and croplands. H. suaveolens seed production and population density were higher in fallows and degraded savannahs. Bush fi{dotless}re reduced plant growth, the amount of seeds annually produced, and the seed germination rate. This study shows that the invasion intensity is related to the farming system. These two exotic species unfavourably altered the vegetation structure available to livestock by favouring the establishment non-forage species.
A discussion is presented of efficient strategies for species preservationin spite of continued human alteration of the environment. Current policy and unresolved questions are included in the discussion. Incentives to maintain seminatural areas as a conservation strategy are recommended: planting of hedgerows or windbreaks to provide corridors for migration of species during climate change; purchase of development rights of natural and seminatural land for conversion to park reserves when climate stabilizes; use of intercropping, traditional forest gardens and crop plantings in the tropics; and maintenance of seminatural habitats on public and private lands.