In order to better understand species distribution and structure in savanna, we examined the patterns of vegetation and how they are related to environmental conditions particularly rainfall in African sub-sahelian savanna. Twelve plots of one hectare each have been set up in wet and dry sites-savanna, in the central and north of Benin, West Africa. Girth at breast height GBH, height, and crown size of individual have been recorded for individuals occurring in established plots and visited two times. A total of 3781 of individuals GBH>15cm distributed in 70 species and 34 families have been recorded. Species composition varies from site to site suggested that many savanna species have preference sites. Although the composition of species is different, the skewness and Shannon-wiener index were not different from wet to dry sites. The maximum height was negatively correlated with coefficient of skewness in savanna.
Analysis of species per canopy position revealed that canopy species had negative skewness with a large number of adults and long tail of juveniles whereas understorey species had positive skewness with a large numbers of juveniles and a long tail of rare adults. This implies that savanna demography is limited for canopy species. The multivariate analysis of species showed three groups in savanna dominated by Isoberlinia doka (Caesalpiniaceae) where the maximum diameter is 89.38 cm and the maximum height is 20.5 m. The first group (abundant) of species was distributed irrespective of sites. The second group (frequent species) was with dominance in wet plots whereas the third group (rare species) was dominant in dry plots.
The detrended correspondence analysis (DCA) applied to datasets from species and plots, showed that both species and plots were dispatching on wet and dry plots. These results suggested that rainfall is playing a key role in savanna areas. Allometry differences of DBH, crown depth and crown width in dry and wet sites supported the previous results. Consequently, the assumption that rainfall is one of the most important factors determining tree species in West African forests has been confirmed. Distribution patterns in sub-sahelian savanna in Benin are dependent on rainfall as a main driver.