Woody plant phenology in the West Africa savanna

In the savanna of West Africa the seasonality of rainfall, with a drought p eriod of at least four months, strongly influences the vegetation. Rainfall is a very critical abiotic variable and therefore plant species must be we ll adapted to survive in this habitat. In our research, phenological patterns of 120 woody plant species have been examined based on the presence of green leaves. According to the patterns found, these species can be classified in phenological groups, which repres ent different strategies for survival. Two extreme strategies are found to resist drought: (1) by using the waterstorage in the deeper soil layers and river beds and by restrictings drought-damage through scleromorphic featur es, and (2) by avoiding the drought through foliage shedding in the dry per iod. The first strategy is represented by the riparian and upland evergreens, an d the semi-evergreens. The evergreens bear leaves the whole year, gradually replacing old leaves by new ones. The riparian evergreens are strictly bou nd to riverbeds and grow in or immediately adjacent to them. The semi-everg reens shed their leaves and start sprouting during a short period (one-two weeks) once a year. Because the evergreens and the semi-evergreens are in l eaf in the dry period they have to protect themselves to drought damage by scleromorphic features. Contrary to these species are the deciduous species which are bare for at l east some months per year. When the dry season starts their leaves dry out and are subsequently shed. They start sprouting before or at the beginning of the first rains. Although much less in number, some deciduous trees also have scleromorphic features to resist drought-damage. The strategy of spro uting just before the rainy season begins indicates that certain water reso urces remain available to these deep-rooting woody plants throughout the ye ar, providing them with a fully operating photosynthetic apparatus when fav ourable conditions arrive.