Introduction

In Africa, the term dry forest covers vegetation types dominated by a more or less continuous tree cover (70%), experiencing pronounced drought during more than three months per year, and occurring within the savanna biome. They may be called (open) woodlands or (dense) dry forests according to tree density and understory structure (Menaut 1983). The Yangambi classification establishes the following (Boughey, 1957a, b; Monod, 1963; Aubréville, 1965).

• A woodland has an upper stratum of deciduous trees of small or medium size, with their crowns more or less touching above a sparse woody understory. Tree density is high enough to affect the herbaceous stratum which differs floristically from the adjacent savanna. The ground layer consists of grasses, herbs and suffrutescent plants in sufficient density to allow for annual burnings. The canopy of a woodland tends to be dominated by one or very few species.

• A dry forest, strictly speaking, is defined as a closed stand with several woody strata. The grass layer, when present, is weak and discontinuous, only allowing for episodic and sparse fires. In most cases, the trees of the upper stratum are deciduous whereas the understory is composed of evergreen and/or deciduous shrubs which differ from the canopy floristically. The canopy is multispecific and often devoid of woodland dominants. In both dry forest and woodland, the tree species which make up most of the canopy are present but never dominant in the surrounding savanna.

Some authors have considered dry forests to be a tropophilous extension of the rain forest, with adaptations to xeric conditions in characteristics of stems but not of leaves (Schnell, 1976–7).