The African rain forest during the Last Glacial Maximum an archipelago of forests in a sea of grass

In centralGabon(Africa) the distribution ecology of the Caesalpinioideae tree species (Leguminosae) were studied in two separate low rainfall areas which straddled the same geological transition from upland to lowland. The principle environmental force arranging their distribution was regional drought stress during the two dry seasons. These drought sensitive tree species were able to survive in the two forest-savannah mosaics because local compensation was provided by the permanent presence of water along some of the streams and by reduced exposure (shade) in narrow valleys. According to the Pleistocene forest refuge theory the Caesalpinioideae in the two study areas arrived during the last 10,000 years (Holocene) from the closest forest refugium- the Chaillu Massif (80 km further south). This part ofGabondid not become deforested during the Last Glacial Maximum when climate was much drier because cloud forest conditions in this elevated area compensated for the shortage of rainfall. Arrival of the Caesalpinioideae trees by migration from this refuge area by their normal dispersal mechanism of explosively opening pods is excluded as their advance over the Holocene was only 36 km. Also arrival through occasional long-distance dispersal events by water along watercourses is not viable, because species composition along streams does not show a decreasing gradient with distance from the refuge area. Long-distance dispersal events in the past now obscured by an equilibrium situation of species distributions with present-day habitat is also not possible as the similar hills and lowland in the two study sites are inhabited by very different species. The only conclusion is that the Caesalpinioideae survived the Last Glacial Maximum in the study areas themselves. The present-day general presence of these Caesalpinioideae in the African rain forest between the former forest refugia shows that small pockets of forest persisted in the then savannah.