Vegetation, climate and soil: Altitudinal relationships on the East Usambara Mountains, Tanzania

Abstract

The floristic composition of forest on the East Usambara Mountains, Tanzania, changes steadily with increasing altitude. Conveniently, two altitudinal zones (lowland, submontane) can be recognised; the altitude between them lying at about 850 m, which is 550 m lower than the equivalent zonal boundary in central Africa. This depression is probably related to lower daytime temperatures, which are 4-5°C lower than in central Africa-attributed to the presence of persistent low-lying cloud at higher altitudes. There is a marked change in topsoil at about 850 m, with a sharp fall in pH and the presence of a thick mor-humus layer at higher altitudes. The vegetation/climate/soil system is dynamic. There is evidence of upward movement of vegetation zones and a warmer and less misty climate over the last 25 years. The mor¬humus layer is lost in tree-fall clearings and under the invasive tree Maesopsis eminii; in the latter case (at least) topsoil pH acidity is over a pH point higher. This is a good site for further investigations of climatic, vegetation and soil changes. Forest persistence during the last ice age (assumed from the large number of endemic species) was probably facilitated by an even mistier (though probably otherwise drier) climate.

Journal of East African Natural History Vol. 87 (1&2) 1998: pp. 85-89