Field Guide to the Moist Forest Trees of Tanzania
Jon C. Lovett, Chris K. Ruffo, Roy E. Gereau & James R.D. Taplin
Illustrations by Line Sørensen & Jilly Lovett
The forests of Tanzania can be divided on the basis of geographical location, altitude, moister gradients and successional stage. Geographical divisions are based on geology and climate. Coastal, Eastern Arc and Northern forests are under the direct climatic influence of the Indian Ocean, but Coastal forests are predominantly on sedimentary rocks, the Eastern Arc are on igneous and metamorphic rocks, and Northern forests are predominantly on volcanic areas (with the exception of the Mbulu highlands). Forests associated with the great lakes of Victoria, Tanganyika and Nyasa are subject to environmental fluctuations associated with variation in the local climates associated with these lakes (Figure 1).
Altitudinal divisions of the forests are arbitrary as, in the absence of disturbance, there is a continuous turnover of species with elevation. The terms used here are Lowland, Submontane, Montane and Upper Montane. Moisture availability is also an important determinant of species distribution. We use the terms Riverine and Groundwater for species that grow by rivers or on a high water table in areas otherwise too dry to support forest. Some species will also grow outside the closed canopy forest environment in open Grassland, which has very few trees, or Woodland, which has an open canopy and grassy understorey, or in Thicket, which is a dense scrub with emergent trees. In closed canopy forest we recognise Dry Lowland and Dry Montane forest types. Tree species in the drier forest types often have a wide elevational range. Approximate elevational ranges and rainfall for the different forest types recognised in the Eastern Arc are given in Table 1. Equivalent forest types are at higher elevations in western and central Tanzania (i.e. around Lake Victoria and Lake Tanganyika) because temperatures are higher compared to the eastern coast.
Disturbance is an important determinant of species composition. Some species are pioneers which become established following disturbance. These species tend to be geographically widespread, occur over a wide altitudinal range and have broad ecological tolerances. Examples include Bridelia micrantha, Harungana madagascariensis and Macaranga capensis. When forests are ecologically dynamic, then they are composed predominately of pioneer species. Fire can be an agent of disturbance and result in particular particular forest types such as those dominanted by Juniperus procera and Hagenia abyssinica. More generally though, fire results in forest being converted to grassland with occasional relictual forest trees. There is also evidence of the reverse taking place. In some forests moribund individuals of fire resistant species such as Agarista (Agauria) salicifolia are found under a closed evergreen canopy, they appear to be relicts of more open conditions.