Soil properties and fast-growing tree species in Tanzania

Abstract

This study examined growth rates and soil conditions of 11–14-year-old forest plantations and adjacent unmanaged natural forests at three climatically and geologically distinct sites in Tanzania. The sites differ markedly in growth rates, soil conditions and potential for management. At the Uluguru Forest Reserve on quartzo-feldspathic granulite and mica with average rainfall of 1058–1377 mm y −1, regenerating evergreen forest and a plantation of Acacia mearnsii have grown at 2.6–7.2 and 3.5 m 3 ha −1 y −1, respectively. At the East Chenene Forest Reserve on granite where rainfall averages 573–625 mm y −1, mature dry woodland and plantations of Cassia siamea and Eucalyptus citriodora have grown at 0.2-2.0 and 1.4-2.2 m 3 ha −1 y −1, respectively. Fertility of the acid soils at these sites depends on litterfall and organic matter decomposition, and it declines when leaching increases after forest clearing. At the Sambasha Forest Reserve on volcanic ash soils with average rainfall of 1200–1800 mm y −1, mature evergreen forest and a plantation of Cupressus lusitanica have grown at 2–9 and up to 15 m 3 ha −1 y −1, respectively. Soil fertility here is much less dependent on organic matter additions. In forest management where neither fertilizer nor irrigation is used, inherent soil fertility has a marked effect on growth. Growth rates in this study are related to rainfall and soil nutrient status at each site, with the fastest growth occurring on wet, recent volcanic soil (Sambasha). The slowest growth was measured at the dry site (East Chenene) where soils are developed from nutrient-poor acid rock. At their present slow rates of growth, plantations will not make a significant contribution in the near future to alleviating deforestation and related scarcity of forest products in semiarid regions of Tanzania. Four actions are urgently needed: (1) identification of the best sites for tree plantations; (2) identification of the most promising exotic and indigenous species; (3) identification of the constraints on growth of promising species; and (4) formulation of economically and physically feasible management techniques to overcome constraints to growth. These can be achieved through increased use of soil surveys; establishment of comparative field trials with varying site/species combinations; recognition of the limited potential of species that require intensive management by local people; and exploitation of the inherent advantages of indigenous species.