The Biological Characteristics and Potentials for Use in Agroforestry of Two Miombo Woodland Tree Species

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Addis Ababa Universty


Two indigenous tree species of Tanzanian miombo woodland were studied to explore some of their biological charactellstics and potential for use in agroforestry. These were selected by farmers from among four most frequently encountered tree species found within the study area using direct matrix ranking approach. The selection of the study species was based on degree of preference by farmers. Semi-structured interviews were administered on 60 informants to explore information on ecological aspects of the study sites, biological characteristics and multipurpose nature of the study species. Studies on the ecology and growth characteristics and their influence on the environment were conducted in four sites i.e. two sites for each study species. A total of 40 quadrats (50 by 20 m), 10 in each of the study sites, were established for study of vegetation information, tree characteristics and estimation of undercanopy vegetation cover. To investigate the influence of the study species on undercanopy soil, samples were collected from three study trees for each site. The soil samples were collected from 0-5 and 25-30 cm depths at 100, 250 cm and outcanopy distances along the transect radiating from tree bole. Bulk density, texture, pH, electrical conductivity, organic matter, total nitrogen, available phosphorus and exchangeable potassium of soil samples were analyzed. Vegetation cover and soil properties outside tree canopies were also investigated for comparison purposes. Sclerocarya birrea (A. Rich.) Hochst. Subsp. caffra (Sond.) Kokwaro. and Brachystegia lIlicrophylla Harms ranked first (1487) and second (1170) respectively in direct matrix ranking and hence were selected for this study. Site two study trees had highest average tree density. B. microphylla had higher mean tree height, canopy depth, and crown cover whereas S. birrea had higher mean DBH. In all sites undercanopy species diversity was significantly higher than species diversity outside the tree canopies. Soil mineral elements decreased with depth and distance from the tree bole. Significantly higher levels of nutdent elements under the tree canopies revealed an increase in input from the study trees. This indicates that the study trees enriched soil under their canopies thus creating suitable microenvironment, which promotes the development of understorey herbaceous layer. Comparison of levels of nutrients under the canopies of the two species indicated that B. lIlicrophylla had significantly higher percentage of total nitrogen at all depths than S. birrea. This may be due to the fact that B. lIIicrophylla, which is a leguminous tree, may fix nitrogen and hence increase nitrogen content under its canopy zones. S. birrea had a wide range of uses, indicating its multipurpose nature. It is concluded that both B. lIIicrophylla and S. birrea display high agroforestry potential and hence are recommended for use in agroforestry practices in miombo zones. However, the results of this study are subject to fUlther verifications.



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