The Biological Characteristics and Potentials for Use in Agroforestry of Two Miombo Woodland Tree Species
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Date
2003-06
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Addis Ababa Universty
Abstract
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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Keywords
Tree Species