DIAMETER DISTRIBUTION OF INDIGENOUS TREES AS INDICATOR OF ADAPTED SPECIES IN SEMI-ARID RANGELANDS OF KENYA

Diameter distributions of indigenous trees as possible criteria for indicating adapted species in semi-arid rangelands were assessed at Nthangu, Kathonzweni and Kibwezi forests of Makueni County, Kenya using existing vegetation, agro-climatic maps and Landsat images. The sites have mean annual rainfalls of 974, 700 and 616 mm respectively, and moisture indices of 49, 35 and 32%. Data on diameters at breast height (DBH) and DBH distributions were collected using transects, 18 sample plots of size 2500 m(2) and 90 subplots. Shapiro-Wilk test was used to determine normality of DBH distributions, and t-tests were conducted on species occurring in two of the three study sites. Statistically significant differences were declared at p < 0.05. Mean DBH values were 7.6, 6.4 and 4.6 cm at Kibwezi, Kathonzweni and Nthangu respectively. Ecologically important trees with high populations of juvenile individuals and near-perfect inverted J-shaped distributions were Searsia spp., Acacia hockii and Diospyros mespihformis at Nthangu; Combretum collinum, Combretum apiculatum and Acacia tortilis at Kathonzweni; and A. tortilis, Acacia mellifera and Commiphora africana at Kibwezi. These species were likely candidates for rehabilitation of degraded sections as their DBH distributions indicated regenerating, sustainable and stable populations adapted to the respective sites.