Evaluation of tree and shrub responses to soil moisture status and their potential for afforestation in the semi-arid rangelands of Makueni district, Kenya

Abstract

This study was conducted to evaluate responses of trees and shrubs to soil moisture status with the aim of determining drought tolerance indicators for their survival in the semi-arid rangelands of Makueni District, Kenya. Study sites were identified in three major agro-ecological zones, namely, Nthangu, Kathonzweni and Kibwezi forests of Makueni district using existing vegetation, agro-ecological maps, and Landsat Imageries. Sample tree and shrub species were identified during field surveys conducted between August and September 2002. Data on vegetation species, soils, and plant and soil moisture parameters were taken for one year between July 2003 and June 2004 to compare the three zones. Collected data was analysed using various parametric and non-parametric tests. The number of tree families, genera and species were largest at Nthangu forest (33, 60, 77) followed by Kibwezi (30, 48, 70) and Kathonzweni (28, 42, 69), respectively. There were significant differences in DBHs (Kruskallis Wallis test, X2=97.56, d.f=2, p<0.05) and heights (Kruskallis Wallis test, X2=100.9, d.f=2, p<0.05) between the three sites. Mean DBHs and heights were 4.6 ern, 6.4 ern and 7.6 ern, and 4.4 m, 5.1 m and 5.3 m, for Nthangu, Kathonzweni and Kibwezi forests, respectively. Plant water potentials ('l') were significantly higher (P<0.05) at Nthangu forest ('l'=-1.23) than at Kathonzweni ('l'=-1.31) and Kibwezi ('l'=-2.64). Similarly, plant water potentials were significantly lower during the dry seasons than during the rainy seasons, and higher in the morning hours than in the afternoon hours. Transpiration rates (Ti) were significantly lower (p<0.05) at Nthangu (Ti=10.24) than at Kathonzweni (Ti=11.35) and Kibwezi (Ti=11.73) forests, significantly lower during the dry seasons than during the rainy seasons, and lower in the morning hours than in the afternoon hours. Stomatal conductances (g) were significantly higher (P<0.05) at Nthangu (g=0.88) than at Kathonzweni (g=0.77) and Kibwezi (g=0.62) forests. It was also significantly lower for the exotic species than for the indigenous species (p<0.05). Soil moisture contents were highest at Kibwezi (54.2 Kg/m3) followed by Nthangu (51.2 Kg/m3) and Kathonzweni (45.1 Kg/m3) forests. Spatial distributions, size characteristics and water potentials of trees and shrubs were identified as potential indicators of stress and adaptation, based on which A. tortilis, and S. siamea were recommended for planting to rehabilitate degraded sites in these areas. It was also recommended that more studies be carried out on a wider variety of species and sites, and with more emphasis on water and osmotic potentials.