Socioeconomic and Biophysical Factors Affecting Tree Richness and Diversity in Machakos County, Eastern Kenya
Trees on farm play a vital role in providing diverse goods and services to those farmers practicing agroforestry in Kenya. Other than the environmental advantages of agroforestry such as carbon sequestration and species conservation, trees provide soil and microclimate enhancement, deliver fruits, medicines, fodder, timber and fuelwood. However, tree species richness and diversity are influenced by numerous socio-economic and biophysical factors as necessary knowledge for promotion of on-farm tree planting activities. The purpose of this study was to assess the effects of different socio-economic and biophysical factors on tree richness and diversity in smallholder farms in the study area. The study covered three broad agro-ecological zones i.e. Lower highlands, Upper midlands and Lower midlands along an elevation gradient in Machakos County, Eastern Kenya. Importance Value Index (IVI) was calculated to get the level of importance of different tree species. Correlations and stepwise multiple regression analysis were used to estimate the influence of the assessed socio-economic and biophysical variables on tree species richness, abundance and diversity. Tree diversity indices were calculated using MVSP to obtain Shannon index and Evenness. Cluster analysis based on the Minimum Variance method was used to partition samples into homogenous classes. A total of 102 tree species were recorded, including 42 exotic and 60 indigenous species. Exotic abundance was at 67% of all counted individuals while mean number of tree species was 12.7 ranging from 3-26 tree species per farm. Farm sizes ranged from 0.1 to 7.1 ha, with a mean size of 1.6 ha. Tree diversity was relatively high with a mean Shannon diversity index of 1.73 (range 0.46-2.53) and a mean Shannon evenness 0.70 (range 0.28-0.97). Based on the IVI, Grevillea robusta and Eucalyptus spp were among the top exotic timber and fuelwood species while Acacia seyal and Terminalia brownii topped the indigenous tree species for fuelwood. Mangifera indica and Persea americana presented fruit tree species with a high IVI and were all exotics. Tree species richness was positively influenced by farm size and market distance, but negatively by elevation, number of plots and gender. Tree abundance was positively influenced by farm size and number of plots. Farm size, however, had a strong negative effect on Shannon evenness. Cluster analysis resulted in six clusters and twelve (12) tree species were responsible for cluster formation. Results of this study can contribute to modify agroforestry programmes for implementing future tree planting activities for different target populations in various economic and environmental circumstances.