ON-FARM TREE SPECIES DIVERSITY, CARBON STOCKS AND SOIL PROPERTIES AS INFLUENCED BY SELECTED Eucalyptus spp IN KAKAMEGA-NANDI FOREST ECOSYSTEM

Abstract

On-farm growing of trees has significantly contributed to easing pressure from natural forests in addition to providing extra income to forest-adjoining farming households. There are however concerns that Eucalyptus species is becoming the dominant trees in this landscape and may have adverse effects on the environment. This study sought to determine the effects Eucalyptus species on the on-farm woody species diversity, carbon stocks and soil properties on farmlands located in the margins of the Kakamega - Nandi Forest Ecosystem. The study targeted farmlands located 0 – 3 km from the forest boundary. It employed a nested experimental design in which the study area was divided into three sentinel blocks. Each block had three sub-blocks, namely: Shamiloli, Mukulusu and Lukala in Kakamega; Makuchi, Makhanga and Blukhombe in Vihiga; and Cheboite, Burende and Mukoyuro in South Nandi. Each sub-block comprised three types of trees stands namely: Eucalyptus dominated tree stands, mixed tree stands comprising of Eucalyptus, indigenous and exotic trees, and pure indigenous tree stands. The study was carried out along three line transects that traversed the three types of trees stands in each block from the forest edge to farmlands. Stratified systematic sampling was used to assess the extent to which the observed variation in the concentration of Eucalyptus trees affected the three variables under investigation (tree species diversity, carbon stocks and soil properties). A sample plot comprised a main plot of 20m by 10m plot for measuring trees with a diameter at breast height (DBH) ≥ 10 cm, a sub-plot of 10m by 5m nested within the main plot for measuring saplings and shrubs of DBH less than 10cm. Data was collected on tree species type, stem DBH for trees, tree height, counts of trees, saplings and shrubs. Composite soil samples were collected within the main plot for analysis of soil organic carbon and other physico-chemical properties. The data was subjected to both exploratory and inferential statistical analysis using R Gui Version 4.2.1. Woody species diversity, carbon stocks, and soil properties were subjected to analysis of variance (ANOVA) at 5% significance level to assess their variation with change in the concentration of Eucalyptus trees on-farm. Post hoc tests were carried out to determine the source of variation among means using the Ryan-Einot-Gabriel-Welsch Multiple Range Test (REGWQ) at 5% significance level. A total of (N=51) species representing 26 families were recorded. Of these species, (n=8) were encountered in the Eucalyptus dominated tree stands, (n=29) in mixed tree species stands and (n=32) in indigenous tree species stands. Among the woody species, mature trees constituted 48.6% while saplings and shrubs comprised 51.4%. Myrtaceae family constituted 50.3% of the woody trees followed by moraceae family with 7.5%. Woody species richness, evenness and diversity were significantly higher in mixed tree stands and indigenous tree stands than in Eucalyptus dominated tree stands (p< 0.05). Trees in the Eucalyptus dominated stands and mixed stands had significantly smaller stem diameter, basal area and above ground biomass than in the adjacent indigenous trees stand (p< 0.05). The highest amount of total carbon was observed in the indigenous trees stand. The percentage soil organic carbon in the Eucalyptus dominated tree stands and mixed tree stands was significantly lower than in the adjacent indigenous trees stands (p< 0.05). The bulk density in the Eucalyptus dominated tree stands and mixed tree stands was significantly higher than in the adjacent indigenous trees stands (p< 0.05). Phosphorus, Nitrogen and potassium did not vary significantly within the three tree stands (p> 0.05). The results suggest that a higher diversity of indigenous trees enhances aboveground carbon stocks and promotes ecosystem conservation