Effects of Bischofia Javanica (Bishopwood) on Stand Structure and Species Diversity in Natural and Planted Vegetation Types in Kakamega Forest, Kenya
Kituyi, Brimas Waswa
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Recent studies have indicated that Bischofia javanica Blume (Euphorbiaceae), a tree species that was introduced in Kakamega forest as a restoration species in the early 1960s, is gradually recruiting and dominating its secondary and plantation forest stands. It is not clear whether this is a transitional succession process or a permanent takeover of the forest. The species has been reported to be invasive in other parts of the world such as Bonin forest and Oceanic Islands in West Pacific where it has been found replacing native species. In Kakamega forest, invasion by B. javanica could lead to significant adverse impacts on its ecological and socio-economic functions. This study sought to determine the distribution, dominance and relative abundance of B. javanica in selected vegetation types of Kakamega Forest. The study also aimedatdeterminingthe recruitment pattern of the species in the vegetation types and how this may have affected species composition and stand structure. The study was carried out in nine vegetationtypes, namely: old-growth secondary forest, middle-aged secondary forest, young secondary forest and mixed indigenous,Maesopsis, Cupressus,Pinusand Bischofiaplantation forests, and thedisturbed primary forest. It employed a nested experimental design; the nine vegetationtypes, which served as treatments of the study, were nested as sub-blocks in Yala, Isecheno and Kibiri forest blocks. Assessment was carried in 30m by 20m sample plots using stratified systematic sampling. Each sub-block had eight sample plots within a forest block making a total number of sample plots to be 216 (3 blocks x 9 vegetation types x 8 sample plots).The plots were located at equidistant points along the transect line laid in the middle of each vegetation type. Data from each plot were collected on tree species types with their local and botanical names and Diameter at Breast Height (DBH) which was measured in cm at 1.3m high.All the data were entered in Microsoft excel and analysed using Analysis of variance (ANOVA) in GENSTAT 18thedition software. Ryan-Einnot-Gabriel-Welsch Multiple Range Test (REGWQ) was used to separate the means between vegetation types. The results indicated thatB. javanicawas the only non-exotic tree species among several native species that recruited in all the vegetation types that were sampled except disturbed primary forest. It was the most abundant tree species, dominating the seedling, sapling and tree densities of plantations and natural forests. In addition, recruitment of the species in the respective vegetation types was found to reduce species diversity of the forest while the species evenness tended to increase. An effective way to manage this invasive tree species is to remove its seedlings and saplings through uprooting and also utilize the mature stems since its wood can provide industrial softwood, pulp and paper. Findings of this study will inform policy makers and resource managers to monitor the range of this tree species.