Interaction of bischofia javanica and its effect on species diversity and structural composition of secondary and plantation forests in a Kenya rainforest

Abstract

We studied the effects of Bischofia javanica on species diversity and structural composition of plantation and secondary forests in Kakamaga rainforest in western Kenya. The invasive exotic tree species was introduced to the area in 1960s and planted as a pure plantation stand to assist in rehabilitating logged primary forest sites. By 1990s, the species had established in nearby secondary and plantation forest stands in which it was not planted. The study sought to determine how invasion of the exotic species had affected species diversity, species evenness and stand density in plantation and secondary forests within the rainforest. The study was carried out in nine forest types, namely: old-growth secondary forest, middle-aged secondary forest, young secondary forest, mixed indigenous plantation, Maesopsis, Cupressus, Pinus and Bischofia monoculture plantations, and disturbed primary forest. These forest types were nested as sub-blocks in three forest blocks. The results indicated that B. javanica was the only non-native woody species among several native woody species that recruited naturally in plantation and secondary forests. It did not grow in the disturbed primary forest. Its recruitment was significantly more aggressive than that of native species. It lowered species diversity but increased species evenness in all plantation and secondary forests. Its relative abundance was 30.6%, dominating the seedling, sapling and tree densities of plantation and secondary forests. It is necessary to remove the tree species from the rainforest before it takes over the whole forest ecosystem.