Species composition and stand structure of secondary and plantation forests in a Kenyan rainforest

摘要

Forest succession has been reported in forest plantations in tropical forests, but little is known about their successional dynamics because most studies have focused on succession in secondary forests. We assessed changes in species composition and stand structure in secondary and plantation forests in Kakamega rainforest in western Kenya. We used a nested experiment to collect data on tree species types, tree height and stem diameter at breast height from secondary forest stands, mixed indigenous plantations and indigenous and exotic monoculture plantations in three forest blocks. Data were analyzed for variation in species diversity, species similarity to the primary forest, stem density and basal area using analysis of variance in Genstat. The results indicated that species diversity and similarity to the primary forest were not different between secondary and plantation forests. However, successional species occupied all the canopy strata in secondary forests, but they occupied only the shrub and understorey layers of monoculture plantations, and the shrub, understorey and sub-canopy strata of mixed indigenous plantations. Mixed indigenous plantations had become nearly indistinguishable from secondary forests, but monoculture plantations maintained a plantation outlook. Old secondary forest had a significantly lower stem density than plantation forests, but their basal area was not significantly different. Middle-aged and young secondary forests had comparable stem density to plantation forests, but their basal area was significantly lower. The results confirmed that plantation forests are experiencing forest succession in tropical forests, their species composition and stand structure are comparable with secondary forests, but they differ in the emergence pattern of successional species and their distribution in forest canopy strata.