Determinants of aboveground carbon offset additionality in plantation forests in a moist tropical forest in western Kenya

Abstract

Few studies have attempted to identify factors that contribute to aboveground carbon offset additionality in forest restoration planting in the tropics. Moreover, those that have compared aboveground carbon offset potential of naturally regenerating secondary forests and plantation forests have yielded conflicting results regarding the ability of the latter to attain carbon offset additionality, thus limiting broad adoption of carbon-driven forest restoration interventions. We assessed woody species diversity, stem density, stem diameter and wood specific gravity of secondary and plantation forests in Kakamega Forest in western Kenya to identify determinants of aboveground carbon offset additionality in plantation forests. Secondary forests comprised old-growth, middle-aged and young vegetation stands. Plantation forests consisted of mixed indigenous, Maesopsis eminii indigenous monoculture and Cupressus lusitanica, Pinus patula and Bischofia javanica exotic monoculture stands. Assessment was carried in 135 sample plots in three forest blocks using stratified systematic sampling in nested plots. Analysis of variance indicated that there was no significant difference in woody species diversity between secondary and plantation forests due to natural forest succession in both forest types. Mixed indigenous plantation had more aboveground carbon stock than secondary forest stands of comparable stand age due to its greater proportion of tree species with high wood specific gravity and large tree diameter. Old-growth secondary forest had more aboveground carbon stock than monoculture forest plantations due to its relatively higher wood specific gravity. Middle-aged secondary forest had relatively lower aboveground carbon stock than plantation forests of comparable stand age because of its smaller tree diameter. The results suggest that stem diameter and wood specific gravity are the most important determinants of aboveground carbon offset additionality. Thus, forest managers and investors in carbon offset projects can achieve aboveground carbon offset additionality in forest restoration interventions by planting tree species with relatively higher wood specific gravity and manipulating them to attain large stem diameter through silvicultural management.