Effects of canopy cover and understory environment of tree plantations on richness, density and size of colonizing woody species in southern Ethiopia

Abstract

To assess the effects of canopy characteristics and associated understory environmental factors of tree plantations on native woody species colonization, we measured canopy closure percent (CCP), leaf area index (LAI), understory air and soil temperatures as well as soil moisture in two broadleaved and two coniferous plantations in southern Ethiopia. The plantation species were Cordia africana (indigenous and broadleaved), Eucalyptus saligna (exotic and broadleaved), Cupressus lusitanica (exotic and coniferous) and Pinus patula (exotic and coniferous). The broadleaved species had significantly lower CCP ( P<0.001), LAI ( P<0.001), higher understory air temperature ( P<0.001), soil temperature ( P<0.001) as well as higher diurnal temperature fluctuations than the conifers. These characteristics were accompanied with significantly higher species richness ( P<0.05) and density ( P<0.01) of colonizing woody species (CWS) under the broadleaved species than the conifers. Cupressus lusitanica, the species with the densest canopy (CCP=94.2%), had the lowest significant understory air and soil temperatures, species richness and density of CWS. On the other hand, Pinus patula, a relatively open canopy conifer, had intermediate CCP, LAI, air and soil temperatures between the broadleaved species and Cupressus lusitanica, but had comparable species richness, density, DBH and heights of CWS with those of the broadleaved species. These results suggest a relationship between plantation canopy characteristics and richness, density and sizes of woody species colonizing under the plantation canopies. Accordingly, we propose that stands of plantation species with open canopies could enhance more native woody recolonization than stands of plantation species with dense canopies at the study site.