The potential of tree plantations to foster regeneration of indigenous woody species in degraded lands of Ethiopia

Tropical forests are declining rapidly mainly due to their conversion to arable lands coupled with overexploitation associated with the ever-increasing demand of communities for wood products spurred by the rapidly growing human population. These events have resulted in serious environmental degradation reflected in the form of degradation of land and water bodies, decline/loss of biodiversity, global warming, etc. These phenomena, in turn, have affected and continue to affect negatively the socio-economic conditions of the communities and the welfare of human beings, animals, plants and microorganisms. In view of these problems, there is an urgent need to conserve and sustainable utilize the remaining natural forests and restore degraded forest area through expansion of forest plantations. With the objectives of meeting the increasing demand for wood, relieving the pressure form the natural forests and revitalizing degraded lands, forest tree plantations have been initiated in the early 20th century in Ethiopia, mainly with introduced (exotic) species of Eucalyptus, Cupressus, Acacia and Pinus. Conversely, plantation establishments using exotic species have often been widely criticized of having serious unfavorable effects on the physical environment and associated life forms. However, as studies in different part of the world and also in Ethiopia indicated the impact of plantations of exotic species on the site conditions and on the biodiversity beneath their canopy depends not only on the type of planted species but also on the history of the site, forest management regime employed and characteristics of planted species. Indeed, several recent studies have shown that tree plantations can have a nurse effect and, thus, be an effective means for rehabilitation of degraded sites by catalyzing the restoration of native vegetation beneath their canopy. In many sites and within various exotic species substantial numbers of indigenous woody species regenerate beneath the canopy of the plantations, thus showing an increase in their biodiversity. As it observed, in some plantation stands the density of regenerating woody plants were over 18,000 plants/ha, e.g., in Eucalyptus stands in Munessa-Shashenene and over 20,000 plants/ha at Menagesha-Suba. According to the present evidences, many tree plantations do not significantly reduce species diversity in the understorey. However, there is a difference among different plantation species. Apparently broadleaved species, including Eucalyptus, are more favorable "nurse trees" than conifers as it is also borne out by comparing Eucalyptus (E. saligna and E. globulus) with conifers (Pinus patula and Cupressus lusitanica) at the two sites, i.e.e Munessa and Menagesha-Suba. these evidences indicated that plantation stands can promote a high density and variety of species in the form of natural regeneration under their canopies.