Effect of prior land use on the recolonization of native woody species under plantation forests in the highlands of Ethiopia

Abstract

Effect of prior land use on the recolonization of native woody species in plantation forests was investigated by assessing naturally regenerating flora (NRF) and soil seed banks (SSB) in plantation forests established on abandoned farmland and cleared natural forest sites in southern highlands of Ethiopia. Eucalyptus saligna and Cupressus lusitanica, two of the most widely planted tree species in the highlands of Ethiopia, were considered in the plantation treatments. About 66 plant species were recorded in the NRF and 55 plant species germinated from the soil samples collected for SSB analysis. Seedlings from the SSB were dominantly herbs, which accounted for 75% of the identified species germinated from the SSB, and native woody species accounted only for 10%. On the contrary, in the NRF native woody species were slightly more dominant (49%) than the herbs (45%). There was high species similarity between the NRF beneath the plantations and the standing vegetation in the adjacent natural forest. On the contrary there was very low similarity between the seedlings emerged from the SSB and the standing vegetation in the adjacent natural forest. Effect of prior land use was apparently stronger on the species composition of the SSB than the species richness of NRF under the plantations. The results also showed that overstory plantation species had stronger influence on the species richness of NRF rather than the pre-plantation land use history. As the SSB of the plantation sites lacked viable seed reserves for most of the naturally regenerating woody plants recorded underneath the plantations of both sites, it was assumed that seed dispersal from the adjacent natural forest has played major role in the recolonization process. From these results it could be shown that establishment of plantation forests either on abandoned farmland or directly on degraded natural forest sites can create comparable enabling environment for the recovery of the native forest flora, even if SSB are devoid of viable seeds of woody species, provided that there is a natural forest in the vicinity to donate seeds.