Forest diversity in fragmented landscapes of northern Ethiopia and implications for conservation

Deforestation and habitat fragmentation that arise largely due to the conversion of forests to other agricultural land-use types and over-utilization of forest resources to satisfy the food and energy requirements of the increasing population are major environmental concerns in northern Ethiopia. Understanding plant species diversity and spatial distribution along environmental gradients is crucial in the management of the remnant forest ecosystems. However, the ecology of the forest remnants in northern Ethiopia is poorly studied. The purpose of this study is therefore to (i) investigate plant species diversity and natural regeneration in relation to selected environmental factors, (ii) quantify the elevation patterns of species diversity and community composition, (iii) examine the extent and spatial distribution pattern of standing dead stems and the effect of mass tree dieback on forest structure and diversity, and (iv) compare the regeneration response of Juniperus procera and Olea europaea subsp. cuspidata in an openaccess forest area to a closed forest management system. The study was conducted in the Desa’a and Hugumburda Afromontane forest remnants, which are the largest forest fragments in northern Ethiopia and are national forest priority areas. A total of 153 species belonging to 63 families was found in the study area; shrub and herb species dominate (ca. 70 %). The vegetation is mainly a dry Afromontane forest type with Juniperus and Olea as the dominant species; a riverine plant community in Hugumburda forest represents a moist forest type. Elevation, slope, soil depth, distance to the nearest stream, soil moisture, and forest disturbance are the main environmental factors influencing species distribution and partitioned plant communities. The diversity of species and the composition of plant communities in Desa’a forest significantly respond to elevation. Species richness and diversity show a unimodal, humpshaped relationship with elevation that peaked at mid elevation (1900 – 2200 m). The beta diversity values indicate medium species turnover along an elevational gradient. The percentages of dead standing trees (snags) due to natural disturbance at Desa’a forest are high for both J. procera (57  7 %) and O. europaea subsp. cuspidata (60  5 %), but show a decreasing trend with increasing elevation suggesting that restoration is more urgent at the lower elevations. Higher tree dieback at the lower elevation has pushed the tree species to the higher elevation by about 500 m, and this can lead to a shift in the forest-shrubland ecotone to higher elevations. Total stand density and basal area are reduced by 30 and 44 % when excluding snags of the two species, respectively. Thus, mass tree dieback of the two key species strongly influences the forest structure. High amounts of dead standing biomass are a particular risk in a fire-prone semi-arid forest environment, and controlling snag densities is of critical concern in the management of the remaining dry Afromontane forests in northern Ethiopia. The natural regeneration of native tree species in both forest remnants is low. Exclosure was found to be an effective management option to improve the regeneration of O. europaea, but it does not improve the regeneration of J. procera. Thus, a closed management system in the open-access and degraded forests may not guarantee a successful regeneration of native woody species. It rather favors grass and herbaceous species and can lead to a gradual conversion of the forest land to wooded grassland. Most of the seedlings in forest remnants are shrubs, while tree species are less diverse and abundant. The standing vegetation is only partly represented in the seedling bank and many of the rare tree species, e.g. Afrocarpus falcatus, show poor or no regeneration. A smaller number of saplings than mature individuals suggest that locally some forest species are experiencing extinction. Thus, it is important to give conservation priority to the last Afromontane forest remnants in northern Ethiopia to achieve local, national and international biodiversity conservation goals.