Effects of Altitudinal Gradient on Diversity and Composition of Farm Plantation Tree Species in Northeastern Ethiopia: Implications for Soil Fertility Maintenance

On-farm tree plantation is a form of land use where trees are planted at the edge or interspersed with crops. It has been practiced in different parts of Ethiopia due to its contribution to the household economy and soil fertility. This study was carried out to evaluate the variation in tree/shrub plantations along altitudinal gradients and plantation niches and farmers’ on-farm tree plantation practices. Semi-structured questionnaire and transect walks were administered to appraise farmer`s tree/shrub plantation practices and composition on agricultural lands across altitudinal gradients and niches at Kobo and Guba Lafto districts, northeastern Ethiopia. A total of 135 farmer respondents’ and sample plots (45 plots in each altitudinal gradient) were simultaneously used to explore the tree/shrub plantation practices and compositions. At each plot, tree/shrub richness, diversity, stem density, and important value index (IVI) were computed. Multivariate analysis, descriptive statistics, and preference rankings were used to evaluate vegetation data and farmers’ perception on tree/shrub plantations. The results showed that most farmers (78.5 %) integrate trees with their crops for household use and soil fertility maintenance. The multivariate analysis revealed a significant reduction in the number of taxa, stem density, richness, and diversity with increasing elevation and from homestead to the boundary and on-farm plantation niches. Ziziphus spina-christi and Cordia africana were the most preferred tree species. Fabaceae was the dominant family, represented by 18.9 % species. The results also showed that there were considerable variations in relative density, relative dominance, and important value index (IVI) across altitudinal gradients and plantation niches. Acacia seyal and Z . spina-christi contributed the highest IVI in the lower and middle elevations, whereas Eucalyptus globulus had high IVI at higher elevation. In the study districts, multifunctional indigenous tree plantations gradually decrease with the entire altitudinal gradients compared to the distribution of exotic trees/shrubs. This calls for actions to be taken on conservation and propagation of native tree genetic resources to maintain soil fertility and productivity.Keywords: Agroforestry, Altitudinal Gradients, Farmers Acuity, Plantation Niches, Soil Fertility