Tree species diversity and spatial distribution patterns on agricultural landscapes in sub-humid Oromia, Ethiopia

Trees are important components of agricultural landscapes in different parts of Ethiopia, and information on their type, diversity and distribution in sub-humid agroecologies is essential for designing interventions. A study was conducted to evaluate tree diversity and their spatial patterns in agricultural landscapes under different land use categories in four selected sub-humid sites in Western Oromia, Ethiopia. Tree inventory was conducted on 100 homesteads (19 ha), 18 crop lands (35 ha) and 11 grazing lands (5.5 ha) belonging to 100 randomly selected households. A total of 82 tree species were identified: 67 in the homesteads, 52 in the crop lands and 29 in the grazing lands. The density of trees varied from 68 trees per ha in crop lands to 801 trees per ha in homesteads. Diversity indices revealed that homestead was the most diverse with Shannon index of 2.42, and Simpson index of 0.84. The density of trees among the tree communities in the four sites varied from 133 in Bako Tibe to 476 in Jima Arjo, but not any one of the sites had more diverse tree community as revealed by the Rènyi diversity profiles analysis. The three dominant tree species in the agricultural landscapes were Eucalyptus camaldulensis, Vernonia amygdalina and Cordia africana. Pearson correlation analysis showed that high tree species density, richness and diversity had high association with homesteads than with crop lands and grazing lands. It also revealed significant positive correlations between land size and evenness, and latitude and evenness whereas there were significant negative correlations between family size and Shannon diversity index, and land size and tree density. The majority (81.6%) of the trees were established through plantation and only 18.4% were regenerated naturally. The proportion of planted trees varied from 68% in Gobu Seyo to 94.1% in Guto Gida. The study showed that agricultural landscapes harbour high diversity of tree species with a spatial pattern, and increasing the tree cover with focus in the crop lands is essential for improved resilience of the agricultural systems and for circa-situm conservation of biodiversity.