WOODY SPECIES COMPOSITION AND RECRUITMENT STRUCTURES OF FOUR COMMON SPECIES IN AN ARID AND SEMI-ARID ECOSYSTEM OF ETHIOPIA

The spatial variations in woody species composition and recruitment structures are less explored in arid and semi-arid ecosystems. We aimed to fill this gap by undertaking the study on three sites which were purposively selected from Awash River Basin based on their area extent and altitudinal gradients. In each vegetation site, we laid out one transect line and on each transect 12 sample plots (each size: 50 × 50 m) were placed with 200 m intervals, resulting in 36 sample plots. Woody species composition difference among the three sites was tested using Adonis2 function. Nonmetric multidimensional scaling ordination analysis was run to visualize the species composition in relation to the vegetation sites in ordination space. The difference in species richness among the sites and the effect of altitudinal variation were tested using a generalized linear model with the Poisson family. The results showed that the species composition was different among the sites and significantly dependent on altitudinal variation (P=0.001). The species richness and number of unique species was higher at Mille site when compared with other two sites, and the alpha diversities of vegetations were reciprocally related to the magnitude of beta diversity. The recruitment structure was poor in sites where the species richness was lower, although the overall patterns vary across sites. Our overall results suggest that restoration schemes need to take onto account the recruitment structure of specific species to enhance the overall health of the vegetation ecosystems, especially in arid and semi-arid ecosystems.