Effect of Senna obtusifolia (L.) invasion on herbaceous vegetation and soil properties of rangelands in the western Tigray, northern Ethiopia.
Introduction: Invasion of exotic plant species is a well-known threat to native ecosystems since it directly affects native plant communities by altering their composition and diversity. Moreover, exotic plant species displace native species through competition, changes in ecosystem processes, or allelopathic effects. Senna obtusifolia (L.) invasion has affected the growth and productivity of herbaceous vegetation in semi-arid regions of northern Ethiopia. Here, we investigated the species composition, species diversity, aboveground biomass, and basal cover of herbaceous vegetation, as well as soil properties of rangelands along three S. obtusifolia invasion levels. Methods: Herbaceous vegetation and soil properties were studied at two locations, Kafta Humera and Tsegede districts, in the western Tigray region of northern Ethiopia under three levels of S. obtusifolia invasion, i.e., non-invaded, lightly invaded, and heavily invaded. Herbaceous plant species composition and their abundance were assessed using a 1-m2 quadrat during the flowering stage of most herbaceous species from mid-August to September 2015. Native species were classified into different functional groups and palatability classes, which can be useful in understanding mechanisms underlying the differential responses of native plants to invasion. The percentage of basal cover for S. obtusifolia and native species and that of bare ground were estimated in each quadrat. Similar to sampling of the herbaceous species, soil samples at a depth of 0-20 cm were taken for analyzing soil physical and chemical properties. Results: Highly palatable grasses and legumes based on the frequency of occurrence were 1.9 and 3.8 times higher in non-invaded sites than in lightly and heavily invaded sites, respectively. Furthermore, unpalatable plant species were 4.0 times higher in the heavily invaded sites than in the lightly invaded sites. Aboveground biomass, basal cover, and species diversity of the herbaceous vegetation were significantly (P<0.01) lower in the heavily invaded sites compared to the non-invaded sites. However, most soil properties were not affected by S. obtusifolia invasion compared with non-invaded sites in our study. Conclusions: Our findings contribute towards better understanding of how S. obtusifolia invasion affects the community structure and composition of native herbaceous vegetation in semi-arid rangelands in northern Ethiopia. Further long-term studies are required to understand the mechanisms of how S. obtusifolia invasion influences soil properties for better control and eradication of its invasion in the semi-arid rangelands of northern Ethiopia.