Typology of the woody plant communities of the Ethiopian Nech Sar National Park and an assessment of vegetation-environment relations and human disturbance impacts

Abstract

Background and aims – Deforestation and forest degradation have hugely affected the Southern Ethiopian Rift Valley, jeopardizing biodiversity conservation and ecosystem service provisioning. Quantifying the impacts of human activities on the remaining woody plant communities and recognizing vegetation–environment relationships provide the basis for targeted conservation and rehabilitation.Material and methods – The study was performed in the Nech Sar National Park (NSNP). Based on a large systematic vegetation survey of 104 plots, we quantified the woody vegetation composition, and we provided a vegetation classification based on Non Metric Multidimensional Scaling, cluster analysis and indicator species analysis. Furthermore, we evaluated vegetation – environment relationships and the effects of human disturbance on community composition and woody plant species richness and diversity.Key results – Our analyses revealed three very distinct woody vegetation types (Acacia mellifera-Combretum aculeatum; Lecaniodiscus fraxinifolius-Deinbollia kilimandscharica and Acacia polyacantha-Ficus sycomorus) which were significantly differentiated by soil pH, electrical conductivity, available soil phosphorus and organic matter, and by elevation. Human disturbance, as quantified by a compound Human Disturbance Index (HDI) significantly affected community composition, species richness and diversity, and was significantly positively correlated with species richness and diversity. The latter is likely due to intermediate levels of disturbance and encroachment of disturbance affiliated shrubs such as Dichrostachys cinerea, Lantana camara, and Acalypha fruticosa. Furthermore, the demographic structure of key woody species such as Acacia polyacantha, Acacia tortilis, Balanites aegyptiaca, Diospyros abyssinica, Lecaniodiscus fraxinifolius and Terminalia brownii, showed impacts of human disturbance.Conclusion – Our results provide a baseline for further conservation actions in the NSNP which should be differentially targeted on the different plant community types. Overall, human disturbance seems not to have resulted yet in species richness declines, although it has started to affect the integrity of the delineated vegetation types and resulted in small scale succession.