A description and conservation assessment of the vegetation in the Eastern Hajar Mountains, Oman
Keywords: arid mountains
plant community description
important plant areas
Oman
Vegetation classification
species distribution modelling
Hajar mountains

This thesis describes for the first time the plant communities of the Eastern Hajar Mountains of Oman and reveals the primary environmental variables affecting their composition and distribution. Following very extensive fieldwork, 208 species were described from the above-ground vegetation; a synopsis of the soil seed bank composition was also presented and compared with the above-ground communities. I then used species distribution modelling (SDM) to predict the current distribution of the key species in unsurveyed sites within the Eastern Hajar to provide a full dataset for robust conservation planning decisions. A systematic conservation planning (SCP) exercise was carried out using the predicted SDMs to highlight the priority areas for conservation. In the first analytical chapter, I describe four well-distinguished plant communities that were identified between an elevation of 1000m and the highest point in the mountains at about 2200m. I identified and quantified the communities using Two-Way Indicator Species Analysis (TWINSPAN) classification, Detrended Correspondence Analysis (DCA) and Canonical Correspondence Analysis (CCA) ordinations. I found annual rainfall, total nitrogen and total calcium to be the most important environmental factors affecting the distribution of plant communities. The sample plots were heavily grazed by livestock, but in contrast to expectations from many studies on the vegetation of arid lands, any grazing effect on plant community distribution appears to be overridden by the other environmental variables.
The vegetation of the Eastern Hajar Mountains is profoundly degraded, and the mountains have been under a prolonged drought for the last 20 years, with more expected dryness in the next 40 years. Also, overgrazing and fast development of the mountains has placed high pressure on their vegetation, which has led to overexploitation of the vegetation and contributed to the lack of regeneration of many of the principal trees of the mountains. The ability of the vegetation of the mountains to regenerate after severe drought or disturbance is mostly dependent on the soil seed bank. I found species richness and diversity to be very low in the seed bank, with a total of 34 species. Twenty-one species (i.e. that were identified to species level) of a total recorded flora of 208 species were found in both the seed bank and the above-ground vegetation. I explain the relative incongruence by factors comprising disturbance (for instance grazing and development), drought and other biotic and abiotic factors.
Although the landscape of the Eastern Hajar Mountains is mostly gravelly and rocky, and the vegetation is concentrated in the wadis, runnels and cliffs, the mountain range supports a diverse flora representing 31% of the total flora of the country. In 2016 the mountains were proposed as an Important Plant Area (IPA) in the Arabian Peninsula (AP), due to this rich flora and the presence of national and regional endemics, using the newly revised IPA criteria. Although the IPA approach proved to be a powerful tool in identifying sites for plant diversity, they are inefficient compared to other prioritisation methodologies and require more land to conserve less biodiversity.
I, therefore, identified areas with high conservation value in the Eastern Hajar Mountains using a combination of my species distribution modelling results and Zonation prioritisation analysis. I made species distribution models using classification tree analysis (CTA), generalised additive and linear modelling (GAM, GLM), MAXENT and Random Forest (RF) modelling techniques, in addition to ensemble modelling techniques. Ensemble models outperformed all of the other methods with an AUC ≥0.9 for 99% of the species (although the modelling performance was generally good for most of the individual modelling techniques). Although the SDMs returned high AUC values for most of the species, due to the inherent uncertainty when projecting ranges from small numbers of observations, the models produced in this study, should be further tested with other evaluation methods, and ideally ground-truthed with a new collection of field data. My analysis showed that the existing protected areas had limited overlap with the highest priority conservation areas. Furthermore, and from the five different Zonation analysis scenarios used to identify the priority areas within Eastern Hajar, the top 10% priority conservation areas were identified to be at mid-elevations and outside the existing protected areas. I believe my analysis will, therefore, help to find more representative and efficient conservation areas and guide further conservation activities in the country.
In the last analytical chapter, I compare the results of the IPAs (a criteria-based selection approach) with those of a complementarity- based approach to assess the compatibility of the IPA as an approach to prioritise areas for conservation, using the modelled ranges. I found that the average area overlap between the two approaches was about 85%. I also found that the top 50% of the priority conservation areas selected by Zonation were almost identical with the proposed IPAs, with equal species richness. Integration of the complementarity-based approach and the IPAs will help in locating high priority areas for the plant conservation.