January 2005
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This study deals with the vegetation and vascular plant flora of the mountain massifs and adjacent lowlands along the border between Uganda and die Sudan to the east of the Nile. The mountains of this area rise abruptly from the Nile Valley plains at 500-700 m., with crests reaching altitudes between 2000 and 3200 m., and consist of rocks of the crystalline basement complex. The rocks of the mountains are much older than the large volcanic highlands of Ethiopia and tropical East Africa (Uganda, Kenya and Tanzania), but the uplifting of the mountain massifs has taken place as part of the East African Rift Valley formation, the same process, which created the volcanic mountains. The three principal massifs dealt with here are the Imatong Mountains group, the Dongotona Mountains and the Didinga Mountains. The first volume of this work dealt with the geographical delimitation of the study area and history of exploration of the flora, and it included the first part of the catalogue (Pteridophyta to Asteraceae). The current volume deals with the remaining families (Gentianaceae to Poaceae), and contains general chapters on vegetation and biogeography. The vascular plant flora of the study area contains 110 species of ferns and fern allies, 4 species of gymnosperms and 2135 taxa (species, subspecies and varieties) of flowering plants. The flora of the study area has been analysed by investigating the geographical distribution of all species both within the study area (by dividing the study area into a western, a central and an eastern part and in altitudinal zones) and in a global context. There is a notable east-west gradient through the study area, with stronger links to the Guineo-Congolean region in the west than in the east, and stronger links to the Somalia-Masai region in the east than in the west. The largest component of the flora is shared with East Africa, which agrees with the position of the study area at the northern limit of the East African highlands. Declining numbers of taxa are shared with Ethiopia and Eritrea, from the highlands of which the mountains of the study area are separated by lowlands, with central and western Africa, with south central Africa, with south tropical Africa and with regions elsewhere. The study area as a whole has a rather low' species density of vascular plants when compared with East Africa and Ethiopia. This is probably due to extensive areas with rather uniform grassland in the northern and eastern parts of the study area, while the montane areas have a high species density. Endemism is moderate, but again with higher values in the mountains than in the plains. The altitudinal zones at 915-1220 m. and 1525-1830 m. have the highest plant taxon richness and the highest number of plant endemics. The altitudes at which the diversity peaks varies between the different vegetation types, and peaks in diversity at altitudes between 915 and 1830 m. are more prominent in forest and grassland than in woodland. But generally, the plant diversity drops off in with increasing altitudes according to a linear function, agreeing with the declining areas of the upper altitudinal zones according to well-known relations between species richness and area. The zonation in the flora is not prominent, but there are slight discontinuities in the interval 1800-2100 m. in the flora as a whole and in grassland, bushland and woodland vegetation, and at 1200-1500 m. in forest vegetation.
