Forest types and biodiversity around the Great Rift Valley in Kenya.

The forests along the Great Rift Valley in Kenya are unique as green islands in dry land and extend from southern Ethiopia to Tanzania. These forests include Afromontane and dry-tropical forests, studied in Kenya by Bussmann and Beck, and in southern Ethiopia by Bussmann. Two taxonomical classes (Ocotetea usambarensis in moist climate and Juniperetea procerae at a dry site) were described prior to our phytosociological studies, which were carried out on the east and west sides of the Great Rift Valley in 2006-2014, and results were compared with Bussmann's outcome from the 1990's. New results are presented in this paper: 1) Forests remain on the west side above 2000 m and are characterized by Ocotea kenyensis (Lauraceae); these are different from the moist Afromontane forest at an eastern site (Ocotetea usambarensis) next to the Rift Valley. Four months with > 100 mm rainfall occur from March or April to August or September. 2) At 1300-2200 m at the eastern site, there are unique dry forests characterized by Brachylaena huilliensis (Compositae), Warburgia ugandensis (Canellaceae), Elaeodendron buchananii (Celastraceae), Vepris trichocarpa (Rutaceae), Calodendrum capense, etc. Some of these are unique species and extend to the rainforest in South Africa. 3) Higher-altitude forests at the eastern site are characterized by Podocarpus latifolius and Juniperus procera, which belong to the Juniperetea procerae. Podocarpus latifolius forests occur at the western site, but these are different from those at the eastern site. 4) The forests of the isolated northern mountains Marsabit and Kulal are different; there is no Podocarpus or Juniperus, and higher moisture permits hanging Usnea usneoides to occur at Marsabit. 5) The forest area of Kenya was potentially 12%, and remnant natural forests only cover 1.9%. This small area permits various forest types along a moisture gradient. 6) Several woody and fern genera of the Afromontane forests are common to tropical montane and subtropical forests in Asia, including Podocarpus, Juniperus, Myrsine, Rapanea, Psychotria, Rumohra, Asplenium, Pteris, and Dryopteris. 7) Our and Bussmann's classifications are not the same at the association or community level. There is thus the question whether the 1990s data were from a quite different area or whether some vegetation disappeared due to human or wildlife affects.