Growth performance of exotic and indigenous tree species in saline soils in Turkana, Kenya

In the arid zone of central Turkana, north-western Kenya, where soil salini ty affects 15-20% of the rangelands, growth performances of trees planted i n saline soil rehabilitation trials have not been evaluated. Tree-planting trials have emphasised exotic species over indigenous ones. However, advant ages and disadvantages of promoting exotic tree species have not been exami ned. The current study was aimed at evaluating growth performance of seven exotic and nine indigenous tree species used in saline soil rehabilitation trials. The tree species were established from 6-month-old saplings using m icrocatchments (FT1) from 1988 through 1990 and Fitting treatment (FT2) fro m 1989 through 1992. The soils in FT1 and FT2 treatments were moderately to highly saline. The exotic tree species produced greater cover and volume d uring the first year (FT1) but by the second year, production was not susta ined due to greater mortality (FT1 8r FT2). The indigenous species in gener al had higher survival rates. Relative growth rates (RGR) of exotic and ind igenous species did not differ (FT1 gr FT2). Tree mortality was negatively correlated with RGR for exotic species in FT1 but not for indigenous ones. However, changes in plant performance were not in response to salinity alon e. Rather, water scarcity superimposed on soil salinity might have influenc ed plant growth performance. Greater water and salinity stress and subseque ntly greater mortality in exotic species provided a more convincing reason for promotion of indigenous tree species. In the future, knowledge of salin ity distribution and selection of indigenous species to match this will be a better way of rehabilitating sires affected by soil salinity in the arid zone of central Turkana, north-western Kenya. (C) 2001 Academic Press.