Effect on the water yield of the Luchelemu catchment in Malawi of replacing indigenous grasses with timber plantations

Abstract

The effect of replacing indigenous montane grass ( Loudetia simplex, Exotheca abyssica, and Andropogon schirensis) and scrub ( Helichyrysum) with exotic pine ( Pinus patula and Pinus kesiya) and eucalyptus ( Eucalyptus saligna) trees on the water yield of the Luchelemu catchment on the Viphya Plateau, in Malawi, was investigated. The results showed that there were no significant differences ( P < 0.05) in the rainfall patterns and the peak stream flows before and after afforestation of the catchment with the exotic trees, but the minimum flows were significantly lower ( P < 0.05) under the exotic pine and eucalyptus plantations than under the original indigenous grass and scrub cover. The lower minimum flows under the exotic vegetative cover were probably due to high evapotranspiration losses of the pine and eucalyptus trees during the dry periods rather than changes in the catchment characteristics. It was concluded that, when converting grassland to forest, careful consideration must be given on the likely effects of such measures on the water yield of the catchment. It was also concluded that appropriate management practices, such as heavy thinning and reduced density of the exotic forest, may help to recover the stream flow under such vegetation.