Environmental Change and Vegetation Succession along an Ephemeral River: The Kuiseb in the Namib Desert

摘要

Forests line the course of the Kuiseb River, an ephemeral river in the Namib Desert, and several areas of these forests are characterized by high concentrations of tree death (Mizuno, 2005; Mizuno & Yamagata, 2005). We sought to clarify the relationship between recent environmental changes and such tree deaths in the region. In November 2007, we examined the roots of a seedling of Acacia erioloba that was germinated by rainfall beginning in January 2006. The Acacia erioloba had grown to a height of 10cm and its roots to over 230cm, within two years. In the sapling (seedling) stage, Acacia erioloba extends its main roots deeply until it reaches a moist, fine-grained soil layer (sandy silt) and can absorb water through lateral roots. When it reaches the stage at which the water supply from the moist, fine-grained soil layer is insufficient for its growing size, the tree extends innumerable lateral roots within a 50cm depth from the land surface, where they absorb water that has been transported to a shallow depth by fog and other sources. Acacia erioloba dies when its lateral roots are unable to absorb water. Until the mid-1970s, successive floods repeatedly deposited fine-grained materials (e.g., sandy silt) that create water-bearing sediments for the growth of Acacia erioloba, and the trees died only rarely. However, from 1980 to 1985 these materials became increasingly scarce due to the decreasing occurrence of flooding, and consequently many trees died. It is reasonable to infer that the trees died because fine sediments were no longer being regularly deposited, and because of the drawdown of the groundwater level, both of which are making it difficult for the shallow roots of the trees to absorb the water necessary to survive.