Abstract
The Namib Desert is located along the western coast of Namibia and is affected by the cold Benguela Current. Although forests are distributed along the ephemeral river, the Kuiseb River in the Namib Desert, many trees are almost dead in certain areas (Mizuno, 2005; Mizuno & Yamagata, 2005). The aim of this research was to clarify the relationship between recent environmental changes and such tree deaths.
Roots of a seedling of Acacia erioloba germinated by rainfall from January 2006 were examined in November 2007 in this study. The Acacia erioloba has grown to a height of 10cm and its root to over 230cm within the two years period from January 2006 onwards to November 2007. Acacia erioloba extends deeply its main roots to the moist fine grained soil layer (sandy silt) and absorbs the water through lateral roots there, in the stage of sapling (seedling). When it reaches the stage at which water supply from the moist fine grained soil layer is not enough for the large body, the tree extends innumerable lateral roots within the 50cm depth lightly from the land surface, and they absorb water brought by fog and so on to the shallow ground. The Acacia erioloba dies when the lateral roots become unable to absorb water near the land surface due to the heavy deposit as a result of the recent decrease of floods. Until the middle of the 1970s, each flood washed away successively the sand deposited by the previous flood, and the trees died only rarely. Recently, particularly from 1980 to 1985, the number of flood days dramatically decreased, many trees died. The most persuasive causation is sand deposition and a drop in the water table caused by the decrease in flooding.
