V. NEW APPROACH TO WATER RELATIONS OF DESERT PLANTS
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The quantitative relationship between the amount of yearly rainfall and
the density of plant cover can be investigated meaningfully only where
there is a gradual increase in rainfall from one part of a region to another
and where additional factors which influence plant cover remain constant
throughout the region. Such additional factors are ( 1 ) the distribution of
rainfall throughout the year, (2) the more or less constant temperature
throughout the entire region, (3) the uniformity of soil composition and
life forms of plants, and (4) the lack of interference by human activity.
A region where these conditions are fulfilled was found in Southwest
Africa. Here, the cold oceanic Benguela stream comes close to the seashore. Just inland along the coast is the Namib Desert, which receives no
rainfall. Further inland the amount of annual rainfall increases gradually
and reaches 500 mm/year at the border of the Kalahari (see Fig. 1 ). RainFig. 1. Distribution of rainfall in Southwest Africa (in mm/year): Test plots for
determination of productivity of the grassland ( □ ) ; locations of samplings for the
analysis of forage plants ( # ) (from Walter and Volk, 1954, p. 97).