THE WATER SUPPLY OF DESERT PLANTS 



H. Walter 

 Botanisches Institut, Stuttgart-Hohenheim, W. Germany 



By deserts we mean arid regions with a very low and unreliable rainfall. 

 It is generally supposed that perennial plants in such regions suffer from a 

 lack of water and need special physiological adaptations such as a physio- 

 logical drought resistance, low transpiration rate, high osmotic pressure, 

 etc. Often it is difficult to understand how perennial plants can thrive in a 

 desert with perhaps only 25 mm of rainfall per year, e.g. around Cairo. 

 But rainfall as such is not the appropriate measure to estimate the water 

 supply of desert plants, which is considerably better than it appears if we 

 consider only the rainfall data. Rainfall in mm means the amount in 

 htres per square metre, therefore it is also necessary to compare it with the 

 transpiring surface of the plants per square metre. To imderstand properly 

 the water economy of desert plants, we must keep two facts in mind: 



(i) The density of the vegetation decreases in arid countries with the 

 decreasing rainfall. 



(2) In extremely dry countries with a very low vegetative cover 



density, the amount of run-off increases and the water is unevenly 



distributed in the soil after a rain. The major portion of the soil remains 



dry (the water runs off and does not penetrate into the soil) and on a 



small part of the area (depressions, runnels, wadis) the water accumulates 



and penetrates quite deep into the soil. 



Let us consider more thoroughly the first factor. Exact measurements of 



the density of plant cover were made in South-West Africa. In this region 



the rainfall is nearly nil on the coast and gradually increases from WSW. 



to ENE. from to 500 mm. The temperature conditions remain more or 



less constant over the entire region (Fig. i) . 



For comparative purposes, it is necessary to utilise similar types of 

 vegetation for which we choose ungrazed grassland. Following the period 

 of the summer rains, it was relatively easy to determine the production 

 of dry matter per hectare by weight. The results showed that the dry 

 matter production increased proportionally with the rainfall (Fig. 2). For 

 every 100 mm of rainfall per year, the dry weight of grass was 1000 kg/ha 



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