208 THK KFFKCT OF SLOPE KXPOSUUE. 



of the trt'C's produces denser shade, the more light-demanding 

 shrubs gradually disappear, and eventually are found only round 

 the margins of the mature bush/ The succession is, in fact, 

 exactly the reverse of that taking place in Tree Veld. 



Close Bush is a much less extensive formation than is Tree 

 Veld, its distribution being much more strictly limited by climatic 

 conditions. Throughout Natal, and, indeed, throughout the 

 eastern region generally, it is very noticeable that bush is confined 

 tc steep slopes facing towards the south-east. Tree Veld, on the 

 other hand, occurs principally on flat ground or on slopes with a 

 northern aspect, the actual type of Tree Veld probably being 

 determined by edaphic factors. In a general way the reason for 

 the lacalisation of bush on the south-eastern slopes is fairlj^ clear. 

 Such slopes are exposed to the rain-bearing winds from the Indian 

 Ocean, and consequently receive a greater deposition of moisture 

 in the form of rain and mist than do the northern ones. In 

 addition, they are protected from the desiccating effects of the 

 dry, hot winds that blow from the north at the end of the winter 

 season."' ^ Even on the favourably exposed slopes, however, 

 bush will not develop except in places with a fairly steep gradient, 

 which permits of rapid cold air drainage, most of the constituent 

 trees being unable to withstand frost. 



Up to the present no precise quantitative data as to the 

 differences in climatic conditions on slopes of different aspect have 

 been obtained in Natal, though, it is obviously of considerable 

 interest to determine the exact conditions which have been 

 responsible for the production of such profound divergences in the 

 character of the vegetation. In other countries the influence of 

 slope exposure on the distribution and successional changes of 

 vegetation has attracted considerable attention, and the analysis 

 of the contributory causes, particularly in America, has been the 

 subject of much investigation. The effects of slope exposure in 

 the Western States, for example, in such regions as the Eocky 

 Mountains, Sierra Nevada and the Coast ranges, are stated to be 

 very striking and definite.'^ Thus Eamaley (quoted by 

 Harshberger^), writing of the plants of the mesas in Colorado, east 

 ol the Divide, states that : 



" the vegetation on the north slopes of the mesas differs, more 

 or less markedly, from that of the south slopes. The varying amount of 

 moisture and sunshine are the important factors in determining this 

 difference. The east and west ends of the mesas are also different in their 

 plants. The west end being closer to the foothills is sheltered from the 

 afternoon sun, while the east end is not thus protected. In spring, small 

 streams from the melting snow on the hills water the west end of the 

 mesa. The snow also lies lower there. Pine trees and quaking aspens 

 grow there, while at the east end there are no trees at all.' 



In the Santa Catalina mountains of Arizona, Shrevo (quoted by 

 Harshberger^) has demonstrated that 



" the direction of slope of a particular habitat is of prime importance 

 in determining the precise character of its plant covering. The differences 

 between the vegetation of a north facing and a south facing slope at the 

 same altitude is rouerhly equal to the difference between two south facing 

 slopes which are 1.000 vertical feet apart."" 



