207 



THE EFFECT OF SLOPE EXPOSUEE UPON THE CLIMATE 



AND VEGETATION OF A HILL NEAK MARITZBUEG : 



A PRELIMINARY INVESTIGATION. 



BY 



R. D. AiTKEx, M.Sc. 

 Departmoit of Botany, Natal Universitu College. 



Bead July 11, 1922. 



Introduction. 



Ecological investigations on the vegetation of Natal, which 

 have been in progress during the last ten or twelve years, have 

 brought very clearly to light the existence of two main types of 

 tree growth, differing conspicuously in their habitat, physiognomy, 

 composition and succession. Of the two, Tree Veld, as it has 

 been termed by Professor Bews,''* is one of the most extensive 

 types of vegetation in South Africa. It has a characteristic, park- 

 like appearance due to the growth of isolated clumps of trees and 

 bushes in the surrounding grass land. The unique feature of this 

 vegetation, as has often been emphasised, lies in the early stages 

 of the succession, where invasion of the grass land is carried out 

 by pioneers which are themselves trees. Once these are estab- 

 lished, other trees and shrubs grow up in their shade, and large 

 clumps may be formed. Naturally the constituents of such a 

 formation are strong, light-demanding and xerophytic trees, the 

 pioneers particularly being able to withstand intense sunlight, 

 even in the early stages of their lives. 



The second type of tree growth, generally spoken of as Bush 

 or Close Bush, presents a decided contrast in appearance to the 

 one just described. Instead of isolated, scattered clumps the trees 

 grow close together, large areas being frequently covered with a 

 dense canopy of trees and shrubs. In the deep shade cast by 

 these, the undergrowth is naturally by no means luxuriant, though 

 mosses, ferns and various Acanthaceae are fairly abundant. 

 Around the margins of the bush, however, a dense, and at times 

 almost impenetrable, growth of tall shrubs and climbers occurs. 

 A study of the changes taking place around the margins of a 

 spreading bush, and of those during the re-establishment of bush 

 after fire, shows a very different type of succession from that 

 characteristic of Tree Veld. The early stages are formed by the 

 growth of tall, somewhat mesophytic grasses and shrubs, in the 

 shade of which trees slowly establish themselves. As the growth 



* The numbers refer to the papers listed in the References at the end of 



this memoir. 



