242 PLANT SUCCESSION. 



The death of Cussonia spicata in the clumps described in this 

 paper cannot, however, be ascribed to shading by the overgrowth 

 of surrounding trees and climbers. Normally it grows to a con- 

 siderable height, and in the majority of the clumps it is quite 

 as high as, if not higher than, the other trees. Again, dead trees 

 frequently occur in situations where it is impossible to ascribe 

 death to shading. Two, at least, occur right out in the open 

 veld, and dead branches are found on trees in clumps where the 

 Cussonia still towers above the younger trees, e.g., Clump 2 in the 

 above notes. 



The writer is now quite convinced that the death of this tree 

 is caused by the attack of white ants. In nearly every clump a 

 termites' nest is found at the foot of the Cussonia. The earth 

 tunnels of the termites can often be seen passing up the bark of 

 living trees, and if the bark be removed from dead branches they 

 are found beneath it and often extend right into the wood itself. 

 Living trees are sometimes found with large portions of the trunk 

 completely eaten away by the white ants. Another noticeable 

 feature of Cussonia spicata in these clumps is that most of the trees 

 except the younger ones have almost completely lost the bark from 

 the lower portion of the trunk. This seems to be one of the first 

 results of the attack of the white ants, whose tunnels, as we have 

 seen, are frequently found below the bark. There is no indication 

 in any of the clumps examined of the white ants extending their 

 operations to other trees, none of which are stripped of their bark 

 as is the Cussonia, though why this should be so the writer is at 

 present unable to explain. 



IX. Summary and Conclusions. 



Examples of the more important types of Tree Veld in South 

 Africa are given, and the possibility of further subdivision is indi- 

 cated. Attention is drawn to a unique feature in the establish- 

 ment of Tree Veld — the invasion of grassland by trees — and the 

 consequent importance of a study of the succession in each type 

 is emphasised. 



A detailed account is then given of a type of Tree Veld, 

 termed a Cussonia-Combretum association, occurring on a hill near 

 Pietermaritzburg. The topography and climate of the hillside is 

 described, and the distribution of Tree Veld in the immediate 

 neighbourhood indicated. 



The dominant grass on the hillside is Aristida junciformis; 

 a list of others which occur is given. 



The main bases of colonisation are a shallow "central vallev" 

 and another valley further east. The trees occurring in these are 

 enumerated. 



Migration from these valleys has taken place principally 

 towards the east, probably because the eastern sides of the valleys 

 are moister than the western. Definite lines of migration from 

 the "central valley" have been formed along tributary depressions. 

 A map showing the distribution of the clumps on the slopes- 

 adjacent to the "central valley" has been prepared. 



