THE PLANT SUCCESSION IN THE THORN VELD. 1 59 



drupes. Azima tetracantha, a somewhat rarer species, was no- 

 ticed once or twice : it has a globose berry. 



The fruits of all those earlier invaders have been noted 

 because it is significant that they are all of the fleshy type which 

 are eaten by animals, and especially by birds. These are the agents 

 of distribution, and wind-distribution seems to play a very un- 

 important part in the establishing of the Thorn Veld as a vege- 

 tation type. The lower branches of the thorn-tree, bare of 

 spines and shaded by the spreading overhead canopy, are the 

 favourite resting-places for birds. They feed in the stream- 

 bank bush, and such species of that bush as have fruits that are 

 eaten are naturally the first to be distributed throug:h the Thorn 

 Veld. Afterwards other species arrive, the majority of them 

 also distributed by birds and other animals, though some of 

 them are not: Dalbergia obovata (woody liane), Calpurnia sp., 

 Chilianthus arhoreus, Vangueria infausta, Sclerocarya caffra, 

 Harpcphyllum caff rum (Kafir plum), Combretum kraussii, 

 Pavetta lanccolata, P. sp., Xanfhoxylon capense, Gymnosporia, 

 (Cclastrus) sp., EUcodcndron (rthiopicum, Grczvia caffra, 

 Roycna paUcus, R. spp.. Plumbago capensis (liane), Brachylcena 

 discolor . 



The young plants of the various species named grow up 

 through the herbaceous and shrubby species which form the 

 preliminary stage, and soon a tangled growth results (see plate 

 8). The soil is loosened, and a certain amount of humus even 

 collects ; other herbaceous species appear, such as Sanseviera 

 thyrsiflora, Kala)ichoc rotundifolia, Barleria obfusa. Asparagus 

 spp.. Scilla sp., and a parasitic species of Melasma. 



Further Stages in the Succession 



Very soon the si>ecies which began under the thorn-tree 

 grow up through it. At a fairly early stage it is common to 

 find Cclastrus or Ehretia towering above it (Fig 2, Stage 8). 

 The lianes, such as Vitis cuncifoUa, V. cirrhosa, Asparagus spp.. 

 sometimes spread all over the top of it. and the thorn-tree may 

 ultimately be killed. On the other band, where herbivorous 

 animals are abundant, unprotected species are kept down as a 

 result oif grazing', and even old thorn-trees have no undergrowth 

 whatever. In this case the succession is limited, and the Thorn 

 Veld remains as pure acacia veld. The thorn-trees themselves, 

 however, grow closer and closer together, especially if the 

 grazing prevents grass-fires, and ultimately we get pure thorn- 

 thickets. The more natural succession is seen where grazing 

 animals are not abundant or are excluded by areas being fenced 

 in.. The clumps which grow round pioneer thorn-trees at first 

 are scattered, as the pioneers themselves were, but the inter- 

 vening areas are colonized by more thorn-trees, which, in turn, 

 produce clumps, and soon a stage is reached where the branches 

 of separate thorn-trees touch. By this time many other species 

 of trees have arrived, and they are not all distributed by animals. 

 .\ new struggle commences, and the climax type is the succulent 



