14] VEGETATIONAL TYPES OF TROPICAL LANDS 465 



may undergo rapid change and the zones of vegetation be actual 

 stages in continuing successions. 



Very widespread in the tropics are ' secondary ' scrubs and forests 

 that form parts of secondary or deflected successions engendered 

 particularly by Man or his domestic animals. When derived from 

 tropical rain forest, such communities are always more or less 

 unstable, whether they consist of weedy herbs, scrub, savanna, forest, 

 or a ' chaotic wilderness of trees, shrubs, herbs and climbers '. 

 When left to themselves and protected from burning, felling, and 

 grazing, they are gradually invaded by primary forest species and 

 proceed towards the climatic climax which with little doubt would 

 ultimately be re-established. But where they are subjected to 

 recurrent fires or persistent grazing, deflected successions set in and 

 lead to biotic plagioclimaxes. Such are, probably, many tropical 

 grasslands and savannas. Even in the forest, according to Professor 

 Paul W. Richards {in litt. binis), ' too frequent cultivation, i.e. 

 shifting cultivation on too short a rotation, is one of the most impor- 

 tant causes of deflected successions ', and, ' especially if accompanied 

 by intermittent burning and grazing, leads to the invasion of forest 

 areas by savanna Grasses and eventually to the establishment of 

 " derived " savanna '. 



Shifting cultivation, which is practised by native peoples in nearly 

 all tropical forested areas, is by far the most important cause of 

 forest destruction there. In its course the trees are felled and 

 burned, after which one or more crops are raised before the fertility 

 of the soil is lost by leaching, erosion, and the exposure of humus 

 to the sun, whereupon the plot is abandoned and another cleared. 

 The secondary succession following abandonment usually starts 

 with scries or quick-growing herbs and continues with more lasting 

 ones. During such re-establishment of vegetation, soil fertility 

 becomes partially or largely restored, so that after some years the 

 ' secondary ' (or tertiary, etc.) forest may be cleared and cultivated 

 again. The practice is, however, liable to be extremely wasteful 

 — especially when clearing and cultivation are undertaken at too 

 frequent intervals. Less drastic in their effects are the abandonment 

 of plantations and selective exploitation of timber, while strong winds 

 may also fell trees and start secondary successions. On the other 

 hand, shifting cultivation that is not too intense and short in rotation 

 is not necessarily wasteful, and may be preferable to some forms of 

 what is intended to be permanent cultivation, because it is less 

 destructive of soil fertility. It may also be less conducive to erosion. 



