

be. ii PURE AND MIXED CROPS 111 



always more difficult to carry out, as the shade-enduring 

 species spring up and cover the ground with more 

 alacrity when the first thinning out of the leaf-canopy 

 |is made. However, these disadvantages are usually more 

 than counterbalanced by the generally more healthy 

 haracter of the forest, and the most undesirable type of 

 :rop is that which is pure and entirely composed of shade- 

 avoiding species. In such a forest the trees are not apt 

 to develop clean boles, and the trees, especially in their 

 later stages, will not be as healthy and vigorous as 

 those which can have their boles protected from the 

 sun and the soil over their roots kept shaded and moist 

 oy other species in mixture. All pure crops are also 

 more liable to suffer from ravages of insects or from the 

 spread of fungoid diseases, which often show a predilec- 

 tion for particular kinds of trees. Perhaps the next 

 most dangerously situated in this respect is a mixed 

 forest composed of trees belonging to the same natural 

 Drder, as many insects or fungi, although avoiding trees 

 belonging to other families, will thrive on allied species. 



The maintenance of pure crops is considered justifi- 

 able only if the species composing them is a soil- 

 improver, or if the soil is of such a natural depth and 

 fertility and has a sufficient moisture not to deteriorate 

 under it, or where the soil will only bear one particular 

 species of trees ; or, again, where only one particular 

 species is of value to the proprietor ; or, finally, where 

 the rotation is so short that the effect of want of 

 mixture on the leaf-canopy and in the soil has not had 

 ;ime to make itself felt. 



In the Tropics there are several instances of naturally 

 pure forests, such as those formed by some species of 

 Dipterocarps, such as Shorea robusta, Dipterocarpus 

 'urbinatus, or by various kinds of Acacia. In the 

 ? ormer case the gregarious nature of the trees is largely 

 iue to their shade-enduring character and the great 

 a eight which they attain, from which they are able to 

 suppress most competitors. In the case of the Acacias, 

 hey are the survivals of the fittest on certain soils. 



