GENERAL INTRODUCTION 11 



Yet, it must be admitted that the limits of conditions within 

 which the varied life of the forest can maintain itself, are 

 comparatively narrow. Many of the native trees are 

 extremely susceptible to frost. Some of the bush-plants of 

 Otago cannot endure the winters of the more Northern 

 Canterbury Plains, but this is not because the forest is 

 decadent. The climate of these plains is an extreme one. 

 Plants that can flourish there must be able to withstand 

 excess of drought, heat, cold, and insolation. Mr. T. W. 

 Adams, of Greendale, has shown that there are but few foreign 

 trees and shrubs which can adapt themselves to these con- 

 ditions. It is, therefore, scarcely a matter for surprise that 

 this district and the somewhat similar one of Central Otago, 

 should be treeless. Undoubtedly, they were both once partially 

 bush-clad ; but the destruction of their forests was probably 

 not in any way due to an increasing severity of climate. It is 

 susceptible of quite another explanation. In these districts 

 the rainfall does not reach thirty inches a year. Now, the 

 experience of many countries, but particularly of the United 

 States, proves that forests cannot exist permanently in regions 

 with a rainfall of less than thirty inches per annum. If, 

 owing to a cycle of wet weather, accompanied perhaps by 

 other contributing causes, they manage to get a foothold in 

 arid districts, they are always liable to be swept off by fire ; 

 and, being once so destroyed, it is difficult for them, without 

 artificial assistance, to become reinstated. 



The soil, unprotected by the shade of the foliage, 

 dries up, and germination is soon made impossible. On hill 

 slopes the spongy mosses no longer retain the moisture. 

 After rain, the rivers and streams become more quickly 

 flooded. Hundreds of acres of soil are thus frequently swept 

 away, and a bare rocky surface replaces the once dense forest. 

 This process is going on throughout New Zealand wherever 

 the bush is being artificially cleared, but the devastation is 

 greatest on the steep hill tops. In America it has been found 



