222 NATURALISATION OF FOREST TREES. 



may be some grounds for such a view, because, generally speaking, 

 similar climates throughout the world produce similar types of 

 vegetation, and a veld vegetation is produced in other countries 

 besides South Africa by a sub-humid or semi-arid, extra-tropical, 

 summer-rainfall climate. This point of view^ however, neglects 

 several important considerations. 



Firstly, there are many cases of successful afforestation in treeless 

 localities. 



Secondly, a grass and forest vegetation are found together in 

 other parts of the world, either in separated patches according to 

 situation, drainage, etc., or with the grass actually as undergrowth 

 under somewhat open forest. This is, at any rate, the case with the 

 coniferous forests of Western America. 



Thirdly, everything that man does is more or less artificial, and 

 while he cannot change the laws of Nature, he is constantly altering 

 Nature herself. For instance, it is said that forest vegetation has 

 advanced appreciably over the American prairies during the last 

 generation, as a result of man's action in keeping the dense grass 

 grazed down, preventing the prairie fires, leaving fallow fields and, 

 in short, by his artificially improving the conditions for the natural 

 extension of the forest. The most obvious way, however, by 

 which man can render natural conditions more suitable for forest 

 vegetation is by himself establishing the forests. All Forest 

 Meteorologists are, I think, now agreed that most forests increase 

 relative humidity, reduce evaporation and increase the capacity 

 of the soil for the absorption and retention of moisture. It is 

 specially these factors which are of vital importance on the border- 

 land between forest and grass land, and Professor Mayr lays stress 

 on this point. He says : — 



" The formation of trees of a more or less dense cover tends to increase 

 the humidity of the air beneath the crown cover by a maximum of io%. 

 We can merely mention here how important this function of the forest must 

 be for the existence of the forest wherever an air humidity of 50% for the 

 four vegetative months (which he states is the essential minimum) is 

 approached (e.g., the successful growth of artificially established plantations.") 



Also as a result of the protection of a forest cover, natural regenera- 

 tion may take place easily where the initial aftorestation has been 

 of great difficulty. 



Fourthly, there is the most important consideration that there 

 has been a large element of what we may call " chance " in the 

 evolution of the world's vegetation. Regions have been isolated 

 from the rest of the world by geological changes, oceans, the tropics 

 and so on, and genera or species which thrive in one region have 

 been physically prevented from spreading to similar regions else- 

 where. It only requires the hand of man to overcome the natural 

 barriers. For instance, the genus Piniis is confined to the Northern 

 Hemisphere, and South Africa is cut off from the latter by the 

 Tropics, and yet we know that several species of pines become 

 readily naturalised here. 



I feel convinced that had South Africa been connected, say, with 

 .Western America, a large part of her interior would have been 

 clothed with pine forests. 



