CLIMATE AND TREE GROWTH 89 



parage British weather, nothing is gained by refusing to 

 look facts in the face, and ascertain as far as is possible 

 the actual state of affairs. Two methods of investigation 

 are available in this respect — one confining itself to the 

 temperature, rainfall, and other climatic features of 

 particular localities in various countries, and the other to 

 the effects of these factors upon tree growth, as seen in the 

 height and development of isolated or massed specimens. 



The principal climatic factors in promoting tree growth 

 and timber production may be considered to be sufficient 

 heat and moisture during the growing season. Heat 

 associated with drought, or moisture with cold, are 

 equally prejudicial to rapid growth, and while the most 

 favourable amounts of heat and moisture vary with 

 species and soil, they can be determined within certain 

 limits in most countries. A favourable combination of 

 heat and moisture, applied to a species in any given soil, 

 results in the maximum production of leaves and wood 

 during the season, and if repeated annually throughout 

 the life of the tree, in the maximum height growth, and 

 heaviest production of timber possible for that particular 

 species. 



In a general way, an excess of heat over moisture 

 favours the production of flowers and fruit at the expense 

 of leaves and wood, while an excess of moisture over heat 

 has the opposite effect, but this is only true in a relative 

 sense. The absolute production of either fruit or wood 

 demands certain quantities of both these factors, and 

 when either falls below the minimum, absolute and relative 

 growth decrease. Mayr ^ has laid down the principle that 

 a mean temperature of 50" F. during the four months 

 of May, June, July, and August is necessary for the 

 growth of trees to a height of 25 feet, and that timber 

 trees will not grow where the rainfall falls below 2 inches 



^ IValdbuu auf naturgeselzlicher Grundlaye. 



