io8 Studies in Forestry [CHAP. v. 



IV. Situation in Relation to Woodland Growth. 



The growth and the regeneration of the different kinds of 

 forest trees are dependent on combinations of various con- 

 ditions regarding temperature and moisture, which are united 

 under the term climate. Some demand for their normal 

 development an amount of warmth which is absolutely fatal 

 to others ; and these again can bear, and often even prefer, 

 a low temperature during winter, which causes the vital energy 

 of the former to cease, never to re-awaken. Some transpire 

 so freely through the foliage (e.g. Spruce) as to need rather 

 a damp soil and atmosphere, whilst others (e. g. Scots Pine) 

 prefer a drier condition. Without light and warmth the action 

 of chlorophyll in absorbing carbonic acid from the air, and 

 consequently assimilation, would be impossible. 



Warmth is in general dependent on, and inversely propor- 

 tional to, the distance of any locality from the equatorial line, 

 although practically the nature of modifying circumstances 

 connected with the distribution of land and water, high moun- 

 tain-chains, &c., is always of more or less influence. Thus, in 

 Britain, owing to our insular climate, and to the humid and 

 essentially equalizing tendency of the Gulf-stream, we have no 

 such extremes of summer heat and wintry cold as annually 

 occur in the continental areas lying along the same degrees of 

 latitude in Europe, Asia, or America. Owing to these local 

 modifying influences, there are nowhere hard and fast zones of 

 woodland crops. With ascent above the sea-level, and conse- 

 quently above the densest and heaviest layers of atmosphere 

 capable of acquiring most warmth in summer, temperature 

 falls at about the rate of i Fahr. for every 300 ft., and Angot 

 has estimated that for every 333 ft. in vertical ascent woodland 

 growth is retarded for about fourteen days in the awakening of 

 vegetative activity during the spring. 



Recent investigations made in the Bavarian Alps (Brenner Pass, 

 4,500 to 5,000 feet above sea-level) have led to the following conclusions 



