48 Studies in Forestry [CHAP. in. 



in atmosphere and soil, with increasing height above sea-level, 

 also causes a more or less irregular and inconstant distribution 

 of species according to vertical zones in the mountains of 

 central Europe, although in this respect the variations due 

 to soil, situation and aspect are so great that it would perhaps 

 be misleading to attempt any classification. It must, however, 

 be remarked concerning the Scots Pine and the Spruce that 

 their limits towards the north, and towards Alpine heights, are 

 not so much due to the actual intensity of cold during winter, 

 as to the fact that transpiration by the leaves is stimulated 

 on bright sunny days in winter at a time when the frost-bound 

 soil can yield no supplies of moisture to replace what is 

 evaporated through the leaves J ; hence the natural consequence 

 is that the foliage grows yellow, the tree turns sickly, and death 

 ensues. These results can most frequently be observed after 

 long, dry, hard winters with frequent sunny days. 



The southern limits of the forest trees of northern Europe 

 are mainly determined by the quantity and the regularity 

 of the rainfall during summer. As the results of careful 

 experiments conducted in Austria, von Honel 2 found that, 

 computed by the unit of weight of their dry foliage, among 

 broad-leaved trees, Ash and Birch require the largest supplies 

 of water for transpiration, then Beech and Hornbeam, then 

 Elms, and finally Maples and Oaks. Among conifers, the 

 order was Spruce, Scots Pine, Silver Fir, Black Pine. He also 

 found that, taken as a whole class, the broad-leaved species 

 consumed on the average about ten times as much water as 

 the conifers, and that, owing to the light foliage of the Pine, 

 this species required very much less soil-moisture than Spruce 

 or Silver Fir. But although the actual total of average annual 

 rainfall may be considerably greater than what is required for 

 transpiration through the foliage during the active period of 



1 R. Hartig, Lehrbuch der Baumkrankheiten, and edit., 1889, pp. 104, 

 361. 



2 Compare, however, the more recent data set forth in Chapter IV. 



