PLANTING OF WATEE CATCHMENT AEEAS 113 



districts, the upper zone of any plantation, no matter of 

 what altitude, is stunted, broken, and battered by the wind. 

 It is advisable, then, to carry planting on hill slopes some 

 distance beyond the favourable ground for trees, as in this 

 way an upper shelter belt is formed, which will be very 

 useful in breaking the force of the wind. Practically, 

 then, the only plantations that should be attempted on peat 

 moors are in the nature of shelter belts. 



The effect of altitude on trees will now be briefly 

 considered. The lowered temperature and short growing 

 season at high altitudes militate against growth of vegeta- 

 tion ; and, as is well known, trees cease to exist above a 

 certain elevation, varying with the climate and latitude. 

 Mere altitude is, however, scarcely so important as other 

 factors, which depress the timber-line, the name given to 

 the limit to which woodland ascends. Trees, on account 

 of their extensive foliage surface, transpire enormous 

 quantities of water; and their growth is checked or 

 stopped altogether when the transpiration becomes excessive. 

 Wind increases transpiration, and the greater the velocity 

 of the wind, the greater the transpiration. At high 

 altitudes the wind is more constant and attains a greater 

 velocity than at low levels ; and the main cause of the 

 low timber-line in the British Isles appears to be the force 

 and constancy of the prevailing winds. 



This view is confirmed by recent investigations in 

 Switzerland by Dr. Brockmann-Jerosch, who states that the 

 upper tree limit is not determined by any one factor, such 

 as rainfall, snowfall, or the average temperature of the year. 

 It is determined for each locality by the physiography or 

 lie of the land, or in other words by the exposure. This 

 agrees substantially with the fact that in a continental 

 climate, with hot dry summers and cold severe winters, the 

 tree limit in the mountains is higher than in an oceanic 

 mild climate, as in the latter there is more wind and 

 consequent increase of transpiration. The best example 

 in Scotland is the great height of the tree -line in the 

 Cairngorm and Deeside Highlands, as compared with the 



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