PLANTING OF WATER CATCHMENT AREAS 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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