TREES FOR WATER CATCHMENT AREAS 163 
Natural birch woods as a rule contain little timber. Those 
in Glen Mor were estimated to produce when felled only 
10 tons of wood per acre, and half a ton of twigs, the 
latter being used for burning the surface scale from steel 
plates in foundries. 
The value of birch woods lies in the protection which 
they may afford to plantations of conifers, lying alongside 
them at a lower level. With the aid of birch, it would be 
possible perhaps to raise the timber line 200 to 300 feet 
higher in many mountainous districts in the British Isles. 
Any natural scrub or wood of birch in the vicinity of a 
high-lying conifer plantation should be enclosed, and be 
carefully preserved as a shelter belt. Birch might also be 
sown freely over wide bands of ground immediately above 
the sites of contemplated plantations at high elevations, in 
cases where the ground could be prepared for the seed 
cheaply and be enclosed at a trifling cost. 
Birch is also very useful as a nurse tree, in frosty 
localities and in exposed situations, where damage to young 
conifers is to be feared; and it may be planted in advance 
for this purpose. Thickets of self-sown birch thinned out 
to five or six feet apart will serve as nurses for spruce 
seedlings, as the latter species usually succeeds in places 
where birch is able to regenerate itself freely. Birch 
can be very cheaply planted by the slitting method. 
In plantations on good sites and favourable situations, 
birch is a weed, and should be eliminated as soon as 
possible, 
There are two distinct species of birch, differing in their 
demand on moisture in the soil; and it is a great mistake 
to plant them indiscriminately. (1) Silver birch (Betula 
verrucosa), with glabrous twigs, pendulous branches, and 
very white bark, is the faster-growing and larger tree of the 
two species. It succeeds in a dry climate, and thrives best 
on a moderately moist soil, and will not grow on marshy 
ground or in an undrained peat-moss. It succeeds on chalk, 
where the other species remains stunted or dies. This 
species occurs in Strathspey, Deeside, and Tayside, which is 
