TREES FOR WATER CATCHMENT AREAS 167 
of small size, ascending in the Highlands of Scotland to the 
upper limit of trees, in Braemar to 1600 feet. It propa- 
gates itself freely by suckers, and is absolutely hardy. In 
very exposed situations it becomes a stunted bush. It 
grows on drier soils than other poplars, but produces in 
such conditions only a very small amount of timber. 
Aspen may be tried on absolutely waste ground at high 
elevations, where there is not much exposure; but it is 
doubtful if it would repay the expense of planting. In 
Sweden, on the best forest soil, pure aspen woods produce, 
at 50 years old, 2240 cubic feet (quarter-girth measure- 
ment) per acre, or an average annual increment of 44 cubic 
feet per acre, the stems averaging 8 inches in diameter. Of 
the total volume of wood at this age and size, about 40 per 
cent is suitable for making matches. See Meddel. Frdn 
Statens Skogsforsoksanstalt, 1917, pp. 1205-1219. 
Willow.—Three species and a hybrid are of interest to 
foresters : 
1. Sallow or goat willow (Salix caprea). This is a small 
tree, rarely exceeding 30 feet in height and 1 foot in 
diameter, common in woods and waste places. It is often 
met with as natural seedlings in plantations, where it is 
looked upon as a weed and is speedily removed in thinnings. 
Sallow has been used for fixing loose and shifting soil on 
river embankments. The wood is useful for hurdles and 
fencing. 
2, 3. White willow (Salia alba) and crack willow (Salix 
fragilis). These two species grow to a large size, and occur 
on the banks of rivers, streams, and lakes, being rare in the 
interior of woods and forests. They require much light 
and space, being similar in this respect to the poplars. 
They do not thrive at high elevations, and refuse to grow 
in acid peaty soil. They are useful for reclaiming and 
holding the soil along streams, and can be planted in marshy 
ground, which they help to drain. White willow grows 
well near the sea, and may be employed for making cheap 
shelter belts. Like all the willows, it is readily propagated 
