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. Fran 

 Statens Skogsfbrsoksanstalf, 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 (Salix 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 



