220 BRITISH FOREST TREES 



shoot up quickly in pure patches or even in groups, but 

 soon thin themselves out, and by the twentieth or thirtieth 

 year are so broken and incomplete in canopy, that the soil 

 beneath them is bound to deteriorate. ' Whilst their retention 

 in patches, or singly in too great numbers, would undeniably 

 be a mistake, yet the selection of good, healthy, individuals 

 of vigorous growth, scattered here and there wherever nature 

 has shown the soil to be congenial to the birch, is in every 

 way justifiable, and often yields very fair returns about the 

 fortieth or fiftieth year, up to which time they can under 

 such circumstances usually hold their own with the beech, 

 and when they can be removed without interrupting the 

 density of the canopy of the crop to any serious extent. This 

 is perhaps still more the case where the silver fir is concerned, 

 and then even the retention of the birch in small patches also 

 often yields satisfactory results, for the former can thrive 

 fairly well on good soils under the light shade of the latter 

 until its removal, which, however, is often necessitated when 

 the silver fir begins to catch up the birch in growth, as other- 

 wise the whip-like twigs and sprays injure the leading-shoots 

 of the fir. The injury thus caused is still more marked in 

 the case of the spruce, into young growth of which the birch 

 very frequently pushes its way ; but until the spruce begins 

 to equal it in height, which happens sooner than in the case 

 of the beech or silver fir, it is generally advisable to allow it 

 to continue forming a portion of the crop, provided always 

 that it occurs merely as scattered individuals, or in lines or 

 rows, and not in patches or groups ultimately unable to pro- 

 tect the soil, and then finally only forming unprofitable blanks. 

 Older patches of birch often give good returns if under- 

 planted with spruce and removed later on when they begin 

 to interfere with the further development of the latter by 

 rubbing off the foliage of the leading-shoots. Along with 

 aspen, ash, and alder, it often forms mixed forests on 



