CHAPTER XXVII 
WOODLANDS 
In these islands the rainfall is everywhere sufficient to 
maintain woods below a certain altitude (varying from 
1,500 to 2,000 feet in different parts of the country). 
Only on moors and in bogs, where the cold, sour, unaerated 
peat is unfavourable to tree-growth, are woods impossible. 
Yet even here on the borders where the moor merges into 
the wood, one is ever advancing upon the other. It 
seems certain that many of our upland moors were once 
wooded, but as the peat accumulated the woods were 
gradually submerged by the moor; in fact, remains of 
birch and pine have been discovered in peat far above the 
present limits of tree-growth (see p. 249). In other 
places where the moor becomes drier and air permeates 
the soil, trees gain upon the moor, following in the track 
of the invading ling and heath. 
Woods are of two kinds—(a) deciduous, (6) evergreen. 
Evergreen trees are xerophytic, their leaves being retained 
during the winter, but evergreen woods are very poorly 
represented in our flora. and these have always been 
planted, except some of the pine-woods of Scotland. 
DECIDUOUS WOODS. 
Deciduous trees shed their leaves at the commencement 
of the cold season, and are leafless xerophytes in winter (see 
p- 60). The soil is generally rich in humus, well aerated, 
and well supplied with earthworms, soil-bacteria, and fungi. 
As soon as the humus so accumulates that it becomes sour, 
the earthworms and soil-bacteria disappear, and the wood 
is in danger of perishing and giving way to moor or heath. 
Our native forest-trees are few in number, and of these very 
few indeed are ever dominant (oak, beech, ash, birch). 
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