16 BRITISH FOREST TREES 



the protection of the soil are such as have a dense crown 

 of foliage, and maintain their growth in close canopy through 

 long stages of their development. 



All species of forest trees are not endowed alike with the 

 qualities requisite for maintaining or improving the pro- 

 ductive capacity of the soil. Among deciduous trees they 

 are conspicuous in the beech, with its close canopy, dense 

 foliage, and heavy fall of leaves rich in potash, and slowly 

 decomposing under the shade of the parent trees into strongly 

 hygroscopic humus of excellent quality. Hornbeam foliage 

 is thinner, decomposes more rapidly, and is less in quantity ; 

 the thickly-foliaged lime shades the soil well, but yields 

 only a small amount of humus ; the chestnut has a some- 

 what full crown of good mould-producing foliage ; but none 

 of these trees occur forming large pure forests in Britain. 

 Oak, elm, ash, maple, sycamore, and the other more valu- 

 able and remunerative broad-leaved trees are unfortunately 

 not naturally adapted on the whole for protecting, far less for 

 increasing, the productive power of the soil, except when 

 grown in admixture with species endowed with such 

 capacity in a higher degree than themselves. Among the 

 conifers, spruce and silver fir especially, and in a less 

 degree the Weymouth, Austrian, and maritime pines, are 

 gifted with soil-improving qualities, which are increased by 

 the fine growth of moss beneath them acting like humus in 

 its sponge-like absorption and retention of soil-moisture. 

 The less thickly foliaged conifers, Scots pine and larch, 

 are least of all endowed in this respect, though during the 

 earlier stages of growth a layer of moss (principally species 

 of Hypnuui) is produced under them, which protects the 

 soil from sun and wind. But as this covering of moss is 

 apt to disappear when the opening up of the canopy reaches 

 a certain degree, such pure forests cannot be worked with 

 long periods of rotation without endangering the quality of 



