44 LOCALITY ANI> FOREST VEGF/FATION. 



rapidity with whicli the leaves are decomposed, a process 

 which is regulated hj the species, degree of cover overhead, 

 and the cliaracter of the locality. Generally speaking, 

 needles decompose more slowly than leaves. Of needles, 

 those of the larch decompose most quickly, next perhaps 

 those of Weymouth pine, Scotch pine, and Austrian pine, 

 last those of silver fir and spruce. Of leaves, those of ash, 

 alder, hornbeam, lime, and hazel decompose quickly ; more 

 slowly the leaves of oak, birch, and sweet chestnut. The 

 leaves of beech stand perhaps half-way, but as beech woods 

 enjoy a dense shade, their accumulation of humus is much 

 greater than that found in oak or birch woods. On cal- 

 careous and sandy soils, humus decomposes more quickly 

 than on loam and clay soils. It is also more rapid in warm 

 low lands than in cloudy mountain regions. The most 

 suitable, or normal, time for the process may be put at two 

 to three years, when the most favourable results in respect of 

 quantity and quality of humus are produced, 



A few words about each of the more important species 

 will not be out of place here : — 



Beech improves the soil in the highest degree, because it 

 has a dense crown and yields a heavy crop of leaves, which 

 decay slowly. Beech woods, if undisturbed, show a thiclcer 

 layer of humus than woods of any other species. 



TlnniJicam approaches beech in this respect, though it 

 does not equal it. 



Lime gives good shade and a heavy crop of leaves, but its 

 timber is of such inferior quality that it is rarely planted 

 for economic purposes. 



Street eheainnt sheds a heavy crop of leaves, but the leaf 

 canopy is comparatively incomplete, admitting too much sun. 



Oah, ash and iciUoir have too thin a leaf canopy to do 

 justice, generally speaking, to the locality. 



Silver fir and spruce are capable of preserving a dense 

 cover overhead up to an advanced age. 



Erert/reen conifers, other than silver la- and spruce, though 



