EFFECT OF FOREST VEGETATION ON THE LOCALITY. 43 



certain species of forest trees possess these two qualifications. 

 During youth most species answer, no doubt, very well ; 

 with advancing age, however, the crowns are not only lifted 

 higher and higher from the ground, but most species thin 

 out considerably. The result is, that the leaf canopy be- 

 comes more and more interrupted and thinner, followed by a 

 crop of noxious weeds, or too rapid decomposition of the 

 humus, accelerated evaporation of the moisture from the 

 upper layer of the soil, and generally a reduction of the 

 fertility of the soil. To prevent such results, the forester 

 must either cut over the woods before the process of thinning- 

 out has proceeded too far, or underplant such woods, or 

 cultivate species which are capable of maintaining a complete 

 cover overhead up to an advanced age. 



Amongst the timber trees with which this l)Ook deals, 

 beech, silver fir, spruce, and hornbeam, are the species 

 which, above all others, preserve a complete leaf canopy 

 until, or nearly to, maturity. These are shade bearing 

 species. All other species are, with certain modifications, 

 less capable of preserving the factors of the locality ; the 

 greater their light requirement and the thinner their crowns, 

 the smaller is their capacity in this respect. Those least 

 suitable are birch, poplars and acacia ; next willows, larch, 

 most pines (with advancing age), oak, ash, elm, Norway 

 maple, sycamore, and alder. 



The production of humus from fallen leaves is greater in 

 woods consisting of broad-leaved species than in coniferous 

 woods, because the more important broad-leaved species are 

 deciduous and shed the whole of their foliage every year, 

 while, with the exception of larch, the conifers are evergreen. 

 The silver fir sheds about one-ninth of its foliage annually, 

 spruce about one-seventh, the pines about one-third to one- 

 fourth. The production of humus from fallen leaves is, 

 generally speaking, greatest when the rate of height growth 

 culminates. 



The accumulation of humus depends greatly on the 



