CHOICE OE SPECIES. 121 



on the soil. B}' changing the crop annually more time is 

 given for the accumulation of the substances which a particular 

 species may require. In the same way it has been suggested 

 that better results might be obtained in sylviculture by a 

 change of species, especially as certain phenomena seemed to 

 support such a view. It has been observed, for instance, that 

 coniferous trees here and there supplant broad leaved species, 

 that spruce frequently usurps the place of silver fir, and birch 

 that of Scotch pine, or that exacting species no longer thrive 

 in certain localities. 



On a close investigation such a theory will be found unten- 

 able except on poor soils. Timber trees take comparatively 

 small quantities of mineral substances from the soil, if the leaf 

 mould be not removed, in fact, only about one-twelfth of the 

 total quantity required by field crops, and about one-twentieth 

 of the rarer substances ;* hence, all except poor soils can go 

 on providing the necessary quantity of mineral matter for any 

 length of time. The trees protect the soil by their foliage, 

 M'hilst the fallen leaves afford a substantial amount of organic 

 matter, in addition to the inorganic materials previously taken 

 from the soil. This amount may be so considerable that an 

 increase of fertility may actually be produced. If, neverthe- 

 less, in some cases a reduction of the fertility should be 

 observed, it will be due to insufficient shelter afforded by trees 

 with thin crowns, to heavy removals of litter, faulty treatment, 

 fires, or other causes. 



The fact that one species sometimes supplants another is 

 generally due to its greater reproductive power, or its greater 

 resisting power against external influences. For instance, the 

 felhng of a spruce or Scotch pine wood may be followed by the 

 appearance of large numbers of insects, which breed in the 

 stools of the trees left in the ground, and destroy any young 

 crop of these species which may spring up ; other species not 

 subject to such attacks may then occupy the ground, if allowed 

 to do so. 



* Sec page "^S. 



