10 A MANUAL OF FORESTRY. 



5. Change of Species. 



The question deserves to be mentioned here, whether 

 in Sylviculture a change of species is advisable, or 

 not, at the end of successive rotations. Change of crops 

 is practised in agriculture, because different species make 

 different demands on the soil. By 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 exact- 

 ing species no longer thrive in certain localities. 



On a close investigation such a theory will be found 

 untenable 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 sub- 

 stances,* so that all except poor soils can go on pro- 

 viding the necessary quantum of mineral matter for any 

 length of time. The trees protect the soil by their 

 foliage, whilst 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 be actually produced. If, nevertheless, in some 



* See pages 140 H3 of Volume I. 



