METHODS AND PRACTICE 155 



species which will aid the development of the permanent 

 crop and create and maintain suitable soil conditions, 

 and protect the crop against wind. Third, a species 

 which will act as nurses, and yield some return in the 

 shape of thinnings before the thirtieth or fortieth year. 

 And fourth, trees which will suppress and kill out surface 

 vegetation during the first ten or fifteen years after 

 planting. The two former species may be regarded as 

 permanent and indispensable ; the two latter as temporary 

 components of a forest crop, which may or may not be 

 regarded as necessary according to circumstances. 



The commercial value of hardy species is dealt with 

 elsewhere, and all that need be stated here are the general 

 principles involved in forming mixtures. It is an 

 accepted principle in sylviculture that a correct mixture 

 should always contain a shade-bearing species to maintain 

 the surface soil in a fertile state. It is equally important 

 that a mixture should contain a wind-firm species, except 

 perhaps in the most sheltered situations. Species for yield- 

 ing valuable thinnings are necessary from an economic 

 rather than a sylvicultural point of view. The ultimate 

 development and maturing of the crop are not affected 

 one way or the other by them, but the nett profits are 

 considerably influenced by species which can save the 

 working expenses of the crop after the twentieth year. 

 Surface-covering species are chiefly necessary on poor, 

 thin, exposed, or peaty soils, on which the trees of the 

 permanent crop have a hard struggle to establish them- 

 selves, either owing to poverty in the first few inches 

 of soil, or to exposure to wind. 



So far as permanent crops are concerned, mixtures are 

 not necessary for soil protection in the case of shade- 

 bearers, but are frequently required for wind resistance. 

 Silver fir and beech, for instance, can stand akne without 

 any danger to themselves, or deterioration of the soil. 



