246 Studies in Forestry [CHAP. xr. 



which forms only a light canopy. Hence underwood may some- 

 times be utilized to the greatest practical advantage if only 

 formed here and there where there is no good layer of dead 

 foliage or moss ; for this may often give the most practical com- 

 bination of protection against sun and wind and of formation 

 of humus with least drawback on account of consumption of 

 soil moisture and soluble nutrient salts. And although the old 

 idea, that soils covered with underwood were invariably moister 

 than similar soils without undergrowth, has not proved itself 

 to be unconditionally correct, yet the advantages of the 

 undoubtedly better layer of dead foliage and humus have in 

 no way been overrated. 



It is a well-known practical fact that the more valuable 

 classes of Oak and Pine timber cannot be profitably produced 

 merely by repeated thinnings and by delay in the utilization of 

 the crop l , since the increased growing-space demanded by the 

 crowns, and the consequent interruption of the leaf-canopy, are 

 incompatible with the conservation of the productive capacity 

 of the soil when it becomes exposed to the deteriorating influ- 

 ences of sun and wind. Strong thinning or partial clearance 2 

 of Oak crops about the sixtieth to the eightieth year, or of Scots 

 Pine about the thirtieth to the fortieth year, after their chief 

 growth in height has been attained on the different classes of 

 soils and situations, with simultaneous underplanting or under- 

 sowing of the shade-bearing species best suited to the soil in 

 question, is the only rational and profitable method of stimu- 

 lating the trees to further production of good annual incre- 

 ment. Mere enlargement of the growing-space will of itself 

 lead to enhancement of increment per individual stem ; but, as 

 has been above shown, its technical quality and its disposal will 

 not be so good or remunerative, whilst this increment will ulti- 



1 Compare London, Arboretum et Fruticetum Britannicum, 1838, 

 vol. iii. p. 1809. 



2 When up to 15 % of the basal area of the stems is removed, it is 

 a thinning, but beyond that it becomes a partial clearance \ see Chapter IX 

 on The Tending of Woods. 



