CHAP, xi.] Effects of Underplanting 245 



experiments, that the quantity of moisture under a soil-covering 

 of living plants is always less during the period of active vege- 

 tation than at a similar depth on naked soil (in consequence 

 of transpiration); that the influence is quite traceable down to 

 the lower layers ; that the quantity found is less when the 

 plants stand densely (without being inversely proportional to 

 the density) ; and that a soil-covering of humus and dead 

 foliage, up to 2 inches in thickness, favours percolation from 

 above downwards as compared with naked soils. 



As Beech, Hornbeam, and Silver, Nordmann's and Douglas 

 Firs have all very much the same sort of heart-shaped, mode- 

 rately deep root-system, the practical effects of underplanting 

 with these different species will ordinarily be about the same ; 

 whilst differences will be noted when the shallow-rooting 

 Spruce is selected as the underwood on soils for which it 

 seems best suited. In the latter case the woods will still 

 show more moisture than pure woods throughout the upper 

 layer of soil, for the Spruce undergrowth will draw its main 

 supplies of water from the layer from 12-18 inches below the 

 surface ; whilst, below that, the quantity of moisture will be 

 much the same, at any rate during the first half of the annual 

 period of vegetation. But, owing to the great density of an 

 underwood of Spruce, a large proportion of the atmospheric 

 precipitations will be intercepted without ever having any 

 chance of percolating to the lower soil ; hence the deeper 

 layers will generally have less moisture than similar soils 

 without underwood. During the autumnal and winter period 

 of rest the soil under broad-leaved, deciduous underwood 

 must (ceteris paribus] show larger supplies of moisture than 

 under a coniferous undergrowth with persistent foliage, which 

 not only intercepts a larger percentage of the precipitations, 

 but also requires water from the soil for purposes of transpi- 

 ration. 



Dense undergrowth throughout a whole wood will of course 

 withdraw larger supplies of moisture from the soil than that 



