198 TRANSACTIONS OF ROYAL SCOTTISH ARBORICULTURAL SOCIETY. 



28. Under planting. 



By Lieut. -Colonel F. Bailey. 



When a young crop of trees of any species has completely 

 closed in over the ground, it will, provided it be of proper 

 density, afford effectual shelter to the soil, conserving moisture, 

 yielding some humus, and keeping down noxious weed growth. 

 But a time comes when the individual trees of such a crop 

 demand more room. The more vigorous of them then begin 

 to crowd out their weaker associates, with the result that the 

 number of healthy stems becomes so much lessened that the 

 density of the canopy formed by the mass of crowns is impaired, 

 and, in the case of light-crowned species, the sun's rays penetrate 

 to the humus layer, while air currents sweep over the soil. 

 Evidence of this is soon afforded by the springing up of grass 

 and other light-loving weeds. Later on, when "thinning out" 

 has proceeded farther, brackens, heather, whins, broom, or 

 other herbs and shrubs appear ; and the layer of decaying 

 vegetable matter on the ground, which is essential to the 

 continued prosperity of the crop, becoming too rapidly oxidised, 

 ultimately disappears. But before this advanced stage has been 

 reached, careful observation, supported by accurate periodic 

 measurements, would have demonstrated that the rate of growth 

 of the remaining, and apparently still healthy, trees had begun 

 to fall off; and the deterioration of the stock may be carried to 

 a disastrous extent. 



All species do not behave identically in this matter, those 

 which suffer most under the natural process of "thinning out" 

 being the species which carry light, shallow crowns, like the 

 oak, the ash, the larch, and the Scots pine. These species 

 begin to "thin out" earlier than do those having deep, dense 

 crowns, like the spruce, the beech, and the silver fir. The 

 following are the approximate ages at which the process 

 described may be expected to commence : — 



Larch, .... 15-20 years. 



Scots pine, .... 20-40 „ 

 Oak, . . . - . 40-60 „ 



One way of dealing with the difficulty is to cut down the 

 crop before serious injury to the soil has resulted ; but larch, 

 Scots pine, and oak do not attain the dimensions which are 



