Effect of Thinning 35 



which have grown from self-sown seed and which require to be 

 removed. Birch on hill pastures is especially troublesome in this 

 respect. 



When an area is planted, say, with saplings at three feet apart, 

 the trees will grow for some years without interfering with each 

 other ; gradually, however, the increasing size of the trees causes an 

 increase in the struggle to obtain light and air. In this struggle, the 

 stronger specimens, having obtained a greater height-growth than 

 the weaker ones, obtain the greatest benefit from the sun and air ; 

 so that the weaker trees are gradually overshadowed and suppressed. 

 For the same reason, the side branches of all the trees will die off, 

 near the ground. This is termed natural pruning, or, as is commonly 

 said, "the trees have begun to clean themselves." 



Effect of thinning. Where thinning has been delayed, the 

 trees are " drawn up." The stems are straight and tall, and the 

 girth tends to be the same at the top as it is at the bottom — in other 

 words, the trees " carry their girth well." These features are 

 excellent, of course ; but if thinning is delayed too long, the timber is 

 less durable, the health of the trees suffers, and they are more 

 liable to fungoid and insect attacks. 



The presence of a branch means the presence of a knot, which 

 will increase in size each year, as long as the branch remains on the 

 tree. Hence the importance of planting a wood sufficiently close, 

 so that the lower lateral branches may be killed at as early a date 

 as possible. Timber is required to be clean, free from knots, and 

 with even-sized rings throughout. 



In dense woods, the annual ring (see p. 44) is small and 

 decreases as the canopy becomes more dense. A thinning can 

 usually be traced in a transverse section of a tree-stem by an 

 increased width of the rings formed after the operation. 



An isolated tree is characterised by larger diameter, smaller 

 height, thicker' and rougher bark ; the timber is more full of large 

 knots than if grown in a dense wood ; and, moreover, the crown, 

 spreading over a larger area, will produce a larger amount of 

 inferior wood. 



