OCIDING PRINCIPLES IN OEDINAHY THINNINGS. 475 



30 years in the case of Scots' pine in Southern' Germany) has been 

 reached. The conijlusion to be drawn from the above facts is that, 

 the density of the leaf-canopy being one and the same, thinnings 

 should be severer in good than in^poor soils. 



(d) Elevation above the sea. — Schuberg's experiments in the 

 Black Forest show that the fertility of the soil and the density of 

 the leaf-canopy being one and the same, the spread of the crown 

 diminishes, that is to say, the number of stems increases, with in- 

 creasing elevation above the sea until the cold becomes so ex- 

 cessive as to require the trees to be more or less isolated. He 

 found that the numbers of stems at the several altitudes of 400, 

 800 and 1,200 metres were respectively as 100 to 126 to 244. 

 The influence of elevation is greatest at a young age and with 

 shade-enduring species, and diminishes as the trees become older 

 xind less tolerant of shade. 



(e) Aspect. — Schuberg's experiments show that in Southern 

 Germany difference of aspect makes only a slight difference in the 

 spread of individual trees, easterly and northerly slopes containing, 

 area for area, only 5 per cent more stems than westerly and south- 

 erly expositions. In this country the extremes of heat and cold 

 and of drought are so much greater than in Germany, that it is 

 probable that the effect of aspect is also much more marked. 



(/) Gradient. — ^The steeper the gradient is, the more light can 

 trees forming a leaf-canopy receive laterally, and therefore the 

 larger the number of trees that can stand on a given area. 



(ff) Condition of the leaf-canopy. — It is obvious that self-sown 

 crops and those resulting from direct sowings require during the 

 pole stage of growth heavier thinnings than crops composed of 

 transplants, which necessarily differ very little from one another in 

 either vigour or stature and all stand at regular intervals apart. 

 Indeed we may in a general manner say that crops consisting of 

 individuals of more or less one and the same height can hardly bear 

 being thinned at all, since they contain next to no overtopped or 

 suppressed trees and the removal of any one individual forms a 

 more or less large gap, which generally takes some time to fill 

 up. Where the overtopping, dominant and dominated trees are 

 not sufficient of themselves to protect the soil, thinnings ought 

 to be light or moderate according to the number and distribution 

 of the overtopped trees. Where the crop is crowded, the thinnings 

 should be heavy. We thus see that in one and the same crop 

 the character of the thinnings may have to vary from point to 

 point, Where the suppressed and overtopped trees occupy a dis- 



