1 88 BRITISH FOREST TREES 



The oak is a deep-rooting species of tree no growth of the 

 forest is more so. It develops a large tap-root, and strong, 

 heart-shaped side roots, which often force their way down to a 

 great depth in deep, easily penetrable soil. With advancing 

 age, after the main growth in height has been completed, 

 the tap-root becomes more or less inactive, and the side- 

 roots near the surface gradually assume a greater share in 

 the task of nourishing and supporting the tree. 



Requirements as to Soil and Situation. Willkomm 

 estimates that the oak can thrive wherever the mean tem- 

 perature from May till October is not less than 54^ Fahr. ; 

 but in any case it, and especially the English oak, makes 

 greater demands on warmth than the majority of other 

 trees. The sessile oak is content with about the same degree 

 of warmth as the beech. 



The oak seems to tolerate both a humid atmosphere and a 

 dry one, but this capacity is naturally regulated by the condi- 

 tions of any particular situation as to soil-moisture and warmth ; 

 in general warm, sunny, southern exposures show the best 

 growth when the soil is fresh and deep, conditions found more 

 frequently on south-eastern than on south-western or western 

 aspects. At the higher elevation even the sessile oak pre- 

 fers the warmer southern exposures to the damp, cold, raw 

 northern aspects. 



The principal demands that are made on soil by the oak 

 are depth, a low enough degree of tenacity to allow of the 

 root-system ramifying easily, and a sufficiency of moisture 

 permeating both soil and subsoil. The English oak can stand 

 more moisture in the soil than the sessile oak, and can even 

 thrive with an excess of it in warm localities, when its 

 growing-space is ample, and its crown well developed, these 

 being the circumstances favourable to rapid evaporation 

 and transpiration through the foliage. In its demands with 

 regard to mineral strength, the oak occupies a position very 



