14 TRANSACTIONS OF ROYAL SCOTTISH ARBORICULTURAL SOCIETY. 



that all the zonal limits of plants are lifted up ; this means, 

 using local examples, that a tree-limit of 2000 feet may be 

 expected in the Grampians where hills rising over 3000 feet are 

 massed together, but on long exposed ranges like the Pentlands 

 where the highest summits are barely 2000 feet, the possible 

 tree-limit will rarely exceed 1000 feet. 



Woodlands in Relation to Soil. 



The geological features of soil need not be dealt with here, 

 as they form the subject of a series of contributions by 

 Professor Geikie in recent volumes of these Transactions 

 (Vols. XX., XXL). The results of the botanical surveys show 

 that, while there is a distinct connection between types of 

 woodland and certain geological formations, still geological 

 change does not necessarily mean change in the woods. The 

 really essential things which give rise to different types of 

 woodland are thus summarised in the "Woodlands of England " : 

 " Briefly, we find that the alder-willow series of woods is char- 

 acterised by a very high water-content, and that the other 

 two principal series which we distinguish, the oak and birch 

 series on the one hand, and the beech and ash series on the 

 other, are related to the lime-content of the soil. When, 

 however, either of these two series is subdivided, we find that 

 the subdivisions are related to the water-content and to the 

 humus-content of the soil. Further, the nature of the humus 

 formed in the various soils has an important influence, and, 

 finally, the available food (in the shape of mineral food-salts) 

 may perhaps be decisive in determining the success or failure 

 of certain tree-associations on sandy soils." 



The amount of water depends, in the first place, on the rainfall, 

 and in this respect Britain is well supplied, having -!0-25 inches 

 per annum even in its drier parts. But if rainfall alone is 

 the only source of water, the place is not specially suitable for 

 trees. The most favourable places for woods are found where 

 the collected rainfall, after being stored amongst the underground 

 strata, issues forth regularly in the form of springs. This water 

 is a constant supply, and it also contains some dissolved mineral 

 food-material. These conditions of water-supply are generally 

 well marked on a valley-slope, since valleys are as a rule cut 

 out of the softer rocks. Valleys are therefore specially suitable 

 for tree-growth, and they have also the advantage of shelter. 



