70 THE SCOTTISH BOTANICAL REVIEW 



physiography, and the soil have largely controlled their 

 present distribution. 



Birchwoods are widely distributed in the moorland region, 

 since they are capable of withstanding the acid condition of 

 the soil associated with atmospheric leaching and moorland 

 drainage. Humid, cool summers are less unfavourable to 

 their growth than to the other tree-species with which they are 

 unable to compete in close canopy on account of their great 

 light requirements. The early post-glacial immigration and 

 spread of the birch was, comparatively speaking, unchecked 

 by the severity of the climate and sterile condition of the 

 soil, but its dominance further south was rapidly limited by 

 competition with other trees. 



Natural pinewoods are at present almost restricted to the 

 Central Highlands of Scotland, but extensive pine-forests 

 formerly covered areas of the moorland peat. The pine 

 appears to have greater capabilities than the birch of with- 

 standing drought and soil-acidity, but requires somewhat 

 higher summer temperatures for its full development, and its 

 former widespread colonisation of moorland peat, probably 

 coincided with a stage of moorland retrogression when the 

 humidity was less, and the summer temperatures higher, than 

 those favourable for rapid moorland growth. 



The oakwoods are subdivided by Moss, Tansley, and 

 others as the chief associations of three separate plant- 

 formations (3). Thus they distinguish : (i) damp oakwoods 

 of Qtiercus pedunculata as the chief association of a plant- 

 formation of clays and loams ; (2) dry oakwoods of mixed 

 Q. pedunculata and Q. sessiliflora with a plant-formation of 

 sand soil ; and (3) oakwoods of Q. sessiliflora alone with a 

 plant-formation of siliceous soils. It must not, however, be 

 forgotten that clays., loams, and sands normally form the 

 lower ground in Britain, while the so-called siliceous soils, 

 derived from the more resistant sedimentary, igneous, and 

 metamorphic rocks, form the steeper-sloping and more 

 mountainous country. It should also be noted that deep, 

 fine-grained soils and coarse, shallow, mechanically formed 

 soils bear parallel relations to the physiography of the softer 

 and harder rock masses. The slopes of the hills have been 

 recently glaciated and the lower ground has been the 



