246 TRANSACTIONS OF ROYAL SCOTTISH ARBORICULTURAL SOCIETY. 



crowberry i^Empetriim), and other species characteristic of 

 " mor." 



It has also been pointed out by P. E. MuUer that the dominant 

 plant association has a considerable influence on the develop- 

 ment of "mor" and "muld" respectively. The Ericaceae 

 (heather, bell-heaths {Erica), and blaeberry, with other species of 

 Vacciniuin) prepare the way for malignant " mor," that is their 

 presence, as on our grouse-moors, gradually destroys the utility 

 of the soil for afforestation. The worst form is where heather, 

 crowberry, and grey crackling lichens cover the ground. Where 

 heather occurs with a covering of mosses, there is still some 

 formation of nitrates and a better chance for trees, especially 

 conifers and birch. 



Beech, especially on dry sandy soils, favours the formation 

 of "mor," and may convert a "muld" soil into a more or less 

 pronounced "mor" within a single generation. This is the case 

 illustrated in the examples of woods given above. The soil 

 gradually becomes impoverished so that the natural regeneration 

 of beech is prevented. This is quite distinct from the prevention 

 of regeneration by rabbits, which might take place on a "muld" 

 soil. Spruce and firs (species of Picea and Abies) are not so 

 favourable for the formation of " mor." Oak and other deciduous 

 trees with open canopy are those which best encourage and 

 preserve the " muld " soil. 



A sequence of forest periods is now recognised in Northern 

 Europe, and this has had a great influence on the humous 

 conditions of the soil. At the close of the glacial period the 

 land became covered with a vegetation of low arctic plants, 

 whose remains have recently been fully demonstrated by 

 F. J. Lewis in Scotland. As the climatic conditions improved 

 trees such as birch and aspen arrived, and these prepared the 

 way for extensive pine forests ; remains of birch and pine are 

 also well known in Scottish peat-bogs, even at high altitudes. 

 Then a segregation begins. On the soils naturally poor, or 

 impoverished by exposure to wind and rain, or where tree-growth 

 is checked by grazing animals and by burning the heather and 

 old grass, the pine and birch woods have gone, and heather or a 

 grassy vegetation has replaced them. But on the deeper and 

 less wasted soils, oak followed the pine. The open canopy and 

 other conditions of oak forest are all in favour of that series of 

 processes which promote " muld " formation, and this, the best of 



