STABLE AND MIGRATORV PLANT FORMATIONS 73 



The peculiarly abundant litter of the beech is stated to 

 contain more nitrogen and potash than most forest litter, and 

 the poverty of undergrowth allows it to quickly come into 

 close contact with the soil. Rocks containing large quantities 

 of lime or iron, such as the limestones and basic-igneous 

 rocks, may retard the formation of acid humus in beechwoods 

 owing to the action of lime and iron on humus. The probable 

 effects of abundant lime and iron in retarding the formation 

 of humus on chalk downs and dolerite bosses have been dis- 

 cussed above. In beechwoods, where the surface is damp 

 and shaded, the result would be rather to promote rapid 

 humification through the retardation of acidity. Forest litter 

 is considered by some to afford little return of nitrogen to the 

 soil, but in the case of beech litter on chalk or decomposing 

 dolerite, with the diminished acidit)% a considerable return of 

 nutriment may perhaps occur. The capability of the beech 

 of growing on poor soil may be explained by its shallow roots, 

 the known relation of the roots to mycorhiza, and the 

 abundant litter and shade afforded by its leaves. 



In respect of the various habitats chosen by British forest- 

 trees the relations between recurrent violent gales and the 

 long life of trees must have full consideration. Oaks are 

 rarely uprooted by storms from sites they have chosen on 

 deep soils, though their branches are frequently broken. The 

 poorly grown oaks planted on dolerite or sandstone exposures 

 in Scotland are, however, often overturned, and an examina- 

 tion of such trees shows a poor development and frequently 

 a rotten condition of the descending roots. Beeches grown 

 on loose soils are easily uprooted, and a hard substratum 

 giving a firm hold to their spreading roots appears necessary 

 for stability. Nor is a deep or shallow-rooting of importance 

 only in respect of the soft or hard nature of the ground and 

 the effects of wind, since a shallow soil is rapidly leached, 

 unless the rock contains sufficient bases to neutralise the 

 acidity incurred by the accumulation of forest litter. The 

 beech can flourish and withstand storms on a firm surface 

 with a shallow soil, and a substratum containing large 

 quantities of lime or other bases would tend to the neutralisa- 

 tion of surface-acidity. The oak stands best on a deep soil 

 where its deep roots can obtain a firm anchorage. Exposure 



VOL. L 6 



