52 TRANSACTIONS OF ROYAL SCOTTISH ARBORICULTURAL SOCIETY. 



5. Plant Indicators. 



By J. M. Murray. 



Observation of the simpler principles of plant succession and 

 ecology is interesting and useful to the forester. He seldom 

 has, however, the training in chemistry and physics that is 

 necessary for the intensive study of ecology as a special science, 

 and even although he had the training lack of time becomes a 

 limiting factor. Nevertheless, during his usual work the forester 

 can gain a great deal of useful knowledge by making simple 

 experiments and by observation. 



Most foresters know by experience the ways in which 

 natural crops of trees or other plants succeed each other. In 

 many parts of the Highlands birch had followed pine, and later 

 had been itself replaced by pine. In the South of England 

 areas of sandy heath clad with dwarf shrubs, such as Calhma, 

 Erica and the like, are now being peopled by pine. Where pine 

 and beech are grown together a change in the soil conditions 

 may so favour one or other of the species that it finally occupies 

 the area. This change occurs not only in the tree inhabitants of 

 an area, but is also a feature of other plant life. The whole 

 trend of succession can be followed from its initiation on bare 

 rock or soil. In a simple case of succession on rock one may 

 find, first a growth of algae or lichens, and then mosses. As the 

 soil becomes more abundant and the water-supply more assured, 

 various flowering plants find conditions favourable for growth. 

 There is a gradual change in the forms of higher plants, from 

 those that have but little in the way of soil or water require- 

 ments through those with greater and greater demands, till the 

 final state is reached. There may come to the mind examples 

 in which the lengthy process of soil formation is already 

 accomplished, but some accidental occurrence lays the soil 

 bare of vegetation. There are instances where the banks of a 

 stream give way and the plant covering is destroyed. Soil 

 may be carried by the stream and deposited elsewhere forming 

 a mud-bank or a gravel strand. It is sufficient that unpeopled 

 land is made available for those numerous spores, seeds, and 

 broken fragments which are ever ready to provide colonisers. 

 On the new ground, for a time, the number of individuals is 

 usually small, and there are many parts untenanted. Later 

 the numbers increase; the diversity of forms becomes greater 



