The Breckland 229 



is not simple, for at any one time cycles in all stages are found, and the 

 period of the rhythm varies from species to species. Up to the present, the 

 data suggest a rhythm explicable partly in terms of the structure and biology 

 of the species, and pardy in terms of the effect produced by its own 

 accumulated humus and litter (and for Agrostis by the carpet of hchen) on 

 the penetration of rainfall during the summer, when the absolute rainfall 

 is low and the evaporation high. Reversal of the soil-moisture gradient 

 in summer is, in fact, a common occurrence. It is relevant to note that 

 Agrostis, Calluna, Pteridium, and Carex form a series with increasmg rooting 

 depths; in suitable soils the roots of Carex descend to 11 ft., and it is the 

 only species for wliich a cycle of change has not been demonstrated. 



SUCCESSIONAL RELATIONSHIPS 



In the edapliic series A to G, nothing has been said about the causes of 

 the change. This does not imply that long-continued occupation of the 

 ground by grassland could bring about the change from a calcareous soil 

 to a well-developed podsol. On the other hand, the presence of the remains 

 of Calluna in D, E, F, and G, and the purple colour of the soils in E, F, 

 and G (sUght stain in D), strongly suggest podsolisation under heather, 

 although the heather no longer dominates. In other places, eroded and 

 similarly free from heather, truncated podsols with recognisable remains 

 of heather have been sealed up by a deposit of blown sand. It may be, 

 therefore, that heather was much more widespread than it is now. What 

 is put forward here as a working hypothesis is that leacliing has proceeded 

 to produce a brown forest soil, whose further change to a podsol is the 

 work ofCalhma, that may, in the last analysis, be dominant owing to man. 



The varying behaviour of the dominants of the four major communities 

 on the different soils makes it impossible to put forward any simple scheme 

 outhning their relationsliips to each other. Further work is needed. At 

 the moment, all that can usefully be said is that the relations Festuca- 

 AgrostisjCarex, Calluna, Pteridium, and Carex j Calluna, Pteridium, and 

 Calluna j Pteridium, vary according to the soil, and for bracken, at least, 

 according to microclimate. 



But the work of E. P. Farrow^ and the facts presented in tliis account make 

 it plain that these communities are not in stable equihbrium with their 

 inorganic environment. The exclusion of rabbits is followed by vegetational 

 change. In different parts of Breckland there are places free, or relatively 

 free, from rabbits and on these areas woody plants have colonised. Of 

 shrubs, the most important is the gorse (Ulex europaeus), and of trees the 

 most important are pine, oak and birch. There is htde doubt that on certain 

 soils, at least, woodland of some kind would eventually be formed. 

 ' E. P. Farrow, Plant-Life on East Anglian Heaths (1925)- 



