The Breckknd 227 



cycle of change — a stage of invasion, followed by dominance for a number 

 of years, then widespread death. During the tenure of the ground, 

 humus accumulates forming a black peaty mor up to 2 in. thick. With the 

 death of the heather and the decay of its stems, the mor disintegrates and 

 erosion exposes the mineral soU, which itself may be eroded until the 

 process is checked by the accumulation of flints forming an erosion pave- 

 ment. On this bare soil a series of communities leads eventually to the 

 estabhshment of grass-heath, which, if the biotic factor permits, is invaded 

 and replaced by heather. Here there is a retreat of heather for which 

 rabbits are not responsible. Large areas of grass-heath of the poorer types 

 occur, where the only evidence of the former dominance oi Calluna is the 

 occasional dead stems under the lichen carpet together with the purple 

 stain typical of Ca//(/«rt-heath soils. 



Pteridium aquilinum. The distribution of bracken in Breckland very 

 strongly suggests a spread from nuclei moist enough for its estabhshment 

 by spores. Large circumscribed areas of bracken contain either woods, or 

 houses surrounded by trees, from which the spread may have taken place. 

 It is excluded from some areas of cold-air drainage by frost, or it invades 

 them marginally only with extreme slowness, and its vigour varies with 

 the microclimate. But even on soils with similar microcUmates variation 

 in height and behaviour is found. Bracken grows in all the seven stages; 

 in height, it varies from approximately 15-18 in. on soil A, increasing 

 through stages B and C to a maximum of about 50 in. in stage D, falhng 

 again to about 14 in. in stage G. Incidentally, on the same soil type there 

 is variation from year to year. The bracken also shows a curious patchiness, 

 the patches varying in size from soil type to soil type, but in any one type 

 forming a series in a cycle of change. Some patches have few, deep-set 

 fronds; others have more numerous taller fronds with the part of the 

 petioles showing above ground of intermediate length; wliile still others 

 carry dense tall fronds with long petioles. In series the average depth of 

 origin of the fronds in the soil becomes less and less. In the last type, 

 death spreads centrifugally from the centre of a patch outwards, and the 

 vacated ground is occupied once more by a scattered population of fronds 

 deep set in the soil. 



It has been shown that the rhizome system of bracken is sympodial and 

 that numerous relatively small individual plants make up an area of bracken. 

 It is a typical travelling geophyte : as the rhizome advances in front, it dies 

 away behind, throwing off live branches which thus become independent 

 plants. The number of fronds carried by any one plant is small— approxi- 

 mately one frond to 8 ft. of rhizome. 



Carex arenaria. The main areas of Carex in Breckland He in the parts of 



15-2 



