226 The Breckland 



of the change is due to the appearance of calcifuges. Deschampsia flexuosa, 

 Nardus stricta and Potentilla erecta, although present in Breckland, are absent 

 from E, F, and G, and also from large areas of Breckland where the soils 

 are certainly acid enough for them. The intimate relation between the soil 

 and the grassland community it bears is thus established. Similar relation- 

 ships can be seen in Breckland between the soil and communities of 

 bracken, heather, and sand sedge. 



CYCLIC PHENOMENA 



Grass-heath. In Grassland G, local disintegration of the lichen mat leaves 

 the soU exposed. The rebuilding of the plant cover is initiated by Aira 

 praecox, Festuca ovina (seedhngs) and Agrostis spp. (vegetative spread). 

 These afford anchorage for the lichens, which re-establish a continuous 

 cover. In time, the grasses at the centre of a patch die, and death spreads 

 centrifugally until the lichen mat, after a temporary dominance (which 

 may last some years), disrupts. 



This cycle of change is a feature of Breckland, and can be well seen in 

 the plant-succession upon bare almost humus-free soil exposed by local 

 erosion in the form of blow-outs.' Details cannot be given here, but 

 periodically during the succession there is built up a stage with Festuca 

 ovina and Agrostis spp. set in a carpet of lichen. As in Grassland G, the 

 grasses die from the centre of a patch outwards, and the pure Uchen carpet 

 eventually disintegrates, exposing the soil to erosion. On a partially 

 eroded soU the fuU succession is telescoped; a new Uchen-grass community 

 is built up only to disintegrate once again; and the wave-hke advance is 

 repeated until a relatively stable grass-heath emerges upon soil containing 

 about 3 per cent of humus. 



But even in a relatively stable grass-heath there is variation from place 

 to place and from year to year. Thus the number of Agrostis shoots counted 

 per square metre, in fourteen plots of 0*05 sq.m. selected at random over a 

 uniform grass-heath was 2320, 870, 3004 in the years 1935, 1936, and 1937 

 respectively: and in two other types of grass-heath the same sequence was 

 obtained. Over the same period the number of shoots of Rumex varied 

 inversely, 263, 1299, and 276. 



Calhma vulgaris, absent from Grassland A, is present on the remaining 

 soil types, and is capable of assuming dominance from C to G. Its ultimate 

 height varies from about 6 in. to about 30 in. according to the soil. The 

 plant lives to an age of about twenty-five years, and (it is important to 

 note) on the poorer soils, at least, the Calhma community goes through a 



' A. S. Watt, "Studies in the Ecology of Breckland. III. The origin and develop- 

 ment of the Festuco-Agcostidetum on eroded sand", Jowr. Ecol. xxvi, i (1938)- 



