The Breckland 223 



soils at one extreme to well-developed podsols at the other. Seven stages 

 in tliis series may be recognised. 



Besides the soil variation brought about in tliis way, there is a further 

 variation resulting from erosion. The leached soUs, having lost their 

 binding material, and supporting a vegetation inadequate to maintain 

 stability, are eroded, often in the form of blow-outs, thereby exposing at 

 the surface different horizons of the podsol profile.' The transported sand 

 forms a blanket of variable thickness covering considerable areas and 

 overlying intact as well as truncated profiles. 



GRASSLAND TYPES 



The recognition of soil variation throws great light upon the distribution 

 and behaviour of the four major communities. They can be illustrated by 

 a brief account of the variation shown by grass-heath on the seven stages 

 in the development of a podsol, and by reference to the communities 

 dominated by Calluna, Carex, and Pteriditim. The "grass-heaths" (grass- 

 lands) and the corresponding soils are provisionally designated by the 

 letters A to G : these symbols have nothing to do with the notation used 

 in soil science. 



The chief features of these seven stages are summarised on the following 

 page. The perfectness of the series is spoiled in the last four members by 

 the deposition of blovwi sand, but the soil has been stable for some time 

 and the blanket of sand seems to have assumed properties appropriate to 

 the underlying soU. The first five stages show a well-marked gradient of 

 fertility: F and G are similar to E. The grassland communities described 

 occur on Lakenheath Warren. They are all heavily grazed by rabbits and 

 are thus comparable within themselves; and they differ in some important 

 respects from ungrazed grassland. 



Grassland A. The liighly calcareous shallow soU bears an open vegetation 

 of species tolerant of chalk or exclusive to it. Festuca ovina is the most 

 abundant species, Agrostis is occasional only. Several species are confined 

 or almost confined to this type: Botrychium limaria, Calamintha acitios, 

 Galium a)igliciim, Ditrichiim flexicaule var. detisum, Bilimbia aromatica, 

 Lecanora kiitigera, Placodiinn ftilgeiis, and Psora decipiens. There are no 

 liverworts. Locally there is more sand, and Cladonia silvatica occurs. 



Grassland B. Of all seven types, this is the richest in species, and its close 

 turf is the nearest approach to chalk pasture found in Breckland. Charac- 

 teristic species include Avena prntensis, Arabis hirsuta, Cirsiiim acaule, 

 Danais carota, and Hypnunt chrysophyllum. The bulk of the turf consists of 



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

 ment of blow-outs ",Jt>«r. Ecol. xxv, 91 (1937). 



