Calaminarian grassland

Calaminarian grassland can be found on soils that have levels of heavy metals, such as lead, zinc chromium and copper, which are toxic to most plant species. It mostly occurs in the north and west of the UK, in one of the following situations:

  • as near-natural, open vegetation of serpentine rock and mineral vein outcrops with skeletal soils, such as at the Keen of Hamar in Shetland;
  • on stable river gravels rich in lead and zinc, such as on the Tyne and Allen river gravels in Northumberland; or
  • most commonly, on artificial mine workings and spoil heaps resulting from past mining activity, such as on the Halkyn Mountain in north Wales.

 

Because of the soil toxicity and paucity of nutrients, the vegetation is normally open, species-poor, slow-growing and has a very open grassy structure with limited vegetation cover.  

 

Characteristic plants include two nationally scarce plants, spring sandwort Minuartia verna and alpine penny-cress Thlaspi caerulescens, along with bird’s-foot trefoil Lotus corniculatus, common bent grass Agrostis capillaris, fairy flax Linum catharticum, harebell Campanula rotundifolia, sheep’s fescue Festuca ovina, sheep sorrel Rumex acetosella, and wild thyme Thymus polytrichus. 

 

Certain species, including sheep’s fescue, sea campion Silene uniflora and thrift Armeria maritima, occur in genetically adapted forms to cope with the toxic soil conditions.

 

Rarer species, such as forked spleenwort Asplenium septentrionale, benefit from the lack of competition from vigorous plants. Some sites hold important populations of rare bryophytes and lichens.