The vegetation of the Stiperstones mines

Shropshire Botanical Society Newsletter 6- Spring 2002 - page 9-10

Sarah Whild

There is a range of plants that are considered to be indicators - they choose only to grow in certain conditions - so, when we see those plants, we can predict the underlying soil and geology. Plants and plant communities respond to climate in the form of rainfall, exposure to wind and sunshine, to altitude, to soil type resulting often from underlying geology and drainage. Contrary to horticultural wisdom, semi-natural plants communities are often more diverse and species-rich on thinner, nutrient poor soils where competitive species do not have the opportunity to dominate.

The Stiperstones range is primarily a quartzite ridge which gives rise to acidic and extremely nutrient-poor soils resulting in species-poor heathland vegetation. The slopes lie mainly on Mytton Flags which are generally acidic in reaction, with some slight base enrichment where springs percolate through to the surface producing flushes of wetland vegetation. Therefore, the typical Stiperstones semi-natural vegetation is heathland and acidic grassland.

Following mining in areas such as the Stiperstones, the spoil mounds that are left are relatively nutrient poor and verging on toxic with heavy metals and salts. Colonization is slow and usually follows a sequence of algae, then lichens, bryophytes, and finally higher plant species such as Festuca ovina, Sheep's Fescue, which shows a degree of lead tolerance. Eventually woody species such as Calluna vulgaris, Heather, which can fix phosphorus and Ulex sp., Gorse, and Cytisus scoparius, Broom, which can fix nitrogen, result finally in a heathy woodland vegetation dominated by Betula sp., Birch, with a Calluna vulgaris understorey as at Gravels and Pennerley mines.

At Pennerley there is extensive colonization by willows and birch, with Quercus spp., Oaks, replacing the birches where deeper soils have formed. There are some interesting botanical features associated with the Pennerley area, resulting from past mining activities. As most miners in this area were also small holders, they managed small hay meadows and pastures on the poor soils of the slopes of the Stiperstones. To reduce the acidity and in an attempt to improve the soils, it is possible that they added calcite in lieu of agricultural lime. Calcite (calcium carbonate) is highly insoluble and does not produce a very basic substrate. However, weak acids such as carbonic acid from rain gradually react to produce calcium bicarbonate, which is soluble, and can produce a basic reaction in the soil. This may have led to a gradual increase in pH in the soils and may help to explain the rather unique hay meadow flora that is found, with many indicators typical of limestone pastures such as Platanthera chlorantha, Greater Butterfly-orchid) and Leontodon hispidus, Rough Hawkbit. Perhaps one of the most curious botanical occurrences in this area is the extent of Laburnum anagyroides, Laburnum, hedges, used in an agricultural context. The only other place where this occurs is in mining areas of south Wales. There is no connection with use of the timber for mining. One anecdotal explanation is that 'flax' was spun from the silky fibres in the seed pods; this was provided by a student, some years ago, who came from south Wales and could remember his grandmother referring to laburnum flax. But I have not found anyone to corroborate that story for the Stiperstones area.

Bog Mine shows a good range of successional processes, from uncolonized waste, through algal crusts and lichen/bryophyte communities, to occasional patches of grass, usually Festuca ovina with some Agrostis capillaris, Common Bent. The least disturbed spoil mounds here have some heather and gorse and birch with Salix spp., willows, on the damper soils. One of the major issues is the friable nature of the soils; problems facing the colonizing plants are not just the toxicity and low nutrient levels but the physical nature of the substrate, not helped by the use of mountain bikes and the general visitor impact on the barer spoil slopes. Even when bryophytes, lichens and the first grasses have colonised, there is still a negligible soil layer - it is only when deeper rooted woody species have colonised that the slopes become stable.

Snailbeach is now a rather interesting area to attempt to interpret. The calcite spoil heaps abandoned in the 1950s did not appear to colonise with any vegetation and these were levelled off with topsoil by Shropshire County Council during the reclamation scheme. The area of grassland just east of the car park was originally sown with a wildflower cornfield mix including Chrysanthemum segetum, Corn Marigold, and Centaurea cyanus, Cornflower. Following this a perennial mix was included with typical seed mix calcicoles such as Daucus carota, Wild Carrot, and other species including Centaurea nigra, Common Knapweed, which is a common species of unimproved neutral grassland - however the variety included here has long ray florets, not normally seen in the Midlands (but reasonably common in Kent and on the continent). Seed mixes such as this make interpretation difficult. Small calcicole species such as Linum catharticum, Fairy Flax, are present around the mine workings. This is not usually a 'conservation seed-mix' constituent and so its presence may be a genuine indication of base-rich conditions.

One of the most surprising finds within the Snailbeach area is of large quantities of Daphne laureola, Spurge Laurel, which is an ancient woodland indicator and also an indicator of base-rich conditions. At Tankerville mine, it picks out the rectangular shape of the old walls, probably picking up on lime-rich mortar, but at Snailbeach, it is present with a number of calcicole ancient woodland indicators such as Brachypodium sylvaticum, False-brome, and Galium odoratum, Woodruff.

Where the walls have not been over-zealously cleaned, a number of species of fern are present, most of which are calcicoles, including Phyllitis scolopendrium, Hart's-tongue, Asplenium adiantum-nigrum, Black Spleenwort, and Dryopteris filix-mas, Male Fern. Again, the presence of these species is undoubtedly determined by the presence of large amounts of lime in the old mortar, and we would not normally expect to find them in this particular area but for these human influences.

 

Map: distribution of Daphne laureola in Shropshire. This species is a strong indicator of base-richness, and is only common along the Wenlock Edge and in the Oswestry uplands. The Stiperstones mines are in the square SJ30.

References

Sinker, C.A., Packham, J.R., Trueman, I.C.T., Oswald, P.H., Perring, F.H., Prestwood, W.V. 1985 Ecological Flora of the Shropshire Region. Shropshire Wildlife Trust, Shrewsbury.

Toghill, P. 1990 Geology in Shropshire. Swanhill Press, Shrewsbury.

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