Thlaspi caerulescens

Thlaspi caerulescens J.S. Presl & C. Presl

Alpine penny-cress

Status: scarce

The distribution of T. caerulescens in England and Wales reflects a distinct association with derelict lead and zinc-mine workings, particularly in limestone areas but also on shales. It is often a pioneer colonist of metalliferous mine wastes and river gravels contaminated with lead, zinc and cadmium, where it is frequently associated with Minuartia verna. It is occasionally found on non-metalliferous substrata (limestone outcrops and scree, whinstone). In Scotland, isolated montane populations occur on a variety of substrata including limestone, serpentine, basalt, shales and porphyritic gravels. Although typically a plant of very open conditions, it can persist in a more closed turf over mine wastes or at its montane sites. It is mainly an upland plant, reaching 940 metres on Caenlochan, but also growing down to near sea-level in western Wales. 

In the British Isles T. caerulescens is a perennial, or rarely a biennial. Plants are almost entirely self-pollinated and populations are inbreeding (Riley 1956). Reproduction is largely by seed but daughter rosettes are produced on old plants and these rosettes can become independent plants. Seed dispersal is not long-range, seedlings frequently establishing on bare soil close to parent plants. Germination occurs in the early autumn. There is no persistent seed bank.

Whilst many of its populations are isolated and often very small, T. caerulescens can be locally abundant at its mine sites. Reworking of lead mine spoil and land reclamation may have already exterminated some populations. The unstable nature of metal-contaminated river gravels confers a precarious status to important populations beside the Afon Ystwyth, Rivers South Tyne and West Allen. It appears now to be extinct at its only serpentine locality at Grey Hill.

This species is widely, although locally, distributed in the mountains of southern, western and central Europe and on metalliferous (calamine, serpentine) soils at lower altitude, extending eastwards to Poland and Yugoslavia. It has been introduced into Scandinavia. The British populations represent the north-western limit of its range. Closely related taxa occur in Asia and in western North America.

This is a very variable plant morphologically, which has led to considerable taxonomic confusion (Ingrouille & Smirnoff 1986). Flora Europaea (Tutin el al. 1993) now recognises only two subspecies: subsp. caerulescens and subsp. virens. The former name should be applied to all British material.

A. J. M. Baker