Arabis petraea

Arabis petraea (L.) Lam.

Northern rock-cress

Status: scarce

This is an extremely local and erratically distributed montane species. It is usually in dry to slightly moist, markedly open habitats, on cliff faces, screen, exposed soil and gravel, river alluvium and shingle. A. petraea is especially abundant on basic and ultrabasic igneous rocks in the western Highlands and Inner Hebrides, but also has another stronghold on the granite of the Cairngorms. There are occurrences on serpentine, limestone, hornblende schist, Lewisian gneiss and Torridonian sandstone, so that range of substrate base-status is extremely wide. Yet it is absent from a great many apparently suitable places. Commonly associated species include Alchemilla alpina, Antennaria dioica, Cerastium arcticum, Festuca vivipara, Luzula spicata, Saxifraga oppositifolia, Silene acaulis, Thymus polytrichus and Racomitrium lanuginosum. The altitudinal range is wide, from near sea level in Shetland to at least 1220 metres on Braeriach.

This perennial herb evidently propagates by seed quite freely, at least in some localities. Any limitation by high summer temperature is unlikely, as indicated by the altitudinal range. Its occurrence on low-lying river gravels and shingle suggests establishment from down-washed seed, but could also represent ancient populations.

Many pre-1970 records of A. petraea are probably still extant. This plant is often in places grazed by sheep and deer, but appears to be less threatened than many montane species. Smaller populations would be vulnerable to collecting, but many are large and not at risk.

A. petraea is an arctic-alpine species of wide distribution in the Old World and extending to Alaska in North America.

D. A. Ratcliffe