Hornungia petraea

Hornungia petraea (L.) Reichb.

Hutchinsia

Status: scarce

This small species has two habitats in Britain: south or south-west facing slopes on Carboniferous limestone, and calcareous sand dunes. In both it is restricted to open vegetation with bare soil which is dry in summer and moist in winter. On limestone it avoids the rockiest and driest situations and continuous grass cover. Its most typical habitat is a shallow humus-rich but skeletal soil on a broken rocky slope with an intermittent cover of Festuca ovina. It is usually associated with other overwintering annuals such as Arabidopsis thaliana, Aphanes arvensis, Arenaria serpyllifolia, Cardamine hirsuta and rarely Draba muralis. On sand dunes it is occasionally found at the early fixed stage, more usually at a later stage but where the turf is open owing to instability or disturbance. H. petraea ranges from near sea-level in South Wales to c. 490 metres near Hawes, but normally grows below 300 metres.

H. petraea is a winter annual. Soil moistening and lower temperatures in the autumn stimulate germination and seedlings surviving the early stages have a high probability of reproductive success, as the leaf rosettes are capable of withstanding low temperature, exposure and snow cover. Flowering can start as early as January in warm spells and continues until May or June but is usually curtailed earlier than this by spring drought. Plants are largest, and subsequent seed production is greatest, following mild winters when much-branched plants continue to grow and flower for long periods in the spring. A certain amount of disturbance by grazing can be beneficial by preventing development of a closed turf, and the species does well in the year following a dry summer when more bare soil is exposed by the death of perennial grasses. Self-pollination always ensures good seed production.

In most localities the population size, which varies from tens to many thousands, has been stable over many years and the only major threat is of habitat destruction, as in the ecologically similar Draba muralis.

It is widely distributed in suitable habitats in southern Scandinavia and the eastern Baltic, western, central and southern Europe, and extends to north-west Africa and Asia Minor. 

For a more detailed account of this species, see Ratcliffe (1959).

D. Ratcliffe