Phyllodoce caerulea

Phyllodoce caerulea (L.) Bab. (Ericaceae)

Blue heath, Fraoch a' Mhèinnearaich

Status in Britain: VULNERABLE. WCA Schedule 8.

Status in Europe: Not threatened.

This calcifuge, dwarf shrub of montane heaths occurs on acidic freely draining soils on very steep ground between 670 and 840 metres, almost always with a northerly or easterly aspect. All localities bear late snow-beds, which normally clear in June. It occurs in communities typical of moderately late snow-beds, dominated by dwarf shrubs, in which Vaccinium myrtillus is the most frequent and generally the most abundant species, though Empetrum nigrum may be locally dominant. More rarely it occurs in open areas dominated by low grasses and herbs such as Sibbaldia procumbens, and is occasionally found at the edge of grasslands dominated by Nardus stricta.

P. caerulea is a low bushy evergreen shrub up to 20 cm tall, normally flowering in July, though some flowers may be found as late as October. Flowering is, however, irregular, and may be sparse or absent in some plants in some years. Both self- and cross-pollination occur. Capsules in all stages of maturity remain at the end of the season and seed production is generally low and variable. Seed capsules have been significantly grazed by ptarmigan in most years on the Sow of Atholl, but Ben Alder populations appear to have remained ungrazed. Plants can spread vegetatively by layering of basal shoots or spread of rhizomes, though monitoring over the past decade has not detected any new plants created in this way. It is likely that late snow cover protects the buds from frost damage in winter. It may be readily confused with Empetrum nigrum when not flowering.

It is known from six locations, in three adjacent hectads. On the Sow of Atholl, plants are scattered over about 1 km of hillside. Although this represents the largest assemblage of plants in one area, the population is probably too fragmented for cross-pollination to occur, since plants flower at slightly different dates apparently depending on their emergence from the winter snow cover. In the remote Ben Alder range, five smaller populations occur in a small area (McBeath 1967). Some years ago, P. caerulea was reported from other sites in the vicinity but these have never been confirmed. However it is possible that it occurs elsewhere since it inhabits difficult terrain buried by snow for more than half the year. Its discovery in 1810, and its subsequent history are described in Nelson (1977).

The total known British population numbers fewer than 300 plants, though some of these may represent several individuals growing together. About half are on the Sow of Atholl. Detailed monitoring since 1985 has revealed just six new plants. Quite a number of other small plants occur which were established sometime before 1985, and which are developing very slowly indeed. The overall picture from monitoring in the past decade is of the survival of long-lived individuals. Losses are mainly because of local erosion which occurs when the soil becomes saturated as the snow melts. Negligible recruitment of new plants at the present time means that this species is undergoing an overall decline. Grazing is slight, despite considerable increases in red deer in the Grampians, and the presence of sheep on the Sow of Atholl. Plants are, however, often damaged in the autumn, and this appears to be caused by deer moving through the habitat in the stalking season (Sydes & MacKintosh 1990).

All known colonies lie within two upland SSSIs, but the protection afforded by this designation is limited. Three substantial landslips on the Sow of Atholl have caused considerable damage over the past twenty years and these may be related to mild winters with rapid thaws. Aerial photographs reveal no evidence of slips there in the previous 30 years. The plant could be severely affected by acid deposition, for snow can concentrate pollutants which are released in bursts of very acid water as melting starts, and this species occurs on soils which have low buffering capabilities. Extreme acidic pollution has been recorded in snow near British P. caerulea sites (Davies, Abrahams, et al. 1984). Spring muirburn is not at present a serious problem as all known plants are normally under snow until well after the legal muirburn season. However, extensive fires have occurred close to the Sow of Atholl. Autumn muirburn could pose a threat but does not currently take place on the two estates with P. caerulea populations. It has been collected by botanists and gardeners ever since it was first discovered, but the close monitoring of the past decade has provided no evidence of the removal of complete plants.

In Europe its centre of abundance is in Scandinavia. It also occurs in Greenland, Iceland and eastwards through Russia, and is very rare, perhaps extinct, in the Pyrenees. In other parts of the world it occurs with other members of the genus and is found in North America and through northern Asia to Japan. 

Further details are given in Coker & Coker (1973).

C. Sydes and M. J. Wigginton