Empetrum nigrum L., Crowberry

Account Summary

Growth form and preferred habitats

This distinctive evergreen, small-leaved, often almost prostrate, creeping, much-branched subshrub has small, inconspicuous flowers and black, drupe, berry-like, fleshy fruits. It is found on windy, peaty mountain summits, in exposed places on cliffs and upland slopes, and very occasionally in drier spots on lowland bogs, or on shallow raw peat over limestone rock.

Fermanagh occurrences

While E. nigrum is definitely uncommon in Fermanagh, it is locally frequent and very rarely dominant on some of the more exposed peaty mountain summits, cliffs and upland slopes. It has been frequently recorded in 31 tetrads, 5.9% of those in the VC, but as the distribution map shows it is very local and apart from several isolated stations on Slieve Beagh mountain in the east is almost exclusively confined to high ground in western Fermanagh.

Although listed by Meikle and his co-workers in the card index that formed the basis for the Revised Typescript Flora for the lime-flushed blanket bogland site of Erica vagans (Cornish Heath) at Black River (ie at or near the Carrickbrawn ASSI) and elsewhere in 'District IV', it is occasionally found growing in thin, raw peat over hard, crystalline Carboniferous limestone in places like Trien Mountain above Florencecourt and around the Monastair Gorge. The vast majority of the post-1975 records are from wind-exposed, damp, but always well-drained, acid, nutrient-poor peaty summits, cliffs and upland rocky blanket bog slopes. These conditions are most often met on N-facing slopes in the Lough Navar, Reyfad and Cuilcagh Plateau areas. Here, Crowberry is often associated with Vaccinium vitis-idaea (Cowberry) and other more common heathers – V. myrtillus (Bilberry), Erica tetralix (Cross-leaved Heath) and Calluna vulgaris (Heather), plus the clubmosses Selaginella selaginoides (Lesser Clubmoss) and Huperzia selago (Fir Clubmoss). Occasionally, Crowberry fruits in profusion and it does so on the exposed summit of Cuilcagh, where indeed it is the locally dominant species.

The lime tolerance of E. nigrum that RHN and the current author have noticed in Fermanagh is often repeated in the famous Burren district of Co Clare (H9) (Flora of Connemara and the Burren) and this ecological property is also remarked upon in the Biological Flora account of the species by Bell & Tallis (1973, p. 298). E. nigrum tolerates a soil reaction ranging from pH 2.5 to at least 7.7 and while it is generally considered a calcifuge species, clearly this is not always the case.

E. nigrum has been found only twice growing on somewhat drier, better drained spots on peaty hummocks on lowland raised bogs in Fermanagh. This habitat type has rapidly declined due to drainage and peat cutting and indeed it has disappeared in many other areas of B & I. The 1952 record from Clontymullan bog (Revised Typescript Flora) has not been confirmed recently, while Moninea bog in SE Fermanagh, where the species was first recorded in 1986, is now a protected ASSI site. The latter bog has been described as one of the top ten examples of surviving lowland raised bogs in NI (Leach & Corbett 1987).

Fossil history

Fossil pollen and macrofossil remains, such as fruit stones and leaves, indicate that in Ireland towards the end of the last glacial period approximately 13,000-10,000 years BP, Empetrum nigrum was one of the first species to colonise de-glaciated ground in the cold, wet, oceanic (but gradually warming) climate of the period. Along with dwarf creeping Salix herbacea (Least Willow) and Juniperus communis (Juniper), it appears to have dominated the tundra heath vegetation widespread over Ireland at the time. This was especially so in the wetter west of the island, during much of the Late Glacial and Early Post-Glacial periods, Zones I to IV of the developing Woodgrange Interstadial and the Littletonian Interglacial, as these warm stages are referred to in an Irish context (Jessen 1949, p. 222; Mitchell & Watts 1970, p. 19). The dominance of Empetrum nigrum is so pronounced in the fossil record that Jessen (1949) initiated what soon became the common practise of using the level of its presence as an indicator of the degree of 'oceanicity' of past climate phases.

However, it is important to realise that between these two warm stages of different duration which were suitable for plant growth and vegetation development, there was a return to Polar climatic conditions during the Nahangan Stadial, which lasted for around a thousand years, from 11,000-10,000 BP. This relatively brief, but very significant, cold stage meant that newly arrived and establishing plant species would have again been destroyed, and vegetation forced to retreat to more suitable warmer, sheltered sites, which it is generally imagined probably lay to the south and west of Ireland (Mitchell 1986). The representation of Empetrum nigrum fossil pollen dropped suddenly from abundance to rarity and disappearance over much of Ireland in Zone III; however, as Watts (1963) has shown, Crowberry gave place to Calluna vulgaris and other heathers common nowadays, all of which appeared in the Betula peak zone accompanied by Cladium mariscus (Sword Sedge).

A very concise summary of the fossil history of Empetrum heaths in Britain is provided by Bell & Tallis (1973) and an expansive account is given by Godwin (1975, pp. 300-5). The basic message of these accounts, which is true for all of B & I, is that Calluna vulgaris finally ousted Empetrum nigrum from most heathlands at the end of Pollen Zone IV. Fossil records of Empetrum nigrum in the later Post-glacial Period are sparse and it becomes progressively restricted to higher ground in the N & W, very much the pattern that pertains to the present day (Godwin 1975).

Variation and two subspecies

Nowadays E. nigrum is recognised as a Circumpolar boreo arctic-montane species of cool, damp climates of the N & W in terms of its distribution in B & I, and indeed beyond our shores, although the distribution is not restricted to areas with true oceanic climates, ie regions where the climate is governed by the proximity of a very large body of water. Circumstantial evidence suggests instead, that while the distribution of E. nigrum is without a pronounced continental or oceanic emphasis, in areas away from the oceans it is controlled by environmental features which are analogous in their effects to a true oceanic climate (Meusel 1943; Brown 1971). At the same time, climatic control of the species distribution is more strict than the effect of soil type (ie edaphic control is weaker than climate).

The overall picture of E. nigrum distribution is made more complicated by the existence of a tetraploid form which has mainly (but not entirely) perfect (ie hermaphrodite), bisexual flowers and which is regarded as taxonomically distinct at either subspecies level, as subsp. hermaphroditum (Hagerup) Böcher (as Stace (1997) and Sell & Murrell (2014) prefer it), or at species level. As indicated by the use above of the word 'mainly', the dioecious (two separate sexes) versus hermaphrodite flower character is not absolute; hermaphrodite forms of subsp. nigrum have been reported in Britain (Blackburn 1938) and monoecious forms (ie, having flowers of different sex on different branches of the same plant) of subsp. hermaphroditum can also occur (Danielsson 1988).

Both these E. nigrum subspecies exist in Britain, the tetraploid being heavily concentrated in Scottish mountain sites, but all Irish material belongs exclusively to the monoecious subsp. nigrum (Preston et al. 2002). Due to considerable variation and overlap, the two subspecies are vegetatively indistinguishable until they flower and fruit (Bell & Tallis 1973). The Plant Crib 1998 gives useful advice for spotting subsp. hermaphroditum (Rich & Jermy 1998, pp. 131-2).

Both subspecies (or species) flourish only under the low light levels associated with predominantly cloudy skies, combined with constant high relative humidity in the atmosphere, low summer temperatures and small annual air temperature fluctuations near ground level where the plant resides (Brown 1971). These features are very characteristic of the oceanic, cool temperate climate current today in NW Europe (Miller 1961) and thus, in the Fermanagh area at least, the presence of Empetrum nigrum plants and their pollen in sediments records an oceanic climate. Elsewhere, the presence of the plant and its pollen records only a direct analogue or substitute for this type of climate, eg through shade and relative humidity provided by snow cover or the canopy of ericaceous and other subshrubs.

British and Irish occurrence

The present-day Irish distribution of E. nigrum in the New Atlas shows it widely scattered throughout the island of Ireland, but chiefly represented in the upland, acidic terrain of the N & W. In Britain, subsp. nigrum is almost entirely confined to oceanic moorland heaths and mountain slopes and cliffs, NW of a line drawn on the map between Whitby and Plymouth.

At the same time, the altitudinal range of this subspecies in B & I is considerable, it being found from near sea level to over 1270 m in the Scottish Cairngorms and reaching a height just over 1000 m in Kerry (G.T.D. Wilmore & D.A. Pearman, in: Preston et al. 2002).

The rarer subsp. hermaphroditum (Hagerup) Böcher has never been found in Ireland, but in Britain it can overlap with subsp. nigrum, although generally occurring at higher altitude (>650 m) and often in drier places than the latter.

Although it is not a rare, or even a scarce species in B & I at present, the advent of 'Global Warming', ie a rapid increase in overall mean annual temperature, represents a real threat to species like E. nigrum which compete much better in cooler conditions (Anon. 1991).

European and world occurrence

A number of other related taxa (either forms or varieties, depending upon which taxonomist is followed), possessing differing leaf characteristics or fruit colour, have been described from distant areas in Japan and eastern N America, but if we take the broad view of the species as E. nigrum s.l., then it has a distribution which is circumpolar (Bell & Tallis 1973; Hultén & Fries 1986, Maps 1463, 1464).

The diploid form, E. nigrum subsp. nigrum, is present today in boreal N, W & C Europe, from Iceland in the NW, stretching across Scandinavia and Russia and ranging southwards to middle European latitudes. At its southern margin the distribution becomes progressively confined to higher altitudes in the French Massif Central, the Alps, the Italian Apennines and mountains ranging further east to reach the Caucasus. Subsp. nigrum is very probably close to its southern geographic limit in the current climate of B & I and it may have retreated northwards and up mountains in historic times in these islands.

Subsp. nigrum also extends through boreal Asia and the coastal mountains of western N America, although the Japanese plant is distinguished as var. or subsp. japonicum (Hultén & Fries 1986, Map 1463).

Competition between the subspecies

In all of the cooler areas of subsp. nigrum's distribution (including the more southerly European mountains), it is overlapped and accompanied by subsp. hermaphroditum and clearly the two subspecies must compete strongly for territory. Leaf longevity is one character in which it is known there is a slight, but perhaps significant difference between these two subspecies. The evergreen, peinomorphic, needle-like, reduced, ericoid leaves of subsp. nigrum persist on average for 1.5-1.9 years, while those of subsp. hermaphroditum survive longer, averaging 1.9-2.1 years (Karlsson 1992).

Winter frost protection by snow

At both higher latitudes and altitudes, E. nigrum, like other dwarf woody shrubs including Rhododendron ferrugineum (Alpenrose), requires the winter frost protection of a moderately thick blanket of snow cover. The association of snow patches with rocks, hummocks, hollows or piles of stones, probably helps account for the small-scale pattern of Crowberry in the more extreme, wind-exposed sites it typically occupies. In common with most arctic-alpine plants, E. nigrum is adapted to low temperature regimes and re-commencement of its growth begins in the early spring. E. nigrum normally flowers in April and May, so the snow must disappear quickly in springtime, or else the already brief growing and reproducing season in these regions and habitats would be significantly shortened, which could prevent fruiting (Bell & Tallis 1973). Snow is a rarity in Fermanagh, never lying longer than a few days, but fortunately its frost-shielding and protective blanketing properties are scarcely required either, since frosts are seldom hard in the hyper-oceanic climate of NW Ireland.

Reproduction

E. nigrum subsp. nigrum is dioecious, both male and female plants flowering freely early in the growing season, with dates in Britain ranging with latitude and altitude from March to May. The pinkish-purple flowers are wind-pollinated and although it is often few-berried or sterile, occasionally the glossy black berry-like fruits (drupes) are borne in profusion from early July onwards. In Fermanagh, they are especially abundant on the exposed, elongated summit of Cuilcagh mountain, where very locally Crowberry is the dominant heath species. The berries are described by Grigson (1987) as "eatable, but poor eating, suitable for crows or crakes", and hence the English common name 'Crowberry' and the alternatives 'Crawberry', 'Crawcrooks' and so on. Grigson lists a total of 17 English name variants, some of which are also applied to Calluna vulgaris and other heather species (see also below under 'Names').

Fruit dispersal

Grouse and other moorland fruit-eating birds, including thrushes, fieldfares and crows are considered the main agents of dispersal, although foxes and deer are also said to eat them. On Danish moors, Hagerup (1946) describes commonly finding dark blue excrements derived from various animals, the colour generally indicating fruits of Vaccinium spp. or Empetrum nigrum. He says that the fruits are eaten in large amounts, not only by birds, but also by men, foxes and bears. Thus the 'stones' and seeds of these genera are easily recovered on Danish moors, and the blue dung is so commonly observed that in many places that it has given rise to numerous vulgar names for E. nigrum!

Seed ecology

Each berry contains up to nine small seeds, which pass through the animal vectors undigested (Bell & Tallis 1973). However, the fruits often persist on the branches over winter, many of them failing to disperse and eventually being dropped around the parent plant (Lang 1987).

The fate of seeds is controversial; according to Hagerup (1946), under natural conditions only very low rates of seed germination are reported in any one year. Passage through an animal's gut is not absolutely necessary in order to break dormancy and seed may persist for four or more years before germinating. On the other hand, the survey of soil seed banks in NW Europe lists seven relevant studies, five of which regarded E. nigrum seed as transient, while the other two could not determine the length of persistence (Thompson et al. 1997). Overwintering low temperature exposure is required to break seed dormancy and low numbers of seedlings, if any, emerge throughout the year (although they mainly appear in spring), over a period of several years.

Seedling development

Seedling development and plant establishment by E. nigrum plants is remarkably slow, another feature shared with members of the genus Vaccinium (Flower-Ellis 1971). As one might expect, seedlings are generally observed in gaps in the vegetation, although in truth the species tends to occupy habitats with little in the way of bare soil (Grime et al. 1988). Perhaps as a result of this, in common with Vaccinium myrtillus (Bilberry), V. vitis-idaea (Cowberry) and most other ericaceous dwarf shrubs, E. nigrum seedlings are rarely observed in the field anywhere in B & I.

Vegetative reproduction

Vegetative increase and dispersal by layering of procumbent or prostrate shoots is undoubtedly the most significant current means of reproduction in these islands, although unlike most of the Ericaceae, E. nigrum clones are long-lived, some individuals surviving for up to 150 years (Bell & Tallis 1973). The overall reproductive strategy of E. nigrum subsp. nigrum is thus parallel to that of other berry-producing, large-seeded, long-lived species, such as Vaccinium vitis-idaea and V. myrtillus, and quite different from the rather more seed orientated reproduction and the shorter lifespans, observed in Calluna vulgaris and some Erica species (Grime et al. 1988).

Grazing pressure

A study of Bilberry-dominated heathland in the Derbyshire Peak District found that E. nigrum increased significantly when the mixed shrub heath was fenced and protected from sheep grazing pressure, yet while a regime of summer grazing allowed Crowberry cover to spread, there was a negligible trend either way in winter-grazed plots. It was concluded that longer term studies would be required to discover whether Empetrum nigrum would continue to replace Vaccinium myrtillus under the given conditions (Welch 1998).

Crowberry is very unpalatable to sheep and it is only ever grazed by them between January and April when the animals are most likely to go hungry, or might otherwise starve (Welch 1984).

Fire and trampling

Crowberry can tolerate moderate, well-controlled burning of heaths, but it takes a long time to recover from severe, hot, deeply penetrating fires. There is some suggestion that it can recover more quickly from some Scottish muirburns than can Calluna vulgaris, Erica spp. and Vaccinium spp. However, this is probably seldom the case, or it represents only a temporary situation, since these Ericaceae are very much more often than not the dominant heath and bogland species that E. nigrum has to compete with for space and survival (Gimmingham 1964; Bell & Tallis 1973).

Most dwarf shrubs suffer considerable damage if trampled and this is also true of E. nigrum although the only experimental evidence the current author (RSF) has come across is rather contradictory of itself! A study was made in four sites in the Scottish Cairngorms of damage caused by simulated human trampling around skiing areas (Bayfield 1979). This suggested firstly that the E. nigrum agg. (ie the two subspecies occur here together and were not differentiated) is among a group of three species, the cover of which recovered fairly rapidly after damage: the other two plants studied were Trichophorum caespitosum (Deergrass) and Vaccinium spp. (Bilberry). Each of these replaced more than half the loss of plant cover within two season's growth and, as the only evergreen among them, the rapid recovery of E. nigrum agg. was speculatively put down to stored food reserves in the extensive subsurface root system of the plant (Bayfield 1979).

However, the particular site where E. nigrum was studied possessed a thick carpet of Rachomitrium moss, the cushioning feature of which was possibly very significant in helping Crowberry absorb the physical impact of the trampling. The situation is greatly complicated by the fact that when discussing the experimental treatments that meted out the most severe trampling and the recovery over an eight year period, Bayfield included E. nigrum agg. along with Calluna vulgaris, Arctostaphylos uva-ursi (Bearberry) and Sphagnum rubellum in a list of species described as "most susceptible − with high initial damage and poor recovery", while T. caespitosum and the Vaccinium spp. were classed as "moderately susceptible to trampling − moderate to high initial damage followed by fairly good recovery" (Bayfield 1979, p. 175).

Names

The genus name 'Empetrum' is derived from two Greek words 'en' and 'petros' meaning 'on a rock', ie a reference to the habitat of the plant. This name was first given by the Classical botanist Dioscorides to a quite different, unrelated widespread Mediterranean species, possibly Frankenia pulverulenta, or another closely related evergreen prostrate species of that genus (Johnson & Smith 1946; Gilbert-Carter 1964). The Latin specific epithet 'nigrum', refers to the characteristic shiny black berry-like fruit.

Threats

The current steady increase in mean annual environmental temperatures we are experiencing in Britain and Ireland in recent years may further reduce the competitive ability of this species and thus the area of habitat suitable for it and, indeed, this is also the case for other 'Arctic-alpine' species at these latitudes.

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