Erica tetralix L., Cross-leaved Heath
Account Summary
Native, common, but very probably declining in area. Suboceanic temperate.
1881; Stewart, S.A.; mountains west of Lough Erne.
Throughout the year.
Growth form and preferred habitats
E. tetralix is readily distinguished from E. cinerea (Bell Heather) by its greyer, hairier leaves, occurring in whorls of four, rather than in threes and its larger, more inflated, paler pink flowers held in terminal umbel-like clusters of 4-12. E. tetralix occupies much wetter, more often waterlogged, poorly aerated, acid, peaty habitats than does either of the two other common heather species, E. cinerea and the usually dominant Calluna vulgaris (Heather), or indeed any of the other ericaceous subshrubs found in the wild in these islands. While E. tetralix is most commonly found on, and is characteristic of, very wet or waterlogged, nutrient-starved, peaty, organic soils of acidity in the range pH 4.0-5.0 (Bannister 1966), the species can sometimes manage to compete and survive in more mesotrophic or even eutrophic habitats, always provided that some factor or group of factors other than nutrient level is constantly acting to limit the competitive growth of any potentially dominant species present, thus preventing the species being ousted.
Fermanagh occurrence
In Fermanagh, E. tetralix is almost three times more frequently recorded than E. cinerea and it is also distributed across twice as many tetrad squares as the latter (ie there are E. tetralix records from 236 tetrads (44.7% of those in the VC), compared to just 98 (18.6%) for E. cinerea). Further numerical comparisons of these two species in Fermanagh and elsewhere in B & I are made in the current author's E. cinerea species account.
Apart from around wet hollows, in flushes and on the lower slopes of hummocks of deeper peat on bogs, on pasture moors, exposed, sunny mountain slopes and summits of lower hills, regular habitats of E. tetralix in Fermanagh also include roadsides, riverbanks, lakeshores and under canopy gaps in upland damp mixed deciduous woodland, including eg that in the Correl Glen NR.
In W Ireland, E. tetralix reaches an altitude of 550 m in Donegal and around 520 m in Fermanagh. This means it is absent from Cuilcagh ridge and its higher scarps and screes. In continental Europe, apart from the Pyrenees, C France and W Norway, where it can reach 2,200 m, E. tetralix is usually confined to low-lying areas (Bannister 1966).
Flowering reproduction
Robust plants produce flowers from the second or third year onwards and seed is set annually. Flowering begins in June and reaches a peak in July and August, a little before E. cinerea and C. vulgaris. Both self- and cross-pollination probably occur, the flowers producing abundant nectar attracting a range of insects including bees, butterflies and flies as pollinators.
Another insect, the thrip, Frankliniella intonsa (Tryb.), is very intimately associated with E. tetralix as it lives and breeds inside the flowers. While thrip species inhabiting flowers tend to be specialised feeders on pollen, piercing the grains with their asymmetric mouth-parts and sucking out the contents, in a few cases and, especially in circumstances where other insect visitors are rare, they may also assist the host plant by being significant pollinators. For example, in the Faeroe islands where the climate is even cooler and wetter than ours, and where flying insects are naturally scarce because of this, flowers of Erica tetralix and Calluna vulgaris harboring thrips were found to be both cross- and self-pollinated by the small insects living and sheltering inside them, pollen being unintentionally transported on the hairs on their bodies (Hagerup 1950; Hagerup & Hagerup 1953).
The normal pollinators of E. tetralix are, however, bees and bumblebees, some of which, in addition to collecting nectar, may operate the technique of 'buzzing' the flowers to extract their pollen (Haslerud 1974). To do this they hold onto and vibrate the flowers by rapid contractions of their indirect flight muscles (Knudsen & Olesen 1993), so that the anthers resonate and almost explode pollen (Proctor et al. 1996, pp. 125, 179).
Nectar is often stolen from E. tetralix flowers by a variety of insects after bumblebees have visited and drilled a convenient hole in the corolla tube near its base. Insects stealing food in this manner avoid the legitimate entrance to the flower and thus they fail to service the evolved pollination mechanism of the plant (Hagerup & Hagerup 1953). It remains to be seen how the plant species will evolve a mechanism to counter this thieving, evidence of which appears very frequent when Erica corollas are examined.
Reproductive capacity
Numerous seeds, between 60-100 per capsule, are produced, although up to about 60% of ovules fail to set (Bannister 1966). The ability to colonise a much wider range of habitats than the physiologically severe environments for which it is particularly well adapted, helps E. tetralix to produce a large annual seed crop. In moist conditions, seeds germinate readily with little need of pretreatment, although germination is much better in the light. Seeds can survive periods of at least three months waterlogging without loss of viability. Germination may be poor, however, on very acid peat and on calcareous soils. Seed longevity can stretch to 33 years or more in the soil seed bank (Thompson & Band 1997).
The performance of Calluna vulgaris is significantly better with regard to the scale of seed production and its longevity in comparison with the two common Erica species, E. tetralix and E. cinerea (Barclay-Estrup & Gimingham 1994; Thompson & Band 1997). The persistent corolla characteristic of the two Erica spp. undoubtedly to some extent hampers seed dispersal from their ripe capsule from October onwards, resulting in the observed rarity or absence of Erica seedlings in all except disturbed (usually burnt) ground. The seed dispersal limitation, together with the ease with which prostrate branches adventitiously root in wet habitats spreading and reproducing the species vegetatively, means that the latter is really the effective means of reproduction of E. tetralix in most situations (Bannister 1966).
Response to herbivory
Sheep, cattle and grouse will all graze E. tetralix to a limited extent, but it is rather unpalatable and the animals much prefer C. vulgaris if it is available. Like E. cinerea, Cross-leaved Heath is tolerant of this mild degree of browsing and it can recover quickly, pruned shoots resprouting from their base (Bannister 1966). Like all dwarf shrubs, however, including E. tetralix's most vigorous competitor C. vulgaris, heavy grazing such as has been the widespread practice in recent years due to an ill-advised and short-sighted European Commission farm subsidy involving animal headage payments that encouraged high levels of stocking, can exterminate the woody heath and moorland vegetation and replace it with grassland. Whenever this occurs, continued heavy, selective browsing of the pasture results in it very quickly becoming dominated by the most unpalatable and aggressive grass species, Nardus stricta (Mat Grass).
Response to burning
As E. tetralix is mainly associated with wet ground and its crown and lower stems tend to be buried in wet moss or peat, whenever the heath is subjected to 'muirburn' management by planned, controlled firing, it experiences lower temperatures and/or shorter burns than other subshrubs that grow on drier soils. As a result, E. tetralix is often less damaged and better able to survive than other evergreen ericaceous species managed in this manner. E. tetralix also regenerates well from its shoot base, so that it may become temporarily dominant after fire until C. vulgaris or other more aggressive plants recover their normal vigour and size. Burnt ground appears to be the one situation where seedling regeneration by E. tetralix may be reasonably frequent and obvious, although experimental results demonstrate that germination and seedling establishment is poor and rather slow in wet peat conditions in comparison to that on well-drained mineral soils (Bannister 1964a). Excessive burning leads to the extermination of Cross-leaved Heath and other dwarf shrubs and their eventual replacement by grasses, sedges and rushes.
British and Irish occurrence
Both common Erica species are abundant and widely distributed throughout the N and W of both B & I, corresponding with where the wetter, often milder, climate conditions apply and the most acidic geology and soils occur. However, while E. tetralix is still very well represented in the inland counties of C Ireland which was, until vast quantities of peat were extracted, the major area of deep and widespread raised bogs in the country, in this same region E. cinerea is scarce, or completely absent.
In S England both Erica species were previously associated with rather drier lowland heaths and bogs, habitats which have contracted sharply in the last 60 years due to pressures from farming and other land use developments. In the English Midlands, neither species has ever had much of a presence and both are now increasingly rare, extinct or absent over a considerable area (Preston et al. 2002).
European and world occurrence
The distribution of E. tetralix in B & I and in W Europe is very similar to that of E. cinerea, but rather surprisingly, despite its better competitive performance in wetter habitat situations, E. tetralix is less rigidly oceanic and coastal, and more continental − or suboceanic in its occurrence than E. cinerea (Hultén & Fries 1986, Map 1445). It penetrates both further north and east, eg to near Narvik, around 68°N on the coast of Norway, east to Latvia and C Finland and scattered further eastwards through south C Europe towards the Carpathian mountains (Hultén & Fries 1986).
On the other hand, of the two species, E. cinerea is alone present in the Faeroes, Scillies and N Africa (Hulten & Fries 1986, Map 1446). Although the map just mentioned also shows E. cinerea present in Madeira, the form of the plant previously known as E. cinerea var. maderensis Bent., is now regarded as an endemic species, E. maderensis (Benth.) Bornm, and indeed was first described as such as long ago as 1904 (Press et al. 1994). Bannister (1964 (b)) has suggested that E. cinerea has only a very limited tolerance of winter and spring reduction in the water content of its tissues, compared to E. tetralix, and to an even greater when compared to Calluna which can tolerate large reductions, and that this factor might well determine the continentality of the distributional range of these three heather species.
Established introductions
In some of its more continental sites in C Europe remote from the oceanic climate influence, E. tetralix is very probably a relatively recent introduction, eg in S Germany, where it is believed to have arrived along with conifer saplings imported from further north (Bannister 1966). E. tetralix is a rare introduction in eastern North America (Bannister 1965, 1966; Hultén & Fries 1986). E. tetralix has been cultivated in New Zealand gardens along with other introduced heathers since the early period of European settlement. While seedlings of most heathers are not often seen there, even in gardens, escapes from cultivation and deliberate attempts to naturalize C. vulgaris and E. cinerea have occurred there and these particular heathers are now naturalised in a few stations on both NZ islands. On the other hand reports of E. tetralix occurring with the other two species in the wild in NZ are discounted as being identification errors for E. cinerea (Webb et al. 1988).
Uses
Similar to Calluna vulgaris and Erica cinerea; see the species accounts of these.
Names
The Latin species epithet 'tetralix', is a name first used by Theophrastus for a thistle-like plant which also had leaves in whorls of four, forming a cross-like arrangement as in E. tetralix; over the centuries the name became a technical term to describe this leaf state and as a name it was transferred to this heather by the Swedish taxonomist, Carl Linnaeus (Gilbert-Carter 1964; Gledhill 1985).
The English common name 'Cross-leaved Heath' is a simple translation of 'tetralix' and it is the only name given by Prior (1879) in his dictionary of popular names. In their similar but much larger work on the subject, Britten & Holland (1886, Supplement 1896), list about 17 English common names from around B & I, many of them also regularly applied to C. vulgaris and to E. cinerea, eg 'Bell-heath', 'Broom-heath', 'Besom-heath', 'Heather', 'Carlin Heather' and 'Ling'. One name from Somerset and Hants specifically given to E. tetralix is 'Honey Bottle', but really this would be much more appropriate if applied to E. cinerea. Another N Yorkshire name is 'Father of Heath', while both this species and E. cinerea are often locally referred to by shepherds as 'She-heather', by which they 'ungallantly' distinguish them as inferior grazing to 'He-heather', which is Calluna vulgaris (Britten & Holland 1886).
Threats
Like all plants of heath and bogland habitats in B & I, there has been a decline in both area and abundance of E. tetralix during the last 60 years or so due to a combination of changing land use and pollution, both atmospheric and water-borne. The increased drainage involved in land improvement for agricultural production has probably more severely affected E. tetralix than the other ericaceous species, since they have a preference for drier soils than it does. Where ground that supported E. tetralix has not been reclaimed for agriculture, planted with conifers, built over or otherwise developed, drainage and soil nutrient enrichment have together led to a one way shift in vegetation from woody dwarf shrubs towards pasture grasslands.
In common with other ericaceous subshrubs, E. tetralix cannot effectively compete with grasses and herbaceous plants under the developed/improved agricultural ground with its less nutrient-starved conditions, nor can it survive heavy grazing, severe or frequent burning, or much human or heavy animal trampling. E. tetralix seems set to decline further unless active conservation management intervenes.
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