Andromeda polifolia L., Bog-rosemary

Account Summary

Growth form and preferred habitats

This small, trailing, hairless evergreen ericaceous subshrub, 10-25 cm tall, with a creeping woody rhizome and attractive pinkish urn-shaped flowers is a characteristic species of the lowland raised bogs of Central Ireland. A. polifolia is usually confined to relatively undisturbed bogs and has always been considered very rare in N Ireland (FNEI 3). Richards described A. polifolia as the raised mire specialist par excellence, growing amongst living Sphagnum mosses in a substrate largely composed of sphagnum peat in the pH range 2.8-4.8 (Stewart et al. 1994). Undisturbed raised bogs, or indeed undisturbed bogs of any kind, are now extremely rare in B & I.

In Britain, favourable conditions for Bog-rosemary are found at low altitude, between 10 and 530 m, with between 800 and 1200 mm precipitation and around 140-180 wet days per year (Rodwell 1991(b)). Nutrient analysis of A. polifolia tissues of plants growing in subarctic peat bogs showed that the species survives there with extremely low values of inorganic nutrients (N, P and K) in the shoot parts, these minerals being transported to the actively growing region of shoots with green leaves (Malmer & Nihlgard 1980). Flower buds are produced the previous season and open in April or early May.

Bogland ecology

In addition to the widespread acidic raised bogs of the Irish central plain, A. polifolia is locally frequent on raised bogs in other, more southern parts of Ireland, yet it also has a restricted distribution on Atlantic blanket bogs (Doyle & Moore 1980). The two father figures of the classical school of European Vegetation Classification, Braun-Blanquet and Tüxen during their joint investigative tour of Irish vegetation in July 1949, reported Atlantic Blanket Bog only in their sub-association with A. polifolia within the overall Vegetation Association Pleurozia purpurea-Erica tetralix, where it was strongly dominated by E. tetralix (Cross-leaved Heath) and Narthecium ossifragum (Bog Asphodel) (see Braun-Blanquet & Tüxen 1952, Table 48).

In Finland, however, A. polifolia is rather common in poor fens and in transitional rich fens and it also grows less frequently in nutrient-rich, eutrophic Pine and Spruce swamps (Jacquemart 1998). This fundamentally different ecological behaviour by the species forms an interesting parallel with that of Schoenus nigricans (Black Beak-sedge, or Black Bog-rush), a species of eutrophic fens in England whose widespread occurrence on blanket bog in Ireland has puzzled British ecologists for generations (Sparling 1968).

Flowering and vegetative reproduction

Flower buds are produced the previous season and open in April or early May. Bog-rosemary flowers are small, more or less spherical, pendulous, pinkish in colour and borne in a very short inflorescence of 2-7 flowers. It usually flowers from April to June. Nectar is secreted by swellings at the base of the superior ovary, but insect visitors are often too late, the stamens and stigma ripen simultaneously (ie the flower is homogamous) and self-compatible, so that self-pollination and self-fertilisation regularly occurs, sometimes while still in bud, thus displaying cleistogamy (ie closed marriage) (Hagerup 1954).

Some open pollinated flowers are pollinated by bumblebees and butterflies and achieve cross-fertilisation. In lowland sites in Europe, some plants may have a second flowering period from September to October. In C Europe, about 27% of ovules develop to maturity, representing about eleven seeds per capsule, but in B & I fruits are seldom observed (Jacquemart 1998; M.C.F. Proctor, in: Preston et al. 2002). Most reproduction in B & I is probably restricted to vegetative spread through active growth and layering of horizontal stems to form clones, although seed production obviously remains essential for any degree of dispersal between bogs.

Fermanagh occurrence

A. polifolia was completely unknown in Fermanagh until the above listed solitary site was discovered in 1995 by members of the EHS Habitat Survey Team, on the same bog near Clontymullan which is the only known station for Rhynchospora fusca (Brown Beak-sedge) in the whole of NI. The more accurate name for this piece of lowland valley bog, lying just S of the Arney River where it enters Upper Lough Erne, is 'Drumanacabranagher Td', since this is the name on the current 1:50,000 map nearest the grid reference given by the recorders. RHN who has visited the newly discovered A. polifolia site on five occasions, described it in July 2002 as being, "scattered over 30 paces by 2 paces, parallel to a narrow drain".

A joint visit to the bog by RHN and the present author in 2009 showed A. polifolia had spread to form two largish clonal patches. Parts of the bog surface have been disturbed by drain cutting and peat removal, but a relatively small area of intact surface remains, approximately 6 ha in size. The micro-topography is subdued, and consists mainly of low peat hummocks and a few small pools, beside which the A. polifolia grows. Apart from the two rare vascular plants mentioned, this site also contains several rare bog mosses, including Sphagnum fuscum and S. imbricatum.

In B & I, A. polifolia is usually confined to relatively undisturbed bogs, a habitat type which is under extreme pressure from drainage, afforestation, heather burning and peat extraction. Several A. polifolia sites in NI are known to have been lost to these operations, and Clontymullan (or Drumanacabranagher) bog is disturbed and continues to be exploited for peat, despite the presence of two exceptionally rare plants growing on its surface (ie Rhynchospora fusca and Andromeda polifolia).

British and Irish occurrence

In NI, Bog-rosemary is a rare and slowly declining species as its very specialised bogland habitat becomes modified or destroyed. Apart from the one Fermanagh site described above, there are between one and three recent records in each of Cos Tyrone, Armagh, Antrim and Londonderry (H36, H37, H39 & H40). Two previous sites in Co Down (H38) near Donaghadee were destroyed by development around 1927 and pre-1938 (FNEI 2, FNEI 3).

Although A. polifolia is very well represented on bogs in the more central counties of Ireland, and remains present to a very much lesser extent in NI, in hectad maps both in the first BSBI Atlas (Perring & Walters 1976) and in the map in the Biological Flora species account (Jacquemart 1998), the 1987-8 BSBI Monitoring Scheme survey showed (within its limitations) a 9.0% decline in occurrence in the RoI, and a 18.0% reduction in sites overall in B & I (Rich & Woodruff 1990). In direct contrast with this, after a complete survey of B & I, the New Atlas map indicates numerous fresh finds of Bog-rosemary during the last 60 years, especially in Wales and in the Clyde Basin (M.C.F. Proctor, in: Preston et al. 2002).

The almost total absence of A. polifolia from the Scottish Highlands is quite the most striking feature of its B & I occurrence. While it may be possible to explain this absence by suggesting that A. polifolia is purely a raised bog species, and that it cannot tolerate the differing, more variable and more testing conditions of shallow blanket bog peat, its widespread and common occurrence in Scandinavia would immediately refute this contention. In her review of the species, Jacquemart (1998) indicates that while the chief habitat of the species in NW Europe is on the main dome of raised bogs, it does extend into wet hollows, wooded bogs and occasionally onto blanket bogs. The isolated station on Mount Keen in NE Scotland, discovered in summer 1979, is on a high-level blanket bog sitting at 735 m (Birse 1980).

In view of its isolated and very rare occurrence in the N of Ireland, the abilities the species has to self-fertilize and tolerate prolonged inbreeding are clearly advantageous, allowing even solitary plants to set seed and create populations. Dispersal of the small seed from the fruit capsule is presumably by wind and must be sufficiently effective for the plant to manage jump-dispersal to Mount Keen in Scotland (Birse 1980) and, of course, to the isolated station at Drumanacabranagher or Clontymullan Bog in Fermanagh.

European and world occurrence

The distribution map for W Europe shows A. polifolia is most frequent in countries around the Baltic and less often found in S Sweden, Denmark, the Swiss-French Alps and the Pyrenees (Jacquemart 1998). It is widespread in the boreal zone of Europe, Asia and N America making it circumpolar. It occurs throughout Scandinavia, making its near total absence in the mountains of Scotland very surprising (Hultén & Fries 1986, Map 1456).

In view of its isolated, and very rare occurrence in the N of Ireland, the abilities to self-fertilize and tolerate prolonged inbreeding are clearly advantageous, allowing even solitary plants to set seed and create populations. Dispersal of the small seed from the fruit capsule is presumably by wind and must be sufficiently effective for the plant to manage jump-dispersal to Mount Keen in Scotland (Birse 1980), and of course to the isolated station at Drumanacabranagher or Clontymullan Bog in Fermanagh.

Threats

Drainage and peat cutting.

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