Dryas octopetala L., Mountain Avens
Account Summary
Native, rare and local. Circumpolar arctic-montane.
1804; Wade, Dr W.; eastern range of Cliffs of Magho.
May to July.
Growth form and preferred habitats
This variable, prostrate or low-growing, large creamy-white flowered, dwarf shrub, 5-20 cm in height, has a stout woody rootstock that penetrates deep into rock crevices and sends out lateral roots. At ground level it produces several spreading shoots from the rootstock, each of which branches laterally and roots adventitiously in suitable base- or lime-rich soil, forming dense or loose vegetative clones that often together carpet the rocks, cliff ledges and limestone pavement over which it grows.
The alternate, simple, leathery, evergreen, 2.5-4.0 cm leaves are very variable in size and shape and they are crowded along the prostrate stems overlapping one another to give an almost total cover of the soil. The rolled-down (revolute) margins of the leaves are crenately lobed or bluntly toothed, giving them a scalloped look reminiscent of a miniature oak leaf. This appearance is reflected in the genus name, since 'dryas' is thought by some botanists to be from 'drus', the Greek for 'oak' (Johnson & Smith 1946). The upper surfaces of the small leaves are dark glossy green, ± hairless and rough-textured (rugose) with deeply impressed veins, while their under-surfaces are densely felted with a grey-white tomentum of hairs.
While described as evergreen above, this may merely reflect the relatively mild oceanic climate of W Ireland where D. octopetala evergreen leaf longevity is the norm. Elsewhere, in habitats suffering more severe, extreme or variable temperature regimes, the plant is more likely deciduous, losing its mature leaves gradually through the winter, although retaining juvenile ones around the shoot apices that will expand in spring. It might be better, therefore, to regard D. octopetala plants as semi-evergreen (Elkington 1971). Nelson goes even further, suggesting that while the rootstock is definitely woody and the oldest stems can be well over 50 years old, the stems are less woody and the plants might be better considered an evergreen, perennial herb (Nelson & Walsh 1991, p. 153). In stating this opinion, the current author feels Nelson is being controversial and is out on a limb on his own.
Factors limiting distribution
The distribution of D. octopetala is confined to the occurrence of basic or calcareous rocks and soils. In Ireland, this means scattered Carboniferous limestones and northern basic basalts, while in Britain it also occurs on metamorphosed sugar limestone in the Cairngorms and Teesdale, mica-schists in the central Grampians, lavas in the Lake District and Wales and on calcareous shell sands on the N & W coasts of Scotland (Lusby et al. 1996).
In addition to being restricted to typically calcicole or base-rich soils in B & I, D. octopetala is climatically limited to cool, humid regions with annual rainfall above 1000 mm (40 inches) (Elkington 1971). On European continental mountains, it is usually well protected from winter frost by a deep blanket of snow cover, while in B & I it is more exposed to freezing low temperatures due to the frequent absence of snow. However, observation shows the species is completely resistant to frost and there is no evidence of drought affecting growth or damaging plants either (Elkington 1971).
A number of studies suggest the modern distribution of D. octopetala is probably primarily limited by climatic factors and only secondarily affected by its edaphic (soil) requirements. It is well known that alpine and northern plants grown in lowland gardens suffer during the hottest spells of summer. A study by Conolly & Dahl (1970) found that the distribution of many arctic-montane species in B & I can be correlated with maximum summer temperatures and, in particular, with isotherms of the estimated mean annual maximum temperature of the highest points they reach, both in terms of altitude and pole-ward proximity. Their map of the distribution of D. octopetala shows that in England, Wales and Ireland localities of the species are all included within the 25°C isotherm of this parameter, while in Scotland all localities are bounded by the 23°C isotherm. Of course, correlation with a single factor does not necessarily indicate or prove a causal relationship and a climatic factor that truly affects the distribution or growth of a species must act in some way on the physiological processes of individual plants of that species. It is not known if high temperatures have a direct effect on the plant or act in a more complex manner involving temperature. Since D. octopetala is closely associated with free-draining substrates, a very likely effect of increased summer temperature is increasing moisture stress on the plant, affecting its growth, reproduction and, eventually, its survival (Elkington 1971).
Interestingly, in the horticultural setting D. octopetala is a very easy subject and can be successfully grown, "in any reasonable garden soil, and in any sunny place, requiring nothing but to be well planted and then left alone (with a top dressing at times) to get larger and wider for ever". (Farrer 1930).
Lowland occurrences
Mountain Avens occurs in coastal areas down close to sea-level in parts of both W Ireland and NW Scotland and also on relatively lowland hill slopes inland, including on Knockmore in Fermanagh. At these lower sites, it typically grows on free-draining slopes of sand and gravel or on steep cliff faces, both situations where erosion helps keep the habitat open and provides soil nutrient renewal by rock disintegration. Thus habitat instability, together with the inherent low soil nutrient levels of unproductive vegetation, limit any overly aggressive plant competition, while grazing pressure, to which D. octopetala is rather sensitive, is much reduced by the inaccessibility of steep ground to sheep and other herbivores (Ingrouille 1995; Lusby et al. 1996).
In the Burren, Co Clare (H9), D. octopetala grows with a spectacular profusion over a wide area that is unequalled anywhere else in NW Europe. Here it occurs at or near the coast, carpeting bare limestone rock on slopes and valleys leading to the shore, sometimes present in enormous abundance. It avoids deep, clayey limestone drift but is perfectly at home and able to dominate vegetation over porous drift and solid rock on N- or S-facing slopes from sea-level to around 300 m (Webb & Scannell 1983). In view of this, it is most surprising that there are large areas of ground in and around the Burren district of NW Clare (H9) and SE Galway (H17), apparently sharing the very same geology, geography and history, where the species is totally absent. It is especially mysterious that D. octopetala is completely absent from the adjacent Aran Islands, which represent a slightly offshore section of the mainland Burren and share much of its geology and characteristic mixed-geographic regional flora.
Fermanagh occurrence
Dryas octopetala is one of Fermanagh's most notable Arctic-Alpine Late-glacial survivors. It has been found as a macrofossil in a full-glacial freshwater deposit of Middle Midlandian age, radio-carbon dated to 30,500 BP, located at Derryvree, near Maguiresbridge, Co Fermanagh (Colhoun et al. 1972). The flora and fauna of the deposit indicated that open tundra and a periglacial climate prevailed when the sediment was laid down.
Outside the Burren region of Cos Clare (H9) and Galway (H16), where it is still common and locally dominant on the extensive karst limestones, this is a very rare plant in Ireland. In NI, it occurs only on the base-rich basalt cliffs of Knock Dhu in Co Antrim (H39) and of Benevenagh in Co Londonderry (H40), plus this surviving Fermanagh station on the Carboniferous limestone summit of Knockmore hill, where S.A. Stewart discovered it in 1881. Here it still grows in abundance on a steep slope immediately south of the trigonometrical point and in lesser quantity around the immediate vicinity of the trigonometrical point plinth.
In July 2020, there was considerable delight and surprise when Hannah Northridge found a small patch of D. octopetala, 1 m × 2 m, in a new site on a south facing slope on limestone at Monawilkin to the east of the lake at an altitude of 190 m. Associated species included: Antennaria dioica (Mountain Everlasting), Anthyllis vulneraria (Kidney Vetch), Briza media (Quaking-grass), Campanula rotundifolia (Harebell), Carex flacca (Glaucous Sedge), Euphrasia spp. (Eyebright), Galium verum (Lady's Bedstraw), Juniperus communis (Juniper), Koeleria macrantha (Crested Hair-grass), Leucanthemum vulgare (Oxeye Daisy), Linum catharticum (Fairy Flax), Lotus corniculatus (Common Brid's-foot-trefoil), Pilosella officinarum (Mouse-ear-hawkweed), Sesleria caerulea (Blue Moor-grass) and Thymus polytrichus (Wild Thyme).
At Dr Wade's other original 1804 Fermanagh site for the species, namely the north-facing Cliffs of Magho overlooking Lower Lough Erne, D. octopetala has not been seen since the 1945-53 survey of Meikle and his co-workers. When Praeger refound Dr Wade's original station here exactly a century later, he described the site as follows, "At the base of the east end of the eastern cliffs of Poulaphuca [an alternative name for the site – appearing on maps and in the literature with several different spellings], where the cliffs are low." Robert Northridge and the present author have searched all over this site several times without any success. It is a large area of cliff, some of it difficult or impossible to access, so that a very small patch of D. octopetala might simply elude us, but we have not seen the plant anywhere from along the cliff base.
Flowering reproduction
The main flowering period in lowland sites in western Ireland such as The Burren, Co Clare and Knockmore in Co Fermanagh is in late April and May. In more mountainous situations, anthesis is delayed until June and July, with much smaller numbers of flowers being produced throughout the summer and into autumn and with a lesser late flush occurring in August (Elkington 1971; Nelson & Walsh 1991). The hermaphrodite and male, large, 25-40 mm diameter flowers are borne solitary on erect, dark-red or green, white-tomentose peduncles 2-8 cm long with purplish-black glandular hairs above.
Unusually for a member of the Rosaceae, most flowers have eight white to cream petals, although the number can vary from 7-16. Stamens and carpels are both numerous, the latter bearing long, feathery styles that in fruit elongate further to 20-30 mm and become hairier. The receptacle is domed or conical and the hypanthium saucer-like. A circular nectary on the mouth of the hypanthium sits between the stamens with their golden anthers and the clustered carpels. The flowers in B & I are usually homogygous (anthers and stigmas maturing simultaneously) although protogyny and protandry have been recorded elsewhere in Europe. While most flowers are bisexual (perfect, hermaphrodite), a small proportion are male only, containing undeveloped carpels. The flowers attract a range of insects including flies, bees, butterflies, moths and beetles, although flies are considered the major pollinators in B & I, Scandinavia and the Arctic. If a flower remains unvisited it may self-pollinate and produce at least some viable seed, although less than if cross-pollination was achieved. This suggests there is some trace of an incompatibility mechanism in the species (Elkington 1971).
An interesting flowering adaptation known as sun-tracking (or heliotropism), allows the flowers of D. octopetala to constantly glow on a sunny day. The dish-shaped flowers constantly turn to face the sun, thanks to a rotating physiological movement of their peduncles. It is believed that in colder arctic and mountain environments, the heating provided within the near-parabolic dish of the blossom helps focus the rays of the sun onto the anthers and carpels. This feature warms the reproductive organs, presumably increasing the rate of their development; it will also warm the nectar, making the flowers more attractive to the relatively few flying insects that remain active in these cold habitats. Insects are also known to sun-bask within some types of arctic and alpine flowers such as D. octopetala, their prolonged flower visits allow them to warm themselves, increasing the efficiency of their flight muscles and assisting their subsequent flight (Hocking 1968).
Seed production and dispersal
Achene counts vary across B & I from 48 per fruiting head in Co Clare to 61 per head in Perthshire. The number of flowers per plant probably varies enormously, perhaps as many as 100 on a large plant, although very often it is impossible to recognise an individual as the species is so very gregarious and mat-forming. The achenes are each c 3 mm in size and have a persistent style 2-3 cm long in fruit. The achene and its style are both covered in long silver-white lateral hairs at this stage, making the fruiting head look like "beautiful silver fluff-whirls", as the rock gardener Reginald Farrer (1930) described them. In the Burren, Co Clare where the plant is locally dominant and abundant, the feathery fruiting heads are described by Webb & Scannell (1983) as, "almost as conspicuous as the flowers". The peduncle that bears the fruiting head elongates to about twice its original length as the achenes ripen. The achenes thus appear very well equipped for wind transport by their feathery styles and by their elevation above the creeping vegetative mat of the plant.
An unknown proportion of achenes disperse on the breeze, distances again unmeasured, but in garden experience many achenes appear to end up around the base of the parent, or close to it. There is a tendency for the long, hairy styles to twist around one another, either when immature or in wet weather, which at least temporarily, must seriously reduce the efficiency of their dispersal (Elkington 1971). The fact that so many populations of D. octopetala appear small, isolated, relict and confined to long-known sites, and in other areas like the Burren, Co Clare, super-abundance rubs shoulders with total absence in very similar habitats, suggests to the current author that under present environmental conditions (including climatic warming), dispersal has become a major problem limiting the distribution and affecting the longer term survival of this once extremely widespread arctic-alpine species.
Seed germination and survival
Seed normally germinates in the subsequent spring after overwintering and buried seed is transient in the soil, surviving for one year or less (Thompson et al. 1997).
Effective reproduction appears to be normally by seed, although in modern times at least, seedlings and young plants seem to be only rarely recorded under natural conditions in the field. This cannot always have been the case since, from the plentiful fossil record, D. octopetala was capable of rapid invasive colonisation of recently de-glaciated, unoccupied soils (silts, sands, gravels) in the Late Glacial and early Post-glacial periods. Indeed, it became so widespread and abundant in the exposed tundra as northern European glacial ice-sheets retreated, that earlier palaeontologists labelled two periods in the Late-glacial 'the Older Dryas'(12,500-12,000 BP) and 'the Younger Dryas' (12,300-11,500 BP), with the brief much colder 'Allerød' glacial snap separating them. This labelling arose because fossil leaves and pollen of the subshrub were so plentiful during these two climatically warmer periods when Devensian (or in Ireland 'Midlandian') glaciers began to melt. The abundance of the species in the Late Devensian and early Post-glacial is evidence of the low intensity of grazing at these times. D. octopetala and other members of the Dryas heathland vegetation community are characterised by low growth rates, so that as the climate continued to warm, they could not compete against a broad range of colonising species. Only in the cooler N & W of Ireland & Scotland did open Dryas-heath continue to cover large areas before the advent of sheep grazing and before leaching made soils more acidic (Ingrouille 1995).
While today, evidence of Dryas seed reproduction and colonisation appears surprisingly scant, an exception was the report of seedlings growing among mature D. octopetala plants and on bare patches in Sesleria caerulea (Blue Moor-grass) dominated grassland in the Burren, Co Clare, recorded in 1959 and 1960 by Prof C.D. Pigott (quoted in: Elkington 1971). Further study is needed to clarify the reproductive capability and establishment of D. octopetala in today's warming climate and changing soil conditions.
Vegetative reproduction and individual longevity
A very limited amount of vegetative reproduction may be achieved by the decay of old stems in existing clones, but this is unlikely to be significant in terms of species increase or spread. Shoot growth on older stems is often asymmetrical, with the pith being formed to one side, often the upper, and with incomplete growth rings. Counts of growth rings on the oldest woody stems in the Kola peninsula in Lappland (68°N) showed the individuals were between 50 and 108 years old (Kihlmann (1890), quoted in Schroeter (1908, p. 185)). Measures like this indicate the plant has a definite, rather surprisingly limited lifespan and its local survival in stations that today are geographically isolated must involve maintenance recruitment from seed or genets (offset plantlets) of the existing stock (Elkington 1971).
Fossil history
The leaves of D. octopetala can be readily identified as macrofossils in peat and silt deposits, even in fragmentary condition on account of their strongly recurved margins, densely hairy under-surface and characteristic venation. There are Middle and Late Weichselian macrofossil records from most of the present day Mountain Avens sites in northern Britain and N & W Ireland, plus a few sites further south in Wales and SE England where the species no longer occurs (Godwin 1975, Map of B & I in Fig. 62). There are also pollen records from the Late Weichselian. The Older and Younger 'Dryas clays' widespread in N & W Europe are deposits now recognised as corresponding with zones I & III of the Late-glacial period (see also above).
A second European map (Fig. 63) reproduced in Godwin (1975), shows how the modern Arctic and alpine European distribution of D. octopetala has become fragmented compared with its fossil record by the loss of intermediate lowland stations from the Netherlands, Denmark, NW Germany, southern parts of Norway and Sweden, plus isolated areas of C Europe towards the Black Sea. The post-glacial Flandrian (Littletonian) decline across the whole of its European range has been very considerable and must be attributed to habitat changes affecting plant competition involving factors such as rising summer temperatures, soil chemistry acidification as it ages, plus disturbance by man and his animals.
Variation
All chromosome counts made to date regard D. octopetala as diploid with 2n=18. However, being a very widespread species, D. octopetala contains considerable variation, both phenotypic with respect to different environments and genotypic across its wide geographic range, so it is unsurprising that people have named forms, varieties and subspecies from time-to-time. The Flora Europaea account (T.T. Elkington, in: Tutin et al. 1968) mentions two forms, plants of the first lacking branched hairs on the leaves named as D. babingtoniana A.E. Porsild, and a second form named D. punctata Juz. from Arctic Russia, which has large glands on the upper surface of the leaves. However, the differences are slight and herbarium studies prove there are intermediate forms, so neither of these taxa are considered worthy of more than varietal rank, and it is probably best to regard D. octopetala as one polymorphic species (T.T. Elkington, in: Tutin et al. 1968; Lusby et al. 1996).
Hybrids
D. octopetala and the closely related northern boreal and Arctic form D. integrifolia overlap and crossbreed in Greenland (Elkington 1965) and Alaska (Hultén 1968), with subsequent widespread introgression taking place (Elkington 1971).
British and Irish occurrence
In Britain, for the most part, D. octopetala is thinly scattered across a few mountain sites in NW England and NW Wales, while in Scotland it is much better represented across a wider range of habitats, altitudes and stations, stretching from sea-level in the Inner Hebrides to 1035 m in the Cairngorms. In Ireland, Mountain Avens is common and dominant in a few areas in the Burren, Co Clare from sea level to 300 m on the hills. Elsewhere in Ireland, it is thinly scattered (although locally abundant in a few sites in SE, NE & W Galway (H15, H16 & H17)), with a few isolated, mainly relict mountain cliff sites in northern and western VCs from Sligo (H28) and Leitrim (H29), Fermanagh (H33), W Donegal (H35) to Londonderry (H40) and Antrim (H39). (D.J. McCosh, in: Preston et al. 2002).
European and world occurrence
In Europe, at least, D. octopetala is considered a single species and it has a very definite Arctic-montane distribution stretching in the north from Greenland, Iceland, Spitzbergen, the Faeroes, W Norway, N Sweden and Finland to N Russia and then SW to B & I. In S Europe, it stretches across mountain ranges from the Pyrenees, Alps and Apennines, to the mountains of the Balkans and Macedonia (Kurtto et al. 2004, Map 3367). Beyond Europe it ranges from the Caucasus to Turkey and from Siberia, Korea to the Yenisei region of Japan. In N America, taking it in a wider s.l. sense to include such sometimes disputed forms as subsp. alaskensis (Porsild) Hult. and subsp. integrifolia (Vahl) Hult. (sometimes recognised as separate species), it is widely distributed from Alaska and the Yukon east to the Mackenzie River and south along the Rocky Mountains to Colorado (Elkington 1971; Hultén 1974, Map 45; Hultén & Fries 1986, Map 1190). In the broad sense, D. octopetala is considered a circumpolar Arctic-montane species.
Threats
Fertiliser 'improvement' of the pastures on Knockmore, or excessive sheep grazing, would thus threaten its slender hold on its solitary surviving Fermanagh site.