Twinflower (Linnaea borealis), a beautiful and delicate flower of the native pinewoods, is now quite rare in Scotland, in contrast to its abundance elsewhere.
Twinflower is circumboreal in distribution, meaning that it occurs all around the world in the boreal forest zone of the northern hemisphere. Its geographic range stretches from Scotland and northern Europe through Russia to Siberia, and from Alaska and Canada to Greenland. It also occurs in much of the USA, where it is found as far south as northern California, New Mexico and West Virginia.
Three subspecies of twinflower are recognised, and the plant which occurs in Scotland, Europe and Asia is Linnaea borealis ssp. borealis . In North America, Linnaea borealis ssp. americana is the widespread variety, occurring throughout the species’ range, while the longtube twinflower, Linnaea borealis ssp. longiflora , has a more restricted distribution in the Pacific Northwest region.
Twinflower’s specific name, borealis , reflects its northern distribution, while the other part of its binomial scientific name honours Carolus Linnaeus, the 18th century Swedish naturalist who developed the modern system of taxonomic classification.
Distribution in Scotland
In Scotland, the distribution of twinflower closely matches that of the old pinewood remnants of the Caledonian Forest, as the species is most closely associated with these ancient woodlands. It is most common in the northeast of the country, in the drier native pinewoods, although it also grows further west, for example near Fort William. In Glen Affric it occurs in the area near Dog Falls, in the eastern part of the glen. Curr Wood, situated southeast of the village of Dulnain Bridge in Strathspey, is claimed to have the largest population of twinflower in Scotland.
Further south, it has been recorded from some locations in the Borders region, and also in the northeast of England, where it was most likely introduced in the 18th century, together with conifers planted from Norwegian sources, but it has subsequently disappeared from there.
Although twinflower is predominantly associated with the old native pine forest, it also grows in Scots pine plantations and occasionally in birchwoods and open heathland. This latter vegetation type is twinflower’s habitat in Greenland, which is virtually treeless.
Twinflower is generally quite abundant throughout its range, so it is not considered threatened at an international level. However, the situation is different in a few specific areas – for example, it has been extirpated from the states of Indiana and Tennessee in the USA, while it is endangered in Iowa and New Jersey.
In the UK twinflower is formally classified as Nationally Scarce, but it has no special legal protection. However, it has declined considerably because of the loss of native pinewoods, and a 64% reduction in records of its distribution has been observed in comparison to those from prior to 1970. As a result, it is now the subject of a Biodiversity Action Plan (BAP), under the government’s response to the Convention on Biological Diversity agreed at the 1992 Earth Summit. The objectives of the BAP are to ensure that all the populations in Scotland are self-sustaining, and to restore twinflower to five sites where it formerly occurred.
Twinflower is a small, creeping, perennial shrub in the honeysuckle family, Caprifoliaceae . It is evergreen and is characterised by above-ground runners known as stolons from which numerous short aerial stems or shoots grow. The shoots are 4 – 6 cm. in height, up to 3 mm. in diameter and turn woody with age. They bear small round leaves which are opposite, meaning that they grow in pairs on opposite sides of the stems, and which persist for 12 – 16 months. The shoots are of two different kinds – the non-reproductive ones only have leaves on them, while the flowering shoots also have inflorescences which are up to 15 cm. tall.
Stolons are first produced when a plant is 5 – 10 years old, and they grow in annual segments which can be as much as 48 cm. long. Over time the stolons can become buried under a shallow layer of forest litter or duff and they take root at the nodes in between the different segments. The roots form a shallow, fibrous network with their growing tips in or just underneath the duff layer.
The stolons also produce branches, and when a branch becomes separated from the main stolon, it grows on to form a new plant. This vegetative propagation is twinflower’s main method of reproduction, and as a result the species usually occurs as clonal patches, consisting of groups of plants which are genetically identical.
Twinflower’s common name is derived from its distinctive and unique inflorescence, which is composed of two bell-shaped flowers growing on a forked stem in the shape of the letter ‘Y’. The flowers are pinkish-white in colour, downward facing and each consists of 5 petals. Their scent is described as very fragrant and almond- or anise-like, and they produce nectar which attracts pollinating insects.
In Scotland twinflower typically blossoms in June or early July, although elsewhere in its range it can flower as late as September. Individual blossoms persist for about 7 days, and after pollination the flower develops into a fruit in the form of a small, dry, one-seeded capsule, which is ripe after 36 days.
The seeds have evolved for dispersal by animals or birds, and are partially-covered in small, hairy bracts which stick to the fur or feathers of wildlife and are thereby transported to new sites. Although seed production is plentiful only a small percentage of seeds have been found to be viable, and seedlings are rarely observed, mainly growing on disturbed ground.
In Scotland, twinflower is considered to be one of the characteristic flowering plants of the native pinewoods, and it usually grows in association with other typical pinewood plants such as heather (Calluna vulgaris), blaeberry (Vaccinium myrtillus) and glittering wood-moss (Hylocomium splendens). The presence of large patches of twinflower is considered to be an indicator of ancient or long-established woodland, because its mainly vegetative method of propagation means it is difficult or slow to colonise new areas.
As a nectar-producing plant, twinflower is a food source for the insects which visit it. In Scotland these include flies, hoverflies and solitary bees, and in the course of feeding on nectar they transfer pollen from one plant to another, thereby fertilising the plants and enabling the production of seeds.
Because it is so rare in Scotland, little is known about the species which feed on twinflower. However, in the Pacific Northwest of North America it forms a significant part of the winter diet of Roosevelt elk (Cervus elaphus roosevelti), which is a close relative of red deer (Cervus elaphus), so it is likely that deer fed upon it here as well when it was more abundant in the past. Also in North America, twinflower was used by the indigenous native peoples for brewing a medicinal tea.
Overgrazing by deer, sheep and cattle has been identified as one of the causes for twinflower’s decline in Scotland. As a shallow-rooted species it is susceptible to drought and it is also vulnerable to fire – even low intensity burns will kill it. These factors, together with twinflower’s mainly vegetative method of reproduction and the loss of its pinewood habitat, have combined to render the species rare here, in contrast to its abundance in similar forests in countries such as Norway. It is unlikely that twinflower will spread again of its own accord, so its future in Scotland depends on restoration projects to return it to suitable areas where it has been lost.