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12 Aug 2020

Arctic-alpine plants of Ben Lawers

Written by Helen Cole, Senior Ranger, Ben Lawers National Nature Reserve
A small white-flowered alpine plant growing in a rocky place.
Ben Lawers is renowned for its arctic-alpine plants, and conservation of them was the reason for its purchase by the National Trust for Scotland. It boasts a great diversity of mountain plants, many of them nationally rare.

As the name suggests, many of these species are found in both arctic regions and within high mountain ranges much further south. These environments provide similar, extreme living conditions for plants in terms of climate, soils and length of growing season, and the plants are specialised to thrive in them.

Most of the species found on Ben Lawers are effectively in the middle of their distribution range, also growing in both arctic and alpine regions. Purple saxifrage (Saxifraga oppositifolia) is the first to flower and is the harbinger of spring at Ben Lawers, with the flowers opening as the snow melts. It’s the northernmost flowering plant in the world, growing at 84ºN in Greenland, and the highest, at 4510m in the Alps.

Bristle sedge (Carex microglochin) is unusual in having an arctic-alpine distribution, but only occurs in Britain within a relatively confined area of nutrient-rich flushes on Ben Lawers. The combination of factors which create a suitable habitat in such a restricted area is not fully understood.

Cyphel (Chelaria sedoides) is found in alpine Europe and is at the northernmost limit of its range in Scotland. It grows in dense moss-like cushions from a woody stock, usually above 900m altitude. It has no petals – the colour of the small yellow-green flowers is given by the sepals.

A close-up of a small yellow-flowered alpine plant.
Cyphel (Chelaria sedoides)

In contrast, alpine fleabane (Erigeron borealis), one of our national rarities, is at the southernmost limit of its distribution in Scotland and grows in a small number of localities with base-rich soils. Its specific name, borealis, gives a clue to its northern distribution and its alternative common name, boreal fleabane. The Ben Lawers population has remained largely stable over the last 40 years but is restricted by grazing; few flowering plants survive below inaccessible cliffs.

Plant names often indicate the conditions in which species grow. The alpine gentian (Gentiana nivalis) is the gentian of the snow. Its small size and the need for warm sunshine for the deep blue flowers to open can make it difficult to spot. Ben Lawers is one of only two locations for this plant in Scotland and monitoring has shown that it benefits from the sheep grazing there.

Snow is an obvious manifestation of the climatic challenges facing arctic-alpine plants. Perhaps counter-intuitively for some, such as woolly willow (Salix lanata), it offers protection from the damaging effects of high winds and severe frosts, maintaining warmth. The related but much smaller dwarf willow (Salix herbacea) is one of the few flowering plants characteristic of areas of late-lying snow. Yellow mountain saxifrage (Saxifraga aizoides) grows beside burns and other areas flushed with snow meltwater.

A close-up of a small alpine plant with oval green leaves.
Dwarf willow (Salix herbacea)

One of the most inconspicuous arctic-alpines, snow pearlwort (Sagina nivalis) can be located at sites which are often the first areas to be covered with snow. Recent surveys of other known Scottish sites suggest that most of the British population is now on Ben Lawers, but there are indications that here too it is in decline. This is likely to be related to climate change, as the low winter temperatures and snow cover to which it is adapted becomes less consistent.

Low temperature is one of the challenges facing mountain plants. As an adaptation many, like mountain sandwort (Sabulina rubella, previously Minuartia rubella), are small and low growing. It’s a pioneer of bare, dry gravelly soil caused by frost heave and other disturbance, and has been seen close to the footpath.

A small arctic-alpine plant growing in snowy conditions.
Mountain sandwort (Sabulina rubella)

Growing close together in a low cushion offers various advantages. The tightly packed leaves of moss campion (Silene acaulis) trap heat so that the temperature is considerably higher than the air, reduces water loss and is aerodynamic against wind and ice abrasion. As a bonus, the mass of pink flowers is more obvious to pollinating insects.

Many mountain plants, particularly the slightly larger ones, are very hairy. Alpine mouse-ear (Cerastium alpinum) is covered in long white hairs, creating a local micro-environment by trapping a layer of still air, also reducing water and heat loss. Its flowers are much larger than its lowland counterpart – an adaptation to the need to ensure pollination in a short growing season.

High winds are common in these habitats, so plants need to be well anchored and often have deep taproots for stabilisation. Alpine saxifrage (Saxifraga nivalis) survives on rock faces and in crevices with apparently no soil.

In response to the short growing season, most arctic-alpines are perennial, an exception being the alpine gentian which is annual. Some like drooping saxifrage (Saxifraga cernua) reproduce vegetatively so pollination is not required. It produces small red bulbils, which drop off to form clones and although it does occasionally flowers, it doesn’t set seed in Scotland.

An upright white-flowered alpine plant growing in a rocky landscape.
Drooping saxifrage (Saxifraga cernua)

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