Dwarf birch (Betula nana) is a little-known but important component of the montane shrub community in the Highlands.
Dwarf birch is circumpolar in distribution – it occurs in Greenland and Iceland and in the northern latitudes of Europe, Asia and North America. Two distinct subspecies are recognised: Betula nana ssp. nana, which grows in western coastal Greenland, Iceland, Scotland, Scandinavia, the Alps and across northern Asia to western Siberia; and Betula nana ssp. exilis, which is found in Siberia, Alaska, northern Canada and Greenland. The principal difference between these two subspecies is that exilis has shorter leaf blades than ssp. nana . In Scandinavia, Betula nana hybridises naturally with downy birch, Betula pubescens, to form B. pubescens ssp. tortuosa, which has a similar habit and growth pattern. Another closely related species of dwarf birch, Betula glandulosa, occurs in Alaska, northern Canada and Greenland.
Distribution in Scotland
Dwarf birch is restricted to the Highlands in Scotland, and it occurs from Perthshire northwards through the Cairngorms and the hills of Inverness- and Ross-shire to the Flow Country of Caithness and Sutherland. It grows in areas with wet and cold climates, and at altitudes ranging from as low as 120 metres up to 835 metres. Further south in Britain, there are two outlying populations in northern England, in the North Pennines and at Kielder in Northumberland.
In Scotland, dwarf birch is classified as being nationally scarce, and it exists in scattered pockets throughout its range, usually in a considerably suppressed form, because of overgrazing by deer and sheep.
Surveys carried out by Trees for Life volunteers have found that it is quite widely but patchily distributed on the hills between Glen Affric and Glen Moriston. Outside the UK, dwarf birch commonly forms thickets and extensive stands.
Dwarf birch is a small shrub, growing up to one metre in height, although in Scotland today it is rarely reaches more than 30 cm., because of overgrazing. Its branches or twigs are a dull dark brown colour and are quite stiff. As with downy birch, the twigs are covered in a fine down.
The leaves are small, reaching a maximum width of 2 cm., round in shape and have a few large, rounded and regularly-spaced teeth on their margins. The top surfaces of the leaves are dark green in colour, while the undersides are a slightly lighter shade.
Like other birches, dwarf birch is deciduous and the new leaves appear in spring, in April or early May, and turn a dull yellow colour before being shed in October.
In Scotland dwarf birch grows exclusively on blanket peat, where the soil conditions are typically wet, acidic and lacking in nutrients. Elsewhere in its range, however, it also occurs in more rocky sub-arctic and alpine locations, growing on steeper slopes and where the soils are better drained.
The vegetation community in which dwarf birch most commonly occurs in the Highlands is that characterised by heather (Calluna vulgaris) and hare's tail cottongrass (Eriophorum vaginatum). Other plants commonly found in this community include blaeberry (Vaccinium myrtillus), cowberry (Vaccinium vitis-idaea), feather moss (Hylocomium splendens) and sphagnum moss (Sphagnum spp.).
Dwarf birch is monoecious, so each individual plant has both male and female flowers. The male flowers are catkins up to 0.8 cm. long, which are erect when young, but then droop as they mature. The female flowers are slightly larger and broader, and are pollinated by the wind, becoming cone-like in appearance as they mature.
The tiny seeds, each weighing less than 0.2 milligrammes, ripen by September or October and have a small crescent-shaped wing on each side of them.
The seeds are dispersed by the wind and usually germinate in mid-May in Scotland, triggered by rising temperatures and increasing daylight. The initial growth rate is very slow and seedlings reach a maximum height of 3 cm. in their first year, with 4-6 true leaves.
Dwarf birch can also reproduce vegetatively, with new shoots growing from underground root sections (which have been recorded in Greenland as being up to 147 years old), or via adventitious roots which grow off stems that have been overgrown by moss.
In common with the larger species of birch, dwarf birch has mycorrhizal associations with various fungi. In these mutualistic or symbiotic relationships between trees and fungi, both partners in the association benefit from their interactions, through an exchange of nutrients between their root systems. However, because of its limited distribution in Scotland, and the fact that until recently it has been little-studied here, not much is known about mycorrhizal fungi which are associated with dwarf birch. The fungal species which have been recorded with it are Lactarius helvus, L. rufus, Leccinum holopus and an unidentified Russula sp.
Elsewhere within its circumpolar distribution, a much larger number of fungi have been recorded with dwarf birch. Some of those species, such as the edible boletes (Leccinum scabrum, L. versipelle and Boletus edulis), also occur in Scotland, so it is likely that they would form mycorrhizal relationships with dwarf birch, if it was more widespread and abundant, as in the past.
Non-mycorrhizal fungi occurring with dwarf birch in Scotland are a Although dwarf birch has quite a number of host-specific insects (ie insects that feed on it and nothing else) outside the UK, there are only two such insects recorded in Scotland: the larvae of the moths Swammerdamia passerella and Stigmella nanivora . Other insects which feed on dwarf birch, as well as other plants, include the larvae of a moorland moth (Argyrotaenia pulchellana) and the northern winter moth (Operophtera fagata). In other parts of its range, a number of aphid species suck the sap of dwarf birch.
Larger herbivores which graze on dwarf birch include sheep and red deer (Cervus elaphus), and their feeding is one of the main factors limiting the natural spread of this species in Scotland. In Scandinavia and North America, dwarf birch is also eaten by moose (Alces alces) and reindeer or caribou (Rangifer tarandus), so it is likely that they also grazed on it in Scotland, before they were exterminated. Siskins (Carduelis spinus) feed on birch seed in winter, and will eat dwarf birch seed where it is available.
It is humans, though, who have largest impact on dwarf birch in Scotland, through past deforestation and the continuing effects of overgrazing and the burning of heather moorland. With a growing interest in dwarf birch today, and a recognition of its important role within the montane shrub community (which has largely disappeared from the Highlands), measures are now being taken at some sites to counter these problems. As a result, this scarce species should begin to make a comeback in the near future to more of its former range.