As the largest and longest-lived tree in the Caledonian Forest, the Scots pine (Pinus sylvestris) is a keystone species in the ecosystem, forming the ‘backbone’ on which many other species depend.
Scots pine is the most widely distributed conifer in the world, with a natural range that stretches from beyond the Arctic Circle in Scandinavia to southern Spain and from western Scotland to the Okhotsk Sea in eastern Siberia. Within this range it grows at elevations from sea level to 2,400 metres (8,000 feet), with the elevation generally increasing from north to south. Despite this wide distribution, the Scots pine forests in Scotland are unique and distinct from those elsewhere because of the absence of any other native conifers.
Distribution in Scotland
After the end of the last Ice Age, approximately 10,000 years ago, Scots pine, like other trees, spread northwards again from continental Europe into Britain. As the climate continued to warm, it spread into much of northern Scotland, reaching a maximum distribution about 6,000 years ago, before declining about 4,000 years ago for reasons that are not entirely understood. Today the Scots pine has a natural range confined to the Highlands in Scotland, with the native pinewoods covering approximately 17,000 hectares in a number of separate, isolated remnants – just over 1% of the estimated 1,500,000 hectare original area. In many of the remnant areas, the pines are growing on north-facing slopes, but the exact reason for this is not clear – the generally-wetter conditions of such northerly aspects may have provided protection from fire, which was used to clear the forest in past centuries.
Within its present-day range in Scotland, there is considerable biochemical variation in the Scots pine, and this has led to the recognition of seven different groupings of native pinewoods, characterised by these differences.
The pinewood remnants which survive today occur in some situations as stands of pure pine and in others of mixed stands of pine and birch (Betula pendula and Betula pubescens). Because of its inability to regenerate under its own canopy, it is likely that the areas where pine predominates changed over time (eg perhaps every 2-3 centuries – the lifespan of a single generation of Scots pines), making our native pinewoods a dynamic, ‘mobile’ forest when viewed over the millennia.
In good situations on mainland Europe, Scots pine can grow to 36 metres (120 feet) in height, but in most of the pinewood remnants in Scotland today the largest trees are about 20 metres (65 feet) tall, with exceptional trees recorded up to 27 metres (90 feet). Maximum girth at breast height is usually up to 2.4 metres (8 feet), although some trees up to 3.6 metres (12 feet) have been recorded. Scots pine usually lives up to an age of 250-300 years in Scotland, although a tree in one of the western pinewood remnants was recently discovered to be over 520 years old!
Scots pine is unusual amongst conifers in having a number of different mature growth forms, ranging from tall and straight-trunked with few side branches, to broad, spreading trees with multiple trunks. Eleven different growth forms, or habit types, have been identified for Scots pine in Scotland, and many of these can easily be seen in the pinewood remnants. Young Scots pines display the characteristically conical shape of conifers, but as the trees mature, this gives way to the flat- or round-topped shapes which are typical of the pines in the ancient Caledonian Forest remnants.
The bark of the Scots pine is also quite variable, with the young bark on small branches being papery thin and often orange-red in colour. The bark on the trunk of a mature Scots pine can vary from grey to reddish-brown and forms layered plates or flakes up to 5 cm. thick, with deep fissures in between. Several species of lichen commonly grow on the bark.
The needles grow in pairs, are blue-green in colour and about 5 cm. (2 inches) in length. They normally remain on the trees for 2-3 years, with the old needles turning yellow in September or October before they are shed. Drops of sticky resin often cover the tree’s buds, and also provide a natural preservative for the wood: if a Scots pine dies while it is still standing, the skeleton can persist for 50 or even 100 years before falling down, because the high resin content in the sap makes the wood very slow to decay.
Male and female flowers occur on the same tree. They appear in May with the females on the tips of the higher and more exposed branches and the males clustered together, often en masse, on the branches just below. Pollination is by wind, and fertilised female flowers take two years to become a fully-grown cone. The cones ripen in April, opening while they are still on the tree, and the tiny winged seeds, each weighing 0.005 grams, are dispersed by the wind. Cone production is variable, with good seasons, in which a mature tree can produce 3,000 cones, occurring every 3-5 years, while in between a tree will produce few cones, or none at all. The seeds are generally carried as far as 50-100 metres from the parent tree, although in some situations, especially when there is snow on the ground and a frozen top layer forms, the seeds have been known to travel several kilometres over the smooth, icy surface.
The seeds require a high level of light to germinate and grow, so seedlings are found in open areas and clearings; as a shade-intolerant species, Scots pine does not regenerate under its own canopy. Although germination will occur in various soil types and conditions, the preferred growing situation is on well-drained mineral soil, which in Glen Affric occurs mainly on the slopes of the glen and on the morainic mounds – raised heaps of ground-up rock left behind by the retreating glaciers of the last Ice Age – which are scattered throughout the valley bottom. In the past, it is likely that the effects of forest fires and the rooting behaviour of wild boar (Sus scrofa) both played an important role in creating the exposed mineral soil which pine seedlings grow best in.
As the largest and longest-lived tree in the Caledonian Forest, the Scots pine is a keystone species in the ecosystem, forming the ‘backbone’ on which many other species depend. In the community of organisms which makes up the forest, the Scots pine has a critical role to play, and has relationships with many plants, insects, birds and animals. Some of these live on the pine itself, particularly epiphytic lichens and mosses. These grow on the bark and branches of the pine, especially in wet areas, but do not take any nourishment from the tree. In fact, many of the lichens growing on a Scots pine add to the fertility of the forest through their ability to absorb, or fix, nitrogen from the air. This is incorporated into the body of the lichen, and when it, or the branch it is growing on, falls to the ground, the nitrogen is absorbed by the soil as the lichen decays, and then becomes available for other plants to use.
Like most trees, the Scots pine has special mycorrhizal associations with fungi, whereby the hyphae, or threadlike filaments, of the fungi wrap around the root tips of the tree, and through this an exchange of nutrients takes place. The fungi, which are unable to make direct use of the sun’s energy themselves, receive carbohydrates and sugars which the pine has produced through photosynthesis, while the tree receives certain nutrients and minerals from the fungi, which it is unable to access directly in the soil. Through this mutualistic or symbiotic relationship, both the tree and the fungi benefit and are able to grow better than they would in the absence of the other. Scots pine is known to have mycorrhizal associations with over 200 species of fungi in Scotland, and these include the chanterelle (Cantharellus lutescens), a relative of the common chanterelle which only occurs in the pinewoods, and the extremely rare greenfoot tooth fungus (Sarcodon glaucopus) – Glen Affric is one of only three locations where this species has been observed in the UK.
A number of rare and special plants are particularly associated with the pinewoods of the Caledonian Forest, and these include twinflower (Linnaea borealis), one-flowered wintergreen (Moneses uniflora) and orchids such as creeping ladies tresses (Goodyera repens) and lesser twayblade (Listera cordata). The shade provided by the canopy of mature Scots pines provides a good habitat for blaeberries (Vaccinium myrtillus) and cowberries (Vaccinium vitis-idaea) to flourish in, and dense carpets of these cover the forest floor in many areas. They also play a successional role in the development of the hummocks which are commonly found in the pinewoods. These hummocks form over extended periods of time in the shade of the trees, when lichens and mosses colonise boulders or tree stumps. As these lower plants grow, humus or organic matter builds up and this allows the blaeberries and cowberries to become established. Eventually a living mat of vegetation is formed, completely covering the underlying boulder or stump, and creating the gently-rounded, hummocky forest floor which is characteristic of many of the native pinewood remnants of the Caledonian Forest.
Like all trees, the Scots pine attracts the attention of various insects. Some of these live in the fissures between the plates or flakes of the tree’s bark, and these form a food source for birds such as the crested tit (Parus cristatus) and the treecreeper (Certhia familiaris), which specialise in winkling them out of the cracks and crevices. Larvae of the pine weevil (Hylobius abietis) burrow into the wood of the tree, and other insects live on the pine’s foliage – aphids suck the sap, and caterpillars of species such as the sawfly (Neodiprion sertifer) and pine looper moth (Bupalus piniaria) eat the needles. Wood ants (Formica aquilonia) feed on these caterpillars, thereby helping to protect the trees from defoliation, and also `milk’ the aphids for the honeydew which they produce. These ants live in large social colonies, and their mounds of fallen pine needles and forest detritus are a characteristic feature of the pinewoods. The mounds are up to a metre high, can contain as many as half a million individuals, and are generally south-facing, to take advantage of the sun’s warmth.
A variety of birds are associated with the Scots pine in Scotland, ranging from common insect- or seed-eating species like the chaffinch (Fringilla coelebs) and siskin (Carduelis spinus) to large raptors such as the golden eagle (Aquila chrysaetos). Black grouse (Tetrao tetrix) and capercaillie (Tetrao urogallus) both live in the pinewoods and eat the buds and shoots of the pines. The capercaillie became extinct in Scotland in the 18th century, but was successfully reintroduced from Scandinavia in 1837 and is primarily associated with the native pinewoods today.
The only bird which is endemic to the UK (ie found here and nowhere else in the world) is the Scottish crossbill (Loxia scotica), which is confined to the pinewoods. It is sometimes called the ‘Scottish parrot’ because of its crossed mandibles, which it uses to prise open the tightly-fitting scales of the Scots pine’s cones. The seeds inside form the mainstay of the diet for this rare bird.
Mammals associated with the pinewoods include the red squirrel (Sciurus vulgaris), which also extracts and eats the seed from pine cones while they are still on the trees; mice and voles, which feed on pine seeds which have fallen to the ground, and the pine marten (Martes martes), which eats voles, red squirrels and small birds, and relishes blaeberries in late summer. Larger mammals found in the pinewoods include the wildcat (Felis silvestris), badger (Meles meles), fox (Vulpes vulpes), roe deer (Capreolus capreolus) and red deer (Cervus elaphus). Both roe and red deer browse on Scots pine seedlings, eating the needles and leader shoot of young trees, and the overgrazing pressure from their expanded numbers in the last 150 years has prevented the natural regeneration of the native pinewoods throughout the Highlands. Red deer also damage or kill sapling Scots pines by de-barking or thrashing them with their antlers, particularly in late spring when the new season’s antlers are shedding their velvet. In a natural, healthy forest ecosystem, the deer numbers would be in balance with the regenerating trees in the forest, but the imbalance in our pinewoods has created a ‘generation gap’ in the Scots pines, with no trees younger than 150 years in most locations, until fencing or intensive deer-culling measures were initiated in the last 10-20 years.
In the past, the pinewoods supported a wider range of large mammals, including the wild boar, European beaver (Castor fiber), lynx (Lynx lynx), moose (Alces alces), brown bear (Ursus arctos) and the wolf (Canis lupus), but in Scotland these have all been extirpated – the wolf was the last to disappear, when the last individual was shot in 1743.
Little-known until relatively recently, the native pinewoods of the Highlands have become the subject of various restoration and regeneration programmes, and the future prospects for this unique part of Scotland’s natural heritage now look better than they have done for centuries. Many of the best remnants of the pinewoods have active restoration measures underway in them and research projects are elucidating more of the interconnections and relationships which make up this boreal forest ecosystem.