Convention on biological diversity

Boreal forests

Distribution and structure

Boreal landscapes are composed of a complex of plant communities that, aside from vast tracts of forest stands, include various wooded and open mires or bogs, numerous water bodies of varying size, rivers, rock outcroppings and natural grasslands and fens (Walter, 1979; Barbour and Christensen, 1993). Local distribution of boreal communities is heavily influenced by topography, hydrology and soils. Spodosoils or podsols dominate upland sites. Glacial features such as eskers, moraines, kettle holes and outwash plains create considerable variation in the landscapes. Paludification can be very extensive, especially in northwestern Europe and Russia, western Siberia, and the Hudson Bay lowlands and central Keewatin, District of Canada. The extensive continental larch forests of eastern Siberia, on permafrost areas with thermokarst formations, show a great diversity of habitats. Most of the conifer species are adapted to regenerate following fire either as a result of serotinous cones, or as a result of their shade-tolerant development beneath a mixed or deciduous canopy. Some species cannot regenerate without this canopy, as they need it for protection from frost and excessive sunlight. The boreal biome is characterized by an extensive dormant period, often of extreme cold, which reduces its annual productivity. Net annual primary production on upland sites varies from 400 g/m² on poor sites to 1300 g/m² on mesic fertile sites (Barbour and Christensen, 1993).

In North America, regional classifications of the boreal forest ecosystems (e.g., Sims et al., 1989) are available and collectively define over 500 forest ecosystems, mostly based on understory species. Many boreal plant and animal species have particular habitat requirements. The fire regime and gap-phase dynamics of these forests provide a set of hydrological, edaphic and microclimatic conditions that create a wide array of habitats. The boreal forests also have structural components that are important to the fauna within them These include uneven canopy structure, size and age variation of the trees, especially the occurrence of tall, old coniferous and deciduous trees, stumps of trees and standing and fallen decaying wood of various sizes.

The southernmost cool temperate, oceanic forests found along the western coast of South America and New Zealand are considered to be a southern hemisphere equivalent of boreal forests. These forests, rich in epiphytic cryptogams, are dominated by both evergreen and deciduous broad-leaved Nothofagus species, while mires or bogs are also characteristic (Walter 1979).

In the boreal forest regions, an average daily temperature of more than 10C occurs on less than 120 days and the cold season lasts longer than six months. The northern transition from boreal forest to tundra (barren lands with frozen subsoil) is related to a short growing season (fewer than 30 days with a daily temperature above 10C) and permafrost conditions unfavourable to tree growth (Walter, 1979). The ecozone between boreal forest and tundra, called forested tundra, is characterised by stunted trees and shrubs and may extend hundreds of kilometres on flat terrain. At its southward extremes, the boreal biome reaches areas where the climate is sufficiently favourable to support deciduous broad-leaved species more typical of temperate forests. The zone between the coniferous boreal forests and the temperate deciduous broad-leaved forests often consists of mixed forests, recognised as a hemiboreal or boreonemoral or transitional zone. The climate in the vast taiga zone varies widely from cold, oceanic with a relatively small temperature amplitude, to cold continental in which, in extreme cases, an annual temperature span of 90C may occur. Temperatures also change from north to south, and several subzones can be distinguished: northern, middle, southern and extreme continental.

Species diversity

Boreal forest stands normally contain no more than a few species, primarily of the genera Picea, Pinus, Abies, Larix, Thuja, Betula, Prunus, Alnus and Populus, and they often form monocultures, particularly in the case of Picea, Pinus and Larix. These genera are panboreal and members of the four deciduous genera (Betula, Prunus, Alnus and Populus) grow more rapidly than the conifers and tend to occupy sites immediately following stand disturbance. Tree species richness in North American forests is greater than in the Euro-Siberian region. In North America, four of the six principle boreal forest species extend across the continent, though no single tree species is panboreal. Picea mariana grows on poor soils and forms the northern treeline continent-wide. Where fire is uncommon, Abies spp. often predominate in the eastern and continental North American boreal zone. In Eurasia, this genus is ecologically largely replaced by two species of Larix. Larch forests, mostly consisting of Larix gmelinii, cover 2.5 million km² in continental Siberia where much of the terrain has deep permafrost. Larix sibirica often forms monotypic stands following disturbance by fire (Schulze et al., 1996), while in North America, Larix laricina is rarely a dominant species, it is found mainly on cold, wet and poorly drained sites such as in sphagnum bogs and muskeg. In Europe, only Picea abies and Pinus sylvestris are true dominants of the boreal zone, and are often mixed in successional phases with broad-leaved deciduous tree species such as Betula pendula, B. pubecens, Populus tremula and Alnus glutinosa and A. incana. In more eastern European regions, Picea abies is replaced by the closely related Picea obovata, with Abies sibirica, Larix sibirica and Pinus cembra subsp. sibirica. There is a broad belt of hybrids, Picea abies x P. obovata, between their natural regions. In Eurasia, the proportion of Picea gradually decreases eastward while that of Larix increases correspondingly. In northern Japan, the number of coniferous species increases again.

Plantations in the boreal biome