Boreal Region

Here I briefly describe the Fennoscandian boreal forests in their supposed natural conditions and also some differences to the boreal forests of other regions.

A short growing season, low solar elevation angles, low soil temperature, moist climate due to low evaporation and low nutrient availability are common attributes of all the boreal forests.

The special characters of the boreal forests of our area include its relatively oceanic climate. The best climate match on other continents is located in western North America from Canada’s British Columbia to Alaska (photo below). The climates of central and eastern Canada, Siberia and the Russian Far East are more continental. In Fennoscandia there is almost no permafrost, with no forest growing on what little there is, whereas over 50% of Canada and Russia, including large forest areas, are underlain by permafrost¹.

Fennoscandia is characterised by shallow glacial till on a shield of bedrock (photo below); whereas on the Russian plains and in North America south of the Canadian Shield there are deep sediment deposits². Soils are normally podzols: the uppermost pale layer of mineral soil has lost a part of its minerals to lower layers by weathering; in the most fertile and less acid soils (in “herb-rich forests”) the layering is absent because earthworms mix the soil³ (however, the earthworms have spread to the previously glaciated areas through agriculture, so those soils are actually “unnatural”! ⁴). The mineral soil is covered by an organic surface layer³.

Just like in the temperate zone, in the boreal zone, too, Europe has fewer tree species than Asia and North America. In one hectare, there are typically 2–6, at most approx. 10 tree species⁵. The same tree species grow in almost all the old forests: the dominants are usually Scots pine (Pinus sylvestris) on the drier and Norway spruce (Picea abies) on the moister sites. The wettest spruce forests grow on peat; less fertile woodlands on peat are dominated by pine, in the north also by Siberian spruce (P. obovata) ⁶. Silver (Betula pendula) and downy birch (B. pubescens), and common aspen (Populus tremula) are usually abundant, too. Here and there one may find grey alder (Alnus incana) and goat willow (Salix caprea). European rowan (Sorbus aucuparia) and common juniper (Juniperus communis) are often abundant in the shrub layer but seldom reach tree dimensions in coniferous forests. Black alder (Alnus glutinosa) may grow in fertile wet sites. You may have difficulties distinguishing between the two birch species but the other mentioned trees are very easy to identify. The tallest native tree species is Norway spruce that has attained 49.4 m in southern Sweden⁷. The spruce is lacking from Scotland’s and Iceland’s natural boreal forests as is the pine from Iceland’s. There is no virgin or near-virgin forest anymore in Scotland and in Iceland and they are not discussed further here.

Shrubs play in the Fennoscandian boreal forests a low significance compared to other boreal forests. If present, the shrub layer is normally formed by tree saplings.

Shallow and infertile soil can be seen in the field layer: it is dominated by subshrubs, especially bilberry (Vaccinium myrtillus, photo 1 below), lingonberry (V. vitis-idaea), crowberry (Empetrum nigrum) and common heather (Calluna vulgaris); only in the most fertile soils do herbs and grasses dominate the field layer^{3 8} (photo 2 below). The ground layer is composed of mosses (e.g. glittering woodmoss (Hylocomium splendens) and red-stemmed feathermoss (Pleurozium schreberi)) in moist and fertile sites and of lichens (e.g. reindeer lichens (Cladonia)) in drier sites³. In the sediment regions of Russia and Canada the undergrowth is remarkably richer (photo 3 below).

The colours of the Fennoscandian boreal forests are various green tones, trunks brownish grey and upper pine trunks orange (photo below). In old forests, birches are usually covered by mosses, lichens and algae, thus appearing rather grey than white. In winter the prevalent colour is white due to snow cover, while autumn is dominated by the yellow of dying birch leaves. Spruce crowns are dense but relatively narrow, usually with space between individual crowns. Pine and birch have very thin crowns. The subshrub and moss/lichen layers are usually continuous.

It may sound paradoxical that although boreal ecosystems are colder and moister than temperate ones, it is the boreal forests where fire is an intrinsic ecological component, affecting the species composition and the structure⁹. This is a result of several factors. Dead plant material accumulates because decomposition occurs slowly due to low temperatures, infertile soils, and the acid and resin-rich debris of conifers. Conifer fuels are inherently more flammable due to significant resins, tars and other easily ignited compounds in their wood, bark and needles. Furthermore, the ground layer of lichens and feather-like moss dries out in rather short dry spells, as do the pine needles on the ground. The relatively open canopy also allows the sun to dry the forest floor in dry periods.

Scots pine forests on dry sites are often characterised by frequent low-intensity fires that produce stands consisting of trees of various ages². Scots pine is the most drought-tolerant and nutrient-stress-tolerant of all the Eurasian boreal tree species¹⁰, and the thick bark of large individuals and the long distance between the ground and the canopy protects them from fire². The cones of Scots pine are not serotinous (i.e. opened by heat) like cones of the North American jack pine (Pinus banksiana) and lodgepole pine (P. contorta) ¹¹; also North American black spruce (Picea mariana) has semi-serotinous cones¹². These North American trees have thin bark and do not thrive in a surface fire regime as Scots pine does¹¹. Instead, the abundant serotinous cones, which have crowded on their branches over the years, release their seeds after a crown fire, resulting in impenetrable thickets. In later development stages, too, North American boreal forests are typically still denser; the densities of fallen trees being also higher (photos below). Only eastern white pine (P. strobus) and red pine (P. resinosa) growing in the southern hemiboreal zone of eastern North America, and Rocky Mountain Douglas-fir (Pseudotsuga menziesii var. glauca) of interior British Columbia have fire-resistant bark¹² but in more northern areas no tree species. In Siberia, larches (Larix spp.) have even better fire-tolerance than Scots pine¹⁰.

Spruce is more shade-tolerant but fire-intolerant¹⁰. Thus, spruce slowly replaces pine in moister forests if fire does not eliminate the spruces. However, Scots pine may live for hundreds of years and has often survived also in mesic communities until fire occurs. Prior to effective fire prevention, also the moister spruce forests have burned frequently. However, these fires were mostly of human origin; particularly in the period which began at the end of 1500s and ended about 1850, the fire interval was much shorter than the natural one, which had been 200–500 years in the (semi)maritime climate of our area^{13 14}, in Norway still longer. Gap-dynamics was the dominant natural disturbance regime² unlike, for example, in western Siberia and central Canada, where crown fires are dominant¹¹.

The southernmost part of the boreal region is called hemiboreal or boreo-nemoral. In natural conditions, Norway spruce and Scots pine would be the dominants here, too (photo below), but southern broadleaf trees – pedunculate oak (Quercus robur), small-leaved lime (Tilia cordata), Norway maple (Acer platanoides), wych (Ulmus glabra) and European white elm (U. laevis), and European ash (Fraxinus excelsior) – would occur as individuals and form small groves in favourable sites¹⁵. Virgin forests lo longer exist in these most fertile sites.

The boreal forest becomes sparser to the north, trees get lower and their crowns narrower. The narrow crowns increase the efficiency of light interception at the low sun angles¹⁵ and reduce snow loads¹⁷. Norway spruce slowly intergrades to Siberian spruce. Wood decomposes slowly in northern Fennoscandia’s cold climate: a dead tree may stand 200 years; when it eventually falls the decomposition may take another 200 years if the trunk is raised above the ground supported by its own branches or stones⁵. Consequently, standing and fallen dead trees are abundant in the northern old-growth forests. Unlike in many other boreal regions, in Fennoscandia a broadleaf tree, downy birch (B. pubescens), extends northwards and upwards beyond the coniferous species in response to the oceanic climate¹⁸. At higher elevations and in northernmost regions downy birch is Arctic downy birch (var. pumila, photo below). This generally less than 10 m tall gnarly tree has arisen from the hybridization of var. pubescens and (sub-)shrub dwarf birch (Betula nana) ¹⁹. Features inherited from dwarf birch have made this tree-like birch suited for northern Fennoscandia’s harsh conditions²⁰. Other trees in these northernmost and uppermost forests are grey alder (Alnus incana subsp. kolaënsis), rowan, bird cherry (Prunus padus subsp. borealis), common aspen and goat willow. The field layer vegetation does not differ substantially from that of the neighbouring coniferous forest. There is no arctic forest limit in Fennoscandia: Arctic downy birch extends to the coast of the Arctic Ocean. Nor is the northern limit of the pine far from that. To the south, in the Swedish and Norwegian mountains at elevations of a few hundred metres, the forest resembles the lowland forest in the north (photos below). The Arctic downy birch woods have also been called subalpine in Sweden¹⁵.

Norway’s coastal forests are very oceanic and moist, having been called boreal rainforests; nevertheless their flora is largely the same as in the forests to the east, while the lichen flora includes rare species of wet forests²¹.

It is easy to walk in European boreal forests. The tree stands are generally rather sparse, undergrowth low and slopes gentle. In comparison, North American boreal forests are remarkably more difficult to penetrate: besides the mentioned abundant fallen trees, very shade-tolerant but short-lived balsam fir (Abies balsamea) ¹² forms dense sapling thickets even in older forests (photo below).

The silence of the northern wildernesses is incredible. If there is no wind, it is possible to hear the blood roaring in one’s veins – perhaps for the first time! Mosquitoes can be very annoying in the north in July; personally I favour early August when day temperatures are still relatively high but the first night frosts have already killed most mosquitoes.

Boreal forests are also called by the Russian word ”taiga”. Note that in Canada “taiga” is used to refer to the northernmost boreal forest zone, approximately equivalent to the European “north-boreal zone” ²².

KR

References:

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