Cold-Temperate Zones: Evergreen Forests

There are large regions of the Earth with a temperate climate that host evergreen forests. In Europe, North America, and Asia these are cold-temperate conifer forests, and in the northwestern United States, more temperate forests (sequoias).

These forests are mainly characterized by conifers with small, narrow, needle- or scale-shaped leaves that can reduce transpiration (passage of watery vapor from a living body through a membrane or pores) to low levels that last through the winter and allow the tree to photosynthesize when the climate permits.

The exceptions are larch that lose their leaves during autumn, and they extend to the farthest latitudes where no other species can survive. From the physiognomical standpoint, the conifer forests do not undergo obvious changes during the course of the year unless a consistent percentage of open-leaved shrub and tree species is associated with them.

The northern forests and taiga (a moist subarctic forest dominated by conifers that begins where the tundra ends) occupy the northern areas of Canada and Eurasia (Scandinavia, Russia, and eastern Siberia) between fifty-five and seventy degrees latitude. Here, in January, temperatures can fall to —70° C, and the ground is covered with frost for a long time.

The summers are short but relatively warm: During the hottest month temperatures range from 10° to 20° C, and the vegetative period varies from five to seven months. Precipitation, between 250 and 750 millimeters per year, occurs mainly in summer, but the climate can, in fact, be considered constantly humid because there is no real hot season. The North American forests are colonized by conifers of the genera Picea (spruce), Picea glauca (white spruce), Picea mariana (black spruce), Abies (fir), Abies balsamea (balm of gilead fir), Larix (larch), Larix laricina (tamarack), Pinus (pine), Pinus banksiana (jack pine), and Pinus murrayana (lodgepole pine) and farther south by Pinus strobus (weymouth pine) and Pinus resinosa (red pine), and Tsuga (hemlock), Tsuga Canadensis (eastern hemlock-spruce).

This last species is not found in Europe where Picea abies (Norway spruce) alternates with Pinus sylvestris (Scotch pine). The pines create pioneer forests on poor soil that can last for a few centuries before they are colonized by common spruce, which forms dense forests on mature soil. Farther east the common spruce is replaced by Siberian spruce (Picea obovata), Siberian larch (Larix sibirica), Siberian fir (Abies sibirica), and Siberian stone pine (Pinus sibirica). In the north where the cover is thinner, there are more lichens (Cladonia and Vetraria). The conifers are associated with broadleaved trees of the genera Populus (poplar), Aims (alder), Sorbus (whitebeam), and Betula (birch).

The undergrowth is uniform because of the permanent forest cover and the slow decomposition of the litter and consists of species of the genera Vaccinium (bilberry), Sedum (stonecrop), Kalmia (sheep-laurel), and Empetrum nigrum (crowberry). Although the trees grow slowly and present narrow annual rings, their longevity is high.

In western North America evergreen conifer forests are associated with broad-leaved forests. Going north along the Pacific Coast one first encounters forests of Douglas fir (Pseudotsgua menziesii), western red cedar (Thuja plicata), and western hemlock (Tsuga heterophilla), mixed with large maples and firs; then coast redwood (Sequoia sempervirens), in areas with high levels of winter rainfall and summer fog. From the thermal standpoint this forest is hot-temperate, and on the south it borders on laurifilli oak woods that mark the boundary with Mediterranean chaparral.

In North America, around the Great Lakes where precipitation ranges from 600 to 1,150 millimeters per year, there is a forest dominated by Weymouth pine (Pinus strobus), red pine (Pinus resinosa), and eastern hemlock-spruce (Tsuga Canadensis) associated with numerous broad-leaved species, including big-toothed aspen (Populus grandidentata), eastern poplar (Populus deltoides), cherry birch (Betula lenta), and old field birch (Betula populifolia).

This type of mixed forest is also found in the Far East north of Hokkaido, Japan, where large conifers Manchurian fir (Abies holophylla) and Korean pine (Pinus koraiensis) are associated with species from the genera Betula (birch), Ulmus (elm), Tilia (lime), and Juglans (walnut).