Forests, Temperate. Cold-Temperate Zone: Broad-Leaved Deciduous Forests

The term temperate forest is applied to heterogeneous (consisting of dissimilar components) broad-leaved deciduous (leaf-shedding) and evergreen forests that are found within the temperate zones of the Earth between the two tropics and the polar circles.

On the basis of temperature variations and precipitation levels occurring within it, the temperate zone is divided into two hot-temperate zones in which the mean temperature exceeds 20° C for a few months and never falls below 5° C; two cold-temperate zones in which the temperature never exceeds 20° C at any time of the year and is less than 10° C for at least four months; and a cold zone in the Northern Hemisphere alone where the mean temperature rises above 10° C during only four months of the year.

Although there are some exceptions, the typology (classification based on types) of the temperate forests is correlated with the duration of the warm season. Where the mean temperature of 10° C lasts for more than five months there are broad-leaved deciduous forests; where it is shorter there are evergreen forests. The exceptions to this "rule" are coast redwood (Sequoia sempervirens) forests in the western United States and the beech forests of the European mountain plains.

Cold-Temperate Zone: Broad-Leaved Deciduous Forests. The broad-leaved deciduous forests comprise mostly trees with open leaves that fall in winter when temperatures can fall below 0° C. Leaf loss is a strategy for adapting to cold and suspends the trees' metabolic activity.

Geographically, in the Northern Hemisphere these forests extend from Europe, between the Atlantic Ocean and the Ural Mountains, the northeastern portion of the United States, and eastern Asia (Ussuri Basin, Manchuria, central-northern China, Japan, and Korea). In the Southern Hemisphere they are limited to two areas: The first straddles Chile and Argentina, and the second is Tierra del Fuego. In all these regions annual precipitation ranges from 500 to 1,500 millimeters, and winters are marked.

When compared with the vegetation in tropical environments, the broad-leaved deciduous forests present a simple "architectural structure" that is stratified on two levels: a dominant layer of trees and shrubs and a dominated layer of trees that is the result of "cultivation" by humans over the course of centuries. The human effect is less evident on the herbaceous (having the characteristics of an herb) and mossy layer and part of the shrub layer, which are good indicators of soil condition and vegetation potential.

In central and western Europe the broad-leaved deciduous forests are uniform from the physiognomical (term deriving from the discipline that tries to interpret the features of an individual from his external aspect) standpoint. They are characterized by a low number of species that create extensive, relatively open stands.

This floristic "poverty" is the consequence of the Quaternary glaciation (an event occurred in the Pleistocene period during the Neozoic or Quaternary geological era, between 0,01 and 2 millions years ago). Paleobotanic studies have shown that there was a greater number of species during the Tertiary period.

A distinction must be made between mixed broad-leaved and beech forests. The species in the mixed broad-leaved forests are Quercus robur (common oak) and Quercus petraea (European oak), associated with Fraxinus excelsior (common ash), Populus sp. pi. (poplars.), Betula sp. pi. (birch), Ulmus sp. pi. (elms), Alnus glutinosa (common alder), and Prunus avium (cherry).

These associations vary according to the type of soil and altitude. On the plains the dominant species are common oak and white beech, whereas ash and common alder forests prevail on the alluvial lands. The mountain zones are characterized by common beech forests (Fagus silvatica) that are either pure or associated with pines (Abies alba and Picea abies). The chestnut forests (Castanea sativa) are a case unto themselves: They are found on siliceous (relating to silica) or volcanic soils and are cultivated by humans for their fruit.

With respect to their European counterparts, the mixed forests of eastern North America are richer in tree species and comprise the genera Quercus (oak), Fagus (beech), Betula (birch), Juglans (walnut), Acer (maple), Tilia (lime), Ulmus (elm), Fraxinus (ash), Liriodendron (tulip-tree), Castanea (chestnut), and Carpinus (hornbeam) and are often mixed with conifers. Because they extend over a vast area that is not uniform in terms of climate, a distinction, on the basis of ground moisture, is made between mesophytic (growing under medium conditions of moisture) and xerophytic (growing under minimum conditions of moisture) forests.

The mesophytic forests consist of lime, maple, and birch forests in the northwestern United States and those associated with Canadian beech (Fagus grandiflora), sugar maple (Acer saccarum), and Canadian hemlock (Tsuga canadensis) in the northeastern United States. The xerophilic forests are dominated by oak (Quercus borealis, Quercus velutina, Quercus alba, Quercus macrocarpa) and hickory (Carya sp. pi.) trees in the western area.

The temperate forests of east Asia, with a humid, continental climate and mainly summer rains, are floristically comparable to those of America. They are broken down into three sectors: Inner Mongolia and northern Manchuria dominated by birch (Betula) associated with poplars (Populus) and willows (Salix) in the humid environments; northeastern Manchuria and southeastern Siberia with birch (Betula), oak (Quercus), alder (Alnus), lime (Tilia), elm (Ulmus), hornbeam (Carpinus), beech (Fagus), maple (Acer), ash (Fraxinus), chestnut (Castanea), and hazel (Corylus); and Japan, where there is a prevalence of beech (Fagus japonica, Fagus crenata), oak woods, and mixed forests that often include conifers.

In the Southern Hemisphere the broad-leaved deciduous forests of the genus Nothofagus (Southern beech) are often associated with cypress (Cupressaceae), araucaria (Araucariaceae), and podocarp (Podocarpaceae). In Chile they occupy two areas: one in the north on the Argentine border and one in the center, known as "Valdivian" after the city of Valdivia, that is rich in floristic terms. In this area the ocean's influence causes precipitation to range from 2,000 to 3,000 millimeters per year, and this permits the growth of epiphytes (plants that derive nutrients and moisture from the air and rain) and vines in a dense undergrowth.

Here various species of Nothofagus (Nothofagusobliqua and Nothofagus procera) are associated with conifers of the genera Fitzroya (fitzroya), Podocarpus_ (podocarp), Araucaria (monkey-puzzle), Libocedrus (incense cedar), and Magnoliacee (magnolie). In the southern part of this area the dominant broad-leaved species is ulmo (Eucryphia cordifolia), which is associated with conifers of podocarp (Podocarpus).