Convention on biological diversity

Temperate forests

Distribution and structure

In Europe, temperate forests extend over some 160 million ha, which represents slightly less than half of the original forest cover. In western Europe (Matthews, 1983), it is estimated that the extent of remaining old growth and semi-natural forest is only 0.8% of the original forest cover (Ibero, 1994). Eastern Europe has more old growth forest than in the west (Ryzkowski et al., 1999). In the United States, less than 2% of the original temperate forests remain, although proportions vary regionally. For example, the states of Washington and Oregon have 13% old growth temperate forests remaining. In British Columbia, Canada, almost 40% of the original natural forests remain, although some of these are subject to intensive forest management (Canadian Council of Forest Ministers, 2000). New Zealand retains less than 24% of its of its native forests (Clout and Gaze, 1984) and in Australia, the amount of the original temperate forest varies from 5-20%. In some temperate areas of developing countries, there is a net loss of forest cover; Chile, for example, loses about 20,000 ha/year [FAO, 2001a/2001b].

The annual productivity of natural northern temperate forests is about 900 to 1000 g/m² and up to 1000 to-1400 g/m² in old southern temperate forests of North America (Lieth and Whitaker, 1975). However, there is obviously a large variation associated with these figures depending on site, elevation, type and age of forest.

Mediterranean forests constitute a distinct sub-zone of the temperate biome and occur between 30 and 40 degrees latitude on the west and south-west coasts of the continents. Their climate is characterized by hot, dry summer and mild, moist winters. The Mediterranean sub-zone in the Americas occupies coastal California in the United States and the coastal region of Chile. In Africa, these forests extend around the Cape of Good Hope; they also occur in the southern part of Australia. However, the largest Mediterranean sub-zone is located around the Mediterranean Sea and includes the southern part of Europe, the south-west part of Asia and the north coast of Africa. In Europe, the Mediterranean sub-zone has been the cradle of several civilisations, one replacing another over centuries, and this has resulted in a long history of extensive environmental change as a result of economic, cultural and social activities. The area surrounding the Mediterranean Sea was originally covered with forests of Cedrus libani, Quercus ilex, Quercus cerris, Arbutus unedo, Pinus halepensis and, Pinus nigra, but the Mediterranean hillsides were transformed hundreds of years ago into terraces of fruit orchards, gardens, olive tree and fig tree plantations, as well as human settlements. Areas that have escaped cultivation are covered with shrubs and bushes, resulting in Maccia (maquis), a woody secondary vegetation cover (Ovington, 1983).

Species diversity⁶

East Asia’s forests are very rich in woody plant species, with almost 900 trees and shrubs. That is almost six times greater than in North America, where the second most diverse temperate forests occur. The temperate forests of Europe are more impoverished, with just 106 tree species and significantly fewer families and genera than in North America. The southern hemisphere generally has even fewer species than Europe (except for Australia with its high diversity of Eucalyptus and Acacia species), but there is a high endemism with most species belonging to different families from those found in the northern hemisphere, suggesting major differences in evolutionary history. Transition zones between tropical and temperate forest biomes, are comparatively species rich. These occur, for example in Japan and the southern United States where temperate lowland forests merge with subtropical evergreen broad-leaf forests. In southern Canada, the maximum tree species richness in temperate forests is approximately 60 species, but by mid-latitudes in the eastern United States, the same biome contains over 100 tree species, illustrating the general latitudinal relationship of species diversity, i.e. diversity increasing towards the equator (Stevens, 1989).

Temperate forests tend to support their largest variety of species on nutrient-rich soils, and species richness also seems to be greater on alkaline and neutral soils than on acid soils (SCOPE, 1996). Local species richness in many of these forests is highly variable, ranging from monocultures to multi-species forests. In many areas of the temperate biome, large stands of deciduous forests may be composed of a single tree species. For instance, Fagus sylvatica dominates deciduous forests in Europe, F. orientalis forms nearly pure stands in the montane region of the Caucasus, and F. crenata is predominant in pure stands in the wetter regions of Japan. In Europe, on calcareous soils with high watertables, Quercus and Carpinus become dominant rather than Fagus. In North America, Fagus rarely dominates forests, but pure stands of Betula and Populus are common, as is the case in Siberia and northern Japan. Nothofagus occurs in monocultures in New Zealand and South America. Quercus and Pinus are global species found in most northern hemisphere temperate forests. In Australia, forests are dominated by the extremely diverse genus Eucalyptus with more than 70 species in 16 forest types (Ovington and Pryor, 1983) whereas Quercus is absent. Although alpha-diversity (patch-scale or within-site diversity) may be low, beta-diversity (regional or among-site diversity) in the temperate biome forests can be quite high.

In North America, an important temperate coniferous forest belt occurs along most of the west coast from Alaska southwards to northern California. The forests lie on the windward side of the coastal mountain chain, which runs the length of the continent. These forests, collectively referred to as temperate rainforests, exhibit a high level of biological diversity with a large number of endemic plants and animals (Ruggiero et al., 1991). They are characterized by several long-lived tree species (>1000 years) and contain the tallest trees in the world (to 95 m), including: Sequoia sempervirens, Sequoia gigantea, Pseudotsuga menziesii, Picea sitchensis, Tsuga heterophylla, Thuja plicata and Chamaecyparis nootkatensis (Maser, 1990). These trees are particularly valuable to the forest industry. Trees of this size and ecosystems as complex as these occur nowhere else in North America (Maser, 1990). The management of the temperate rainforests forests has generated more controversy than that of any of the other North American forest types because of their species diversity, complex functioning and the particularly majestic characteristics of the old-growth trees, which can exist for many centuries in a gap-phase dynamic condition.

As with boreal forests, the fauna of temperate forests, especially the birds and mammals, can have a wide distribution and even extend to other biomes. For example, Neotropical migrant birds of North America, numbering about 250 species, make the annual trip from the tropics to the temperate regions, and changes in the extent and condition of either forest biome can affect the populations of these birds in both continents. Survival of these birds is important because smaller numbers may allow defoliating insects to reach epidemic proportions more frequently and this further endangers the survival of some species (UNEP, 1995). Not all temperate forests host fauna with such a wide distribution. In the forests of southern South America, South-East Asia, Australia and New Zealand, there are many endemic species of mammals and birds that are highly localized (WCMC, 2000).

More animal species have become extinct in the past 1000 years, or have had their range and population substantially reduced, in the temperate forest biome than in the other biomes (Hilton-Taylor, 2000). Falling particularly into this category are the large ungulates including extinct aurochs (Bos taurus) and tarpan (Equus gmelini silvaticus), endangered bison (Bison bonasus) and declining fallow-deer (Cervus dama) and moufflon (Ovis musimon) in eastern Europe. The general reduction of forest cover, combined with hunting and/or trapping, has caused the reductions of many large carnivores such as the brown bear (Ursus arctos), lynx (Felis spp.), cougar (Puma spp.), glutton or wolverine (Gulo gulo) and wolf (Canis spp.) (Hilton-Taylor, 2000; Pimm et al., 1995). Within the past 200 years in North America, the passenger pigeon (Ectopistes migratorius), Carolina parakeet (Cornuropsis carolinensis), ivory-billed woodpecker (Campephilus principalis), Bachman’s warbler (Vermivora bachmanii) and the eastern cougar (Puma concolor) have become extinct (Pimm et al., 1995).

Plantations in the temperate biome

Plantations are often locally and regionally important in influencing floral and faunal richness (Peterken et al., 1992). The recent dramatic increase in the area occupied by forest plantations will have an increasingly important long-term effect on biodiversity. In many areas of the temperate biome, human activities have considerably influenced forest stands and landscape structure. This is especially true for countries of Europe and Asia, with their long history of habitation, where human use of forests has reduced diversity and many stands may now have fewer than 10 species of trees and shrubs. Conversion of broad leaf forests into coniferous plantations began in Europe more 120 years ago and is still common today. For example, the forest cover of Germany has changed from what was formerly more than 90% broad leaf forest (mostly Fagus sylvatica), into about 80% coniferous plantations (largely Picea abies). In a similar manner, much mixed deciduous forest in Japan has been converted into Cryptomeria japonica plantations in the south of the country and into Abies and Picea plantations in the north. Natural Nothofagus forests in Chile have been replaced by plantations of Pinus radiat, and the practice continues today (Gajardo, 1994). [Possible addition: In New Zealand planted forests of Pinus radiata began to be established during the first decades of the 20^th Century. Production from these planted forests has largely replaced that from the podocarp, hardwood and beech (Nothofagus) natural forests enabling conservation of these natural forests ] In North America, mixed forests were often converted into Pinus taeda plantations in the south, and P. banksiana and/or P. resinosa in southern Canada and the northeastern USA. In Europe, large areas of the temperate native forests have been replaced by plantations of introduced tree species, such as Pseudotsuga menziesii, Picea sitchensis and Pinus contorta. Many new plantations in European countries are established by planting species from non-local provenances to enhance wood productivity (Ryzkowski et al., 1999).

The potential for forest plantations to partially meet demand from natural forests for wood and fibre for industrial uses is increasing. Although accounting for only 5% of global forest cover, forest plantations were estimated in the year 2000 to supply about 35% of global roundwood, with an anticipated increase to 44% by 2020. In some countries, forest plantation production already provides the majority of industrial wood supply.