CHAPTER 1

AN OVERVIEW OF TEMPERATE BROADLEAF FORESTS

THE TEMPERATE ZONE

The Earth’s surface is subdivided into five broad latitudinal bands: the Arctic and Antarctic polar zones, the tropical zone and the north and south temperate zones. The temperate zones comprise those regions of the Earth’s surface that are located between the Tropic of Cancer and the Arctic Circle in the Northern Hemisphere and between the Tropic of Capricorn and the Antarctic Circle in the Southern Hemisphere. This is the area between north and south latitudes 23.50° and 66.32° (Lewis, 1977 - Figure 1-A).

The latitudinal bands described in the preceding paragraph coincide roughly with three major thermally defined zones. The “winterless” thermal zone is characterized by having a climate in which no month of the year has a mean monthly temperature lower than 18°C and coincides roughly with the tropical zone. The two polar thermal zones are characterized by having a summerless climate where no month of the year has a mean temperature higher than 10°C. The 18°C isotherm coincides more or less with the distributional limits of plants characteristic of the tropics whereas the 10°C isotherm coincides roughly with the northern limit of tree growth in the Northern Hemisphere and the southern limit of tree growth in the Southern Hemisphere. The mid-latitude regions of the Earth’s surface, those that generally lie between the 10°C and 18°C isotherms, are characterized by having distinct summer and winter seasons, and are designated as the temperate zones [Bailey, 1996]. The 10°C and 18°C isotherms do not coincide perfectly with the Arctic and Antarctic Circles or the Tropics of Cancer and Capricorn. They are influenced by the oceans and continents and by changes in elevation of the Earth’s continents [Bailey 1996]. Therefore, it is possible for areas south of the Tropic of Cancer to have a temperate climate and, conversely, for areas north of the Tropic of Cancer to have a tropical climate (Figure 1.1B).

TEMPERATE FORESTS

Temperate forests are diverse ecosystems composed of mixtures of conifers, broad-leaved evergreen and broad-leaved deciduous trees. Latitude, temperature, moisture and elevation define the distribution of various temperate forests and trees. A large number of families and genera of trees and shrubs are represented in these forests (Table 1.1). Within the temperate zones in the Northern and Southern Hemispheres, there are a number of distinct plant communities or biomes whose distributions are further defined by temperature and moisture. Many attempts have been made to classify the forest regions of the world [Bailey, 1996]. According to data obtained by the FAO Forest Resources Assessment 2000, “temperate” (broadleaf) forests are composed of temperate mountain forests (120 million ha), temperate continental forests (260 million ha) and temperate oceanic forests (30 million ha) (FAO, 2001). For the purpose of this paper, a broad classification of temperate forests into four forest biomes presented by Hora (1981) is used.

Figure 1.1: Comparison of the Earth’s climatic zones as defined by latitude (A) and temperature (B) (adopted from Bailey, 1996): Sclerophyllous-, Warm temperate evergreen-, Temperate deciduous -, and Boreal forests. (a)

Figure 1.1: Comparison of the Earth’s climatic zones as defined by latitude (A) and temperature (B) (adopted from Bailey, 1996): Sclerophyllous-, Warm temperate evergreen-, Temperate deciduous -, and Boreal forests. (b)

Table 1.1 Principal families and genera of temperate broad-leaved trees

Source: Harlow and Harrar,1950; Hora, 1981.

SCLEROPHYLLOUS FORESTS

Sclerophyllous or Mediterranean forests occur in areas where the climate is characterized by warm, wet winters and hot, dry summers. They are located along the western coastal regions of the continents. Annual precipitation ranges between 500 mm and 1000 mm, with an irregular pattern and prolonged periods of low humidity. Much of the vegetation of sclerophyllous plant communities has hard leather-like leaves, an adaptation to periods of prolonged dry weather. These forest communities are known by several names: maquis in Mediterranean Europe, chaparral in California, matarral in Chile, malee scrub in Australia and fynbosch or karroo in southwestern Africa. Heavy cutting of the forest overstorey results in domination of formerly forested sites with dense, evergreen woody shrub vegetation. The vegetation of sclerophyllous forests tends to be rich in species composition but subject to periodic fires and degrading by heavy grazing [Hora, 1981].

The Mediterranean basin of Europe and North Africa is undoubtedly the world’s best-known example of sclerophyllous forest. Prior to the development of a heavy human population that severely affected the vegetation of this region, a canopy of holm oak (Quercus ilex) and other evergreen broad-leaved trees (Figure 1.2) dominated the original forest cover. Other areas of sclerophyllous or Mediterranean vegetation in the temperate zones include southern California (United States), portions of Chile, the Cape region of South Africa and the southwestern tip of Australia [Hora,1981].

Figure 1.2 A Quercus ilex dominated sclerophyllous forest on the island of Sardinia, Italy.

WARM TEMPERATE EVERGREEN FORESTS

Warm temperate forests cover about 100 million ha of land area worldwide and are among the most productive of the world’s forests. These forests are found where the mean temperature is between 0^oC and 18^oC for the coldest month of the year [Dansereau, 1957].

One form of warm temperate forest is an extension of sclerophyll forest into areas where there is no summer drought. A classic example is the Pacific Coast of North America, north of 36°N latitude, which remains moist during the summer due to summer fogs. This forest is dominated by giant conifers including redwood (Sequoia sempervirens), Douglas fir (Pseudotsuga menziesii), hemlock (Tsuga heterophylla) and western red cedar (Thuja plicata). Broad-leaved trees (both evergreen and deciduous) that occur in this forest include big-leaf maple (Acer macrophyllum), vine maple (A. circinatum), golden chinquapin (Chrysolepis chrysophylla), cascara (Rhamnus purshiana) and Oregon myrtle (Umbellularia californica). Other examples of warm temperate evergreen forests that extend from sclerophyllous forests include the Nothofagus dominated forests of Chile (Figure 1.3) and the Eucalyptus diversicolor forests of western Australia. In the Mediterranean region, a warm temperate evergreen forest occurs around the Black Sea east as far as the Caspian Sea and includes the species rich Tertiary-relict Colic forest of Transcaucasia. This forest lies in a region where summers are mild and wet enough for tea cultivation that has replaced much of the natural forest [Hora, 1981].

Figure 1.3 The warm temperate, Nothofagus dominated forests of southern Chile contains both deciduous and evergreen species. In this scene, taken in Parque Nacional Villarica, the evergreen tree is N. dombeyi and the deciduous tree is N. antarctica.

Many of the world’s warm temperate forests are found along the eastern coastal regions of the continents that are exposed to monsoons or trade winds. Rainfall is abundant (150-300 mm/a) and is well distributed throughout the year. In Southeast Asia, eastern Australia and southern Brazil, there is a continuous gradation with increasing latitude from wet tropical to subtropical to warm temperate conditions. It is, therefore, difficult to distinguish vegetation zones in these forests. Characteristically, penetration of these forests is difficult due to the abundance of vegetation; they are rich in tree species, including some conifers, epiphytes and climbers although less so than tropical forests. Some broad-leaved trees are deciduous. In eastern Australia, it has been shown that tropical forest types extend farthest south on the better soils and more humid sites. Australian moist temperate forests, dominated by Nothofagus, occur in Tasmania and Victoria. In Africa, only the Drakensburg mountains of South Africa have suitably moist conditions to support warm temperate forests. The boundaries of the moist temperate forests of eastern North America are also poorly defined because cold air masses move south as far as the Gulf of Mexico. They are generally found along the Atlantic and Gulf of Mexico coastal regions from east Texas and Louisiana north to North Carolina. The tree flora of the North American warm temperate forests is rich and includes both evergreen and deciduous species of oaks (Quercus spp.), Liquidambar stryaciflua), as well as conifers such as pine (Pinus spp.) and bald cypress (Taxodium distichum). Another region where warm temperate forests are predominant is New Zealand; they include various species of Nothofagus and kauri pine (Agathis australis), mixed with subtropical broad-leaved species [Hora, 1981].

Islands of warm temperate forests are also found at high elevations in the tropics. For example, the high elevation cloud forests found in Central America and extending as far south as Colombia in the northern tip of South America contain species of Quercus (author’s observation; Ramierez Correa, 1988; Figure 1.4).

TEMPERATE DECIDUOUS FORESTS

The temperate deciduous forests are undoubtedly the best known of the various types of temperate zone forests. They occupy a total area of approximately 800 million ha worldwide and formerly covered most of western Europe and portions of the Near East and Asia. They are still extensive across eastern North America. Temperate deciduous forests are restricted to the Northern Hemisphere apart from a small area at the southern tip of South America and are found where the average temperature is below 0°C for the coldest month of the year but with an average temperature above 10°C for the warmest month [Dansereau, 1957].

Figure 1.4 - High elevation Central American cloud forests, such as this one near the summit of the Volcan Mombasha in western Nicaragua, contain several species of Quercus.

A characteristic of deciduous temperate forests is leaf fall in autumn, which is an adaptation to the marked but not very prolonged cold season when liquid water is either restricted or unavailable to the plant. Leaf fall is preceded by an often spectacular autumn coloration of foliage, a characteristic that makes many trees of these forests desirable ornamental and garden plants (Figure 1.5). Annual rainfall ranges between 70 cm and 150 cm and is more or less evenly distributed. Evergreen broad-leaved trees do not tolerate winter drought or prolonged cold and are generally absent from these forests.

Figure 1.5 - Spectacular coloration of deciduous foliage prior to leaf fall in autumn is a characteristic of trees in many temperate forests; upper left, Acer circinatum, Oregon; upper right, Populus tremuloides, Colorado; lower, mixed Quercus forest, Umbria Region, (Italy).

The temperate deciduous forests of western Europe have been reduced to fragments of the original forest due to agriculture, grazing and other human activities. These forests are poorer floristically than other temperate deciduous forests because of the extinction of many species during the Pleistocene ice ages. Pure forests of beeches (Fagus sylvatica) (Figure 1.6) dominate the higher elevation forests while Quercus spp., Tilia spp. and Fraxinus are dominant components of lower elevation forests. In Asia, North America and the Near East, there are many more species of trees including representatives of the genera Acer, Aesculus, Carya, Liriodendron, Liquidambar, Magnolia and Juglans, as well as outliers of some tropical families (e.g. Diospyros) [Hora,1981].

For example, in the temperate deciduous forests of the southern Appalachian Mountains of the eastern United States, there are approximately 140 different species of trees, of which about 60 are commercially important (Figure 1.7; Westveld, 1949).

BOREAL FORESTS

The boreal forest region encircles the globe at the northern limits of forests and covers vast areas of North America and Eurasia. The winters are colder and longer than in the temperate deciduous forest zone and much of the region is dominated by extensive conifer forests composed of species of Abies, Larix, Picea and Pinus or extensive areas of bog or peatlands [Hora, 1981]. Broad-leaved trees are poorly represented in boreal forests although those species that do occur often cover large areas. Families and genera of broad-leaved trees that are found in boreal forests include the Salicaceae (Salix, Populus), the Betulaceae (Alnus, Betula) and Rosaceae (Sorbus) [Vasilevich, 1996]. In the northern limits of the boreal forest, trees are typically reduced to low, shrubby krumholz due to the short growing season and severe weather.

Figure 1.6 - Pure forest of European beech in the Apennine Mountains, Tuscany Region (Italy).

Forests composed of typically boreal species extend south into the higher elevations of the Appalachian, Rockies and Cascade mountain ranges in North America and in the Alps, Carpathian and Pyrennees ranges of Europe.

Figure 1.7 - A mixed deciduous temperate forest in the southern Appalachian Mountains of the eastern United States. Forests such as these may contain as many as 140 species of broad-leaved trees (Pisgah National Forest, North Carolina, United States).