As may be expected, there is, in general terms, a correlation between the climatic conditions of a particular area and the type of forest vegetation occurring there. Consequently, there is a broad correlation between latitude and type of forest. However, a rise in altitude may to some extent compensate for a low latitudinal position, and forest types which may be expected to occur in more northerly regions are sometimes found in mountainous southerly areas. Further, climate is not determined by latitude alone, and edaphic and other factors may strongly influence the vegetation.
(i) Boreal forests 1
Evergreen coniferous forests which are adapted to areas in which there are very cold winters and relatively short growing seasons dominate the great northern belts of forest which stretch from east to west across Eurasia and North America, and extend almost to the Artic shore in relatively sheltered areas. These northern forests are usually regarded as a single type known as the boreal forest, and consist of two separate formations: one in Eurasia and the other in North America.
The species-composition, and floristic richness of this forest type vary considerably. Thus, in the Eurasian formation there are fewer species in the European section than in the Asiatic. From western Norway to the Urals Pinus sylvestris and Picea abies dominate. However, eastwards through European Russia the frequency of other species increases and species such as Abies sibirica, Larix sibirica and Picea obovata are found. Additional species occur further eastwards.
The North American boreal forest is similar to that of Asia in the relative richness of its flora. However, there is again a gradual transition between the western and eastern parts of the formation. In the east, the dominant species are Picea glauca, Abies balsamea, Picea mariana, Larix laricina and Pinus banksiana. In the west, the eastern dominants are less frequent, and are ultimately replaced by Pinus contorta var. murrayana, Abies lasiocarpa and other species.
Distinct associations are found in the boreal forests, and there are also frequent occurrences of pure stands (consociations).
(ii) Sub-alpine forests
Because air temperatures decrease with increasing altitudes, climatic conditions on the mountain ranges of the middle latitudes may be somewhat similar to those of the boreal forest belt. Thus, evergreen coniferous forests are to be found on mountains further south than the Tropic of Cancer in both the Old and New Worlds. Some of the species of these Sub-alpine zones are the same as those of the boreal forest. Generally, however, most species are quite distinct.
Pinus sylvestris, Picea abies, Larix decidua and Abies alba compose the sub-alpine forests of western and central Europe. Forests of pine and silver fir still remain between 1,500 and 2,100 m. on the Pyrenees; and throughout the Alps, spruce forest persists between 1,000 and 1,800 m. while larch and Pinus cembra form closed stands up to 2,100 m. in the central Alps. In the western Caucasus forests dominated by Picea orientalis and Abies nordmanniana are extensive between 1,500 and 2,000 m. Further east, in the Tien Shan, Picea schrenkiana forests dominate at elevations between 1,200 and 2,200 m.
The sub-alpine forests of Europe extend southwards to the Mediterranean and even as far as the Atlas Mountains in North Africa and the mountains of southern Turkey and Syria. In Algeria and Morocco, Pinus halepensis and Abies numidica may still be found between 750 and 1,200 m and above these, to a height of about 1,800 m are the famous cedar forests of Cedrus atlantica. Although the cedars of Lebanon, Cedrus libani, have almost entirely disappeared from the Lebanon Mountains, they are still plentiful in the remoter areas of the Taurus.
In North America, there is an almost continuous belt of sub-alpine forests which extends along the Sierras from Canada to southern California, and along the Rockies as far south as New Mexico. Picea engelmanni and Abies lasiocarpa occur throughout, except in California, and Pinus contorta is to be found everywhere from the mountains of the Yukon to the San Pedro Martir Range in California and to the Front Ranges in Colorado.
(iii) North American lowland and mountain coniferous forests
In the past, apart from the boreal forests, there were in North America vast areas of lowland coniferous forests belonging to different formations. Much of these forests have been exploited, but large areas still remain. However, during the last two decades of the 19th century, the so-called “lake forests” which extended from Minnesota and northern Pennsylvania to southern Ontario were rapidly exploited, and to-day the vegetation of the area is generally scrub.
There still remains extensive coniferous forests in western North America. These do not fully conform to the general pattern of coniferous forests occurring at high latitudes and high altitudes. They stretch southwards from southern Alaska down the coastal lowlands as far south as California. The forests are dominated by different species in different places and are regarded as belonging to two distinct formations, each composed of a number of associations.
In the coastal lowlands of Alaska and northern British Columbia, the forest is dominated by Picea sitchensis. Southwards, in southern British Columbia and Washington, Thuja plicata and Tsuga heterophylla gradually assume dominance. Eastwards from the Cascades, these species tend to disappear and are replaced by Pinus monticola, Larix occidentialis, Abies grandis, Pinus ponderosa and P. contorta.
One larger area of coniferous forests is found in the middle latitudes, over much of the coastal plain of south-eastern U.S.A. from New Jersey southwards to northern Florida and Alabama and then westwards to Texas. Pure stands of single species are of frequent occurrence, the most common being Pinus taeda, P. echinata, P. rigida, P. palustris and P. elliottii.
(iv) Ecotone mixed conifer and broad-leaved forests
With the exception of the anomaly in western North America which has been described above, the high latitudes and high altitude coniferous forests of the northern hemisphere give way, in humid regions, to broad-leaved deciduous communities. The changes in species content are often imperceptible, in some cases the zone of transition (or ecotone) in which the conifers are intermingled with broad-leaved species is very wide, and conifers and broad-leaves occur in almost equal quantities.
In European Russia, southwards from the boreal forests, pure stands of pine and spruce become fewer and are progressively invaded by Quercus robur. Many of these forests have now been clear-felled, as the soils were found suitable for mixed farming.
The mixed conifer and broad-leaved forests of European Russia are part of a forest belt which extends across Europe. The belt begins with outliers in the Highlands of Scotland, the Massif Central and on the slopes of the Cantabrian mountains in northern Spain. It then becomes more continuous, extending eastwards from the slopes of the northern Alps, central and north-east Germany and south-central Sweden.
The mixed forest ecotone between the boreal forest and the summer deciduous forest does not reappear to any considerable extent in Asia until the middle Amur valley is reached. This type of forest is found in much of northern and eastern Manchuria, northern Korea, southern Hokkaido and northern Kyushu. The flora is here far richer and more varied than in Europe.
In North America also, the lake forest and the boreal forest give way to the summer deciduous forest formation through a zone of mixed forest. However, much of these forests were cleared by the early settlers and the brown forest soils utilized for crop production.
(v) Evergreen mixed conifer and broad-leaved forests
In several lowland areas of the world, there are extensive mixed (conifer and broad-leaved) evergreen forests which appear to be true climatic climax formations and not ecotone. Such forests are widespread in the southern hemisphere in South America, Australasia and southern Africa, but there are relics in the Mediterranean.
It seems clear that much of the area now cultivated, or covered with maquis in the Mediterranean was once lush mixed conifer and broadleaved forest. To-day, what remains of the original vegetation are scattered clumps and isolated individuals of Quercus ilex (e.g. in the Dardanelles in Europe and westwards from Cyrenaica in Africa), Q. suber (around the western Mediterranean basin), Pinus pinea, P. pinaster and P. halepensis.
In South America, south of the scrublands of north-central Chile, forests are first encountered at about 30°S. They are confined to the middle slopes of the western Andes between 900 and 1,300 m; the dominant species in Araucaria araucana which is found in association with Nothofagus spp., and Quillaja saponaria. On the southern part of the Brazilian Highlands, where forests occur frequently, tropical vegetation gives way to a discontinuous mixture of evergreen broadleaves and conifers. The life-form is similar to that of Chile, but the forest is less luxuriant. This type of forest appears at about 23°S at a height of about 1,200 m. and gradually extends to lower elevation southwards. Araucaria angustifolia is dominant.
Forests of very similar structure are found in the North Island of New Zealand and in eastern Australia though they have been considerably reduced. In the northern-most part of New Zealand, Agathis australis is the important tree. Mixed stands of other species which were originally interspersed with Agathis australis survive in various areas in northern New Zealand as far south as the northern and north-western coastlands of the South Island.
In southern Africa, the so-called temperate forests are now represented by mere remnants. The narrow coastal belt between Knysna and Humansdorp in South Africa is now the only area where continuous belts of these forests are to be found. Patches remain, however, between 900 and 1,200 m. on the seaward slopes of the mountains to the east and to the west.
(vi) Deciduous summer forests
In the past, in the lowlands of west and central Europe, large areas of broad-leaved deciduous forests were to be found. These types of forests also extended around the coast-lands of the Baltic as far as the Gulf of Gdansk in the south and Scania in the north. They intruded as far north as the southern coastlands of Norway and eastern Scotland, and as far south as the Cantabrian mountains, the slopes of the Ebro Lowland and the middle slopes of the Appenines and Dinaric Alps. They continued eastwards, south of the mixed coniferous and evergreen forests, across the Danube basins, the lower slopes of the Carpathians and the Northern Ukraine.
Much of this deciduous forest has been razed to the ground. However, in the lowlands of the British Isles and France there are still some Quercus robur forests, with Fagus sylvatica and Fraxinus excelsior on the calcareous or on richer, well-drained soils. In southern Europe, there are forests of Quercus lusitanica, Q. cerris, Q. pubescens with Acer platanoides, Castanea sativa and Fraxinus ornus.
In America, the position is very much the same. Most of the deciduous summer forests have disappeared, but there are remnants of this formation which formerly formed a massive block extending from the Appalachians to beyond the Mississippi. Certain species of Quercus and Carya are still very frequent, however, while Tilia americana, Acer saccharum and Fagus grandiflora occur throughout the eastern part of the formation.
Around the Yellow Sea in western Korea, Shantung and the Kwantung Peninsula, extending into the lowlands of Hopeh, Manchuria and south-eastern USSR, the formation is also present. The dominant trees nearly all belong to the same genera as those of the corresponding forest type in Europe and North America.
In the northern hemisphere isolated stands of evergreen species such as Quercus virginiana and Magnolia grandiflora occur. They are found in Florida, on the coastal fringes of the deep south, and in north-east Mexico.
In the southern hemisphere, the essentially temperate formation type is found mainly in the South Island of New Zealand and in Australia. Along the southern part of the western coastlands of New Zealand's South Island, the windward slopes of the Southern Alps, and across the lowlands of southern Otago, the forests are dominated by Nothofagus spp. In Australia, the dominant genus is Eucalyptus.
Throughout most of tertiary times, the middle latitudes experienced much higher temperatures than they do now. They consequently supported plant communities which must have been similar to those now occupying tropical regions. Not surprisingly, therefore, many of the genera and species which are found in the temperate latitudes belong to families which are predominantly tropical.
A series of forest types, mainly adapted to high temperatures and various humidity regimes, is to be found on all the main continents which extend into the tropics. These are described below:
(vii) Tropical rain forests
Wherever rainfall is heavy and reliable in the tropics, a forest type of remarkably constant structure occurs under undisturbed conditions. This tropical rain forest type comprises three formations: the American, African and Indo-Malaysian tropical rain forests.
The American formation completely occupies the Amazon Basin and extends along river valleys southwards on to the Mato Grosso, south-westwards along the sub-Andean region of central Bolivia and within Argentina, north-westwards along the foothills of the Andes in north-central Colombia and south-west Venezuela, and north-eastwards to occupy almost all of the Guyanas and eastern Venezuela.
Although large areas of the African formation are still to be found in the Congo Basin and in Cameroon, much of this formation which extended in the past throughout southern West Africa has been cleared. So too has the tropical rain forest of eastern Madagascar, Mauritius and the main river valleys of south-eastern Tanzania.
The Indo-Malaysia formation is the most widely distributed of the tropical rain forests. It is found outside the Tropic of Cancer in Assam and outside the Tropic of Capricorn in southeastern Queensland; it occurs as far west as the Western Ghats of India, and as far east as Fiji. It also occurs in Malaya, Sumatra, Borneo and New Guinea.
The tropical rain forest is a closed, tall, evergreen type with trees which sometimes reach 60–70 m. Many of the trees are buttressed, and there are numerous woody epiphytes. The flora is extremely rich. The total number of identified species of larger trees in the Indo-Malaysian rain forest has been estimated to be as great as 3,000. The American tropical forests contain about 2,000 species of large trees. The least rich of the tropical rain forest floras is the African. But this, by temperate standards has a prodigious amount of species: just under 1,000 woody species. One significant factor about these tropical forests is that very few species, and not very many of the genera and families, are represented in all regions. As a consequence, the total tropical flora is even more complex than that found in individual continents.
(viii) Semi-evergreen seasonal forests
Tropical rain forests yield to semi-evergreen seasonal forests as the drier seasons become longer and more pronounced. The semi-evergreen forests are generally found where there is a dry season of about five months, with a mean monthly dry-season rainfall of less than 10 cm. but more than 3 cm. In this forest type, the dominants include deciduous species, but evergreens predominate. Forests of this type occur throughout the tropical world.
In the Americas, they are widespread throughout the West Indies and South America. They have been described in Trinidad, Venezuela, the interior of Guyana, and the north-eastern interior of Colombia. They form a wide belt around the southern edge of the Amazon Basin, and also run parallel to the coast of south-eastern Brazil from Recife to the southern end of the Brazilian Plateau.
In Indo-Malaysia, these forests flank the tropical rain forests in many places to the north and south. They are to be found in north-eastern India, Burma, Thailand and Indo-China, and from eastern Java to northern Australia.
In Africa, the formation is not as extensive. Perhaps the nearest approach to the types found in South America and Asia are the dry evergreen forests of West Africa which almost imperceptibly merge into the tropical rain forests.
In general, the semi-evergreen forests are poorer in floristic composition than the tropical rain forests, and the structure is less complex.
(ix) Deciduous seasonal forests
As the dry season becomes even more severe than in the semi-evergreen regions, deciduous seasonal forests occur. These plant communities are better adapted to withstand periods of drought, and although evergreens largely from the sub-dominant and lower storeys, the dominant trees are deciduous. The forests are poor in lianes and there are very few epiphytes.
These forests are found locally in Venezuela, in Guyana, in the western parts of Central America from Panama to Guatemala, and in many rain-shadow areas in the West Indies.
Much of the deciduous seasonal forests have been removed from the Asiatic-Australasian zones. However, extensive areas still remain in central Burma, Thailand and Indo-China. A similar type is also found in northern Australia.
Some stands of deciduous trees still exist over vast tracts of tropical Africa. However, there are perhaps not true seasonal deciduous forests of the types described in America, Asia and Australia. Woodland sometimes described as “dry forest”, “miombo forest” or tree steppes extends from Angola across Zambia and Malawi into western and central Tanzania. Most of Katanga in the Congo and parts of the north of Rhodesia are included in this belt.
In 1965 the total area of man-made forests for which figures were reported to the FAO World Symposium on Man-made Forests (Australia and New Zealand, April 1967) amounted to about 34 million hectares. Adding to these figures those areas believed to be in existence, but for which no figures were actually reported, the total area of man-made forests was reckoned to be in 1965 about 80 million hectares. This area was expected to be increased by about 5 million hectares a year over the following 20 years (about 2 million hectares in the reporting countries). Thus the total area of man-made forests should have amounted to about 100 million hectares in 1970, of which probably roughly half in China (Mainland) and the USSR (other major individual countries in this respect are the USA, Japan, Korea, Spain, United Kingdom, France, India, South Africa, Indonesia, Italy, Poland, Brazil, China (Taiwan), New Zealand, Burma, Chile).
As for the species planted, figures are available only for those man-made forests for which data have been reported. The picture of species composition may thus be far from representative of the true world picture if all areas could be analysed. The most widely planted group of species, though, appears to be that of the conifers: together these comprise about 70 per cent of the reported plantations. The greatest proportions are in the temperate regions of Europe, North America and Australia. In the warmer zones of the tropics and sub-tropics the predominant species are broadleaved, but in Africa the conifers nearly match them.
With regard to species, the most important conifers are as follows:
In the colder zones: Pinus silvestris, P. koraiensis, P. tabulaeformis, P. massoniana, Picea sitchensis, Pseudotsuga menziesii, Larix spp. In the warmer zones: Pinus radiata, P. patula, P. pinaster, P. elliottii, P. merkusii, Auraucaria spp. The great preponderance of pines is evident.
Of the broadleaved genera and species the principal are the eucalypts (mainly E. globulus, E. camaldulensis, E. saligna/grandis), poplars (mainly P. nigra, P. deltoides and their hybrids), wattle (Acacia mearnsii) and teak (Tectona grandis). The eucalypts are probably the most extensive.
