1.6.1 Distribution in relation to climate: biomes
Living organisms have very distinct distributions; there are at least 400 000 species of flowering plants but not one occurs everywhere in the world. There are, however, communities of plants and animals found over often extensive regions of the world that have a similar and characteristic appearance (or physiognomy) as defined by their life form (see Section 3.1.1) and principal plant species. These major ecological communities are known as biomes (Fig. 1.12) and their distribution depends very greatly on regional climate, particularly temperature and rainfall. Similar regional climates produce similar morphological adaptations in plants. The term biome was first used by Clements and Shelford in 1939; an interesting team as Clements was a botanist and Shelford a zoologist. Whittaker (1975) lists six major terrestrial physiognomic types - forest, woodland (see Section 1.1.2 for definitions of these), shrubland, semi-desert scrub, desert and grassland. Trees dominate the first two of these types, and occur so widely around the world that more than one biome type is defined within the wooded areas of the world on the basis of climate. Species in separate biomes sometimes belong to the same genera (e.g. Nothofagus in Chilean and New Zealand rain forest) suggesting a former proximity of the continents.
Tropical rain forests, which are of especial interest because of their extreme species richness (including the greatest animal diversity of any terrestrial biome) and the fact that all other forest biomes are thought to have evolved from them, are described in Section 2.5. It is estimated that 50% of all Earth's species are found in the 7% of land covered by tropical forests, and predominantly in rain forests (see Woodward, 2003 for a more detailed description of terrestrial biomes). They are at their best in the warm, moist circumtropical lowlands where there is little seasonal variation in climate. These are the lowland or equatorial evergreen forests, sometimes referred to as aseasonal or perhumid rain forests (care is needed with perhumid since it is sometimes used
to mean very wet and sometimes aseasonal). The vegetation is dominated by a rich variety of multi-layered broad-leaved evergreen trees, with up to 100 or more tree species per hectare (2.47 acres), though some forests are dominated by one family of trees, such as the Dipterocarpaceae in Indo-Malayasian rain forests.
In areas where rainfall is more seasonal (sometimes referred to as the moist or humid tropics), two types of forest are found. Above 1000 m the forest changes to a more seasonal montane or cloud forest where much of the precipitation comes as mist or fog and consequently epiphytes (see Section 5.5.1) are abundant. In the lowlands, the evergreen trees are progressively replaced by deciduous species that drop their leaves during the dry (or drier) season, resulting in semi-evergreen rain forest and, where drier still, tropical deciduous forest, also called seasonal tropical rain forest or monsoon forest. Here, the forest structure becomes simpler with fewer epiphytes and lianas. As moisture declines further and becomes even more seasonal, especially where there is summer drought and cool moist winters, evergreens again come to dominate. Fire also becomes increasingly common and gives rise to Mediterranean ecosystems dominated by sclerophyllous trees and shrubs (evergreens with hard, thick, small, leathery leaves, resistant to hot, dry conditions). The biome is named for the largest area of this evergreen forest centred around the Mediterranean Sea where much of the former sclerophyllous chaparral woodland of holm oak (Quercus ilex) has been replaced by dense scrub (maquis) or more open heathy garrigue with many short aromatic shrubs. Similar Mediterranean-type vegetation is also found in the chaparral of California, the matorral of Chile, the fynbos of the Cape Province of South Africa (as distinct from the shrubby veld) and finally the bush vegetation of southern and southwestern Australia. All these areas have a similar appearance, dominated by hard-foliaged and twiggy shrubs and small trees. Yet the species they contain are quite distinct (that is, there is a large number of endemics, species found only in that area and nowhere else); for example, of the 113 woody genera (with 169 species) in Chile, only 13 genera are amongst the 109 genera (with 272 species) in California. In fact, the Mediterranean ecosystems contain about 20% of the world's plant species.
Where the climate remains warm but becomes drier still with several months of severe drought, the tree cover thins out into the tropical savannas. These cover 20% of the world's land surface and are the home of many large grazing and browsing animals, especially in East Africa with elephants, zebras, antelopes and hippos. The largest savannas are indeed in Africa, where they cover 65% of the continent (see Box 1.1), but are also found in South and Central America and northern Australia. Fire is one of the main forces that controls the distribution of savanna, so much so that in areas of recurrent fire, savanna may push into areas that would otherwise be rain forest. The difference in vegetation is quite distinct and abrupt as demonstrated by the Olokomeji Forest in Nigeria. This rain forest has a life-form spectrum in which over 90% of the species present are phanerophytes (growing points - perennating buds - more than 25 cm above ground - see Section 3.1.1). The trees and shrubs have very thin bark so that the cambium is very easily damaged by fire. In grassy savanna woodland only 5 km away, less than one-third of the species are phanerophytes and most plants either avoid fire by being annual (one-quarter are annual plants or therophytes) or hide the growing points at ground level (hemicryptophytes -one-fifth of the flora) or below it (geophytes). The percentage of geophytes is four times that of the forest, where it is already higher than average because the climate is dry. Bark of tree and shrub savanna species is usually more than a centimetre thick, giving protection from low-intensity fires, and many woody savanna species have enormous root systems which sucker readily when the shoots have been burnt to ground level. All these features of savanna plants show great adaptation to fire not found in the rain-forest plants.
The Miombo savanna, which lies to the south of the East African plains, the nyika thornbush, and the Congo forest is one of the most extensive African biomes. Measuring 2575 km (1600 miles) from east to west, and over 1287 km (800 miles) from north to south, it covers southern Tanzania, most of the southern Congo, Angola, Zambia, Zimbabwe and Malawi. It is largely found on the elevated inland plateaus of southern central Africa on deep, semi-podzolic sandy soils of little use to cultivation. Typically it rains for one half of the year while the other is dry and dusty. The miombo is sparsely populated, and is in a part of Africa where sleeping sickness, caused by the protozoan Trypanosoma, continues to spread despite all attempts to eliminate its vector, the tsetse fly. This savanna is effectively a woodland with a field layer dominated by grass, though in many places the trees are tall and may form contiguous forest. At the onset of the dry season tree leaves wither and fall, the grass dies back and fire often sweeps through the area, baring the ground and charring the lower trunks of the trees. Just before the rains come, the trees burst into leaf, those of Brachystegia and Julbernadia, the two most dominant genera, being a brilliant red. Julbernadia globiflora often grows with Brachystegia spiciformis, which is very common and often dominant. Ziziphus rhodesica and Gardenia spatulifolia commonly occur in Brachystegia forest; their saplings often commence growth on termite mounds.
Scattered through the miombo landscape are prominent bosses of rock known as inselbergs or kopjes. These dark, strongly weathered rocky outcrops have floral and faunal communities distinct from those of the surrounding savanna matrix. Two fig trees, Ficus burkei and F. capensis, occur here; the former is also found in woodland and the latter in riverine vegetation. Whilst the miombo savanna supports almost twice as many species of animals as that of Guinea, it comes a poor second to the East African plains, in part because of the poor quality of its grasses, most of which are unpalatable 'sourveld' in the dry season. However, on the fertile flood plains of the rivers the biomass of buffaloes, hippos and elephants must rival that of the river basins of East Africa.
Dry grass-poor mopane Colophospermum mopane woodland, a special type of miombo found on heavy clay soils, borders the flood plains of the Zambezi, Kafue, Luangwa and other rivers of this region. This forms a favourite feeding ground for elephants, herds of which work their way through it, pushing over big trees, and snapping boughs and branches offsmaller ones. This large-scale disturbance pattern plays a major role in the ecology of the grass-impoverished mopane, many of whose tree species readily produce substantial epicormic growth following stress or damage. Bushy growth on trees damaged by fire or elephants provides good protein-rich feed for animals that cannot reach the top of the big mopane trees. Impala and other herbivores rare in Brachystegia forests are frequently found here. Browsing of the black rhino, an animal particularly fond of the large fruits of the sausage tree Kigelia africana, is largely dependent on the activities of elephants. The very wide-lipped white rhinoceros is, in contrast, a grazer rather than a browser. Neither rhino is black or white: white was an imperfect translation of the Boer word for wide.
Despite its apparent dry and harsh nature, mopane supports a number of animals that are rare or absent in typical miombo. Impala and giraffe are the most obvious; others include greater and lesser kudu, bushbuck and duiker. The presence of these smaller herbivores is due to both the drier and more open character of the woodland and to the way the ecosystem has been changed, almost 'engineered', by elephants.
The dominant trees can be very different in different parts of the world. In parts of Mexico the savanna is dominated by a palm Paurotis wrightii, in Belize it is the Caribbean pine Pinus caribea, and in Africa acacia species.
At higher latitudes where the climate becomes cooler, especially in winter, and more seasonal but still moist, temperate broadleaf forests are found. In the northern hemisphere this seasonality has produced the mixed deciduous forests of eastern North America, western and central Europe (described in Section 2.4.1) and eastern Asia. These three areas were once part of a continuous forest belt and so share many closely related plants, dominated by a range of trees including beeches, oaks, limes, ashes, maples and a good number more. East Asian forests are the most diverse (suggesting they were the least affected by Quaternary cold periods), and those of Europe are the least rich (e.g. 729 species in 77 genera in temperate east Asia, 253 species in 90 genera in eastern North America, 68 species in 37 genera in western North America and 124 species in 43 genera in Europe - Rees et al, 2001). In the southern hemisphere and parts of Asia (Japan, South Korea, southern China) the same conditions have favoured broadleaved evergreen forests, typified by the eucalypt forests of eastern Australia and the evergreen Nothofagus forests of South America and New Zealand. However, pockets of deciduous forest are found in Chile and Argentina dominated by other southern beeches such as roble N. oblique and rauli N. procera.
At higher altitudes still in the northern hemisphere, where the climate is too unfavourable for deciduous trees, the boreal forest takes over across huge swathes of North America (Shugart et al., 1992) and Eurasia (Hytteborn et al., 2005), making up almost a third of the world's forest area. The southern boundary is defined as where the summer becomes too short (less than 120 days with daily average temperatures of >10 °C) and winters too long (> 6 months) and harsh for deciduous forest. The transition is fairly broad, giving rise to the transitional mixed or hemiboreal forests of places such as Prince Edward Island in Canada (Section 4.1.2), and the pre-settlement southern forest of north-east Minnesota, USA.
Boreal forests show a number of transitions at their borders; that with wetland leads to mire and bogs in the northern regions. Such transitions tend to be in a state of flux, with forests becoming mires at one point in time while bogs and mires become afforested at another. Stratigraphically mires are important; the pollen trapped within the peat has told us much regarding Holocene changes.
The circumpolar boreal forest is dominated by evergreen cone-bearing, needle-leaved trees. Four genera of conifers cover the bulk of both regions: evergreen fir Abies, spruce Picea and pine Pinus with some deciduous larch Larix. Some broadleaved deciduous species are of secondary importance, including alder Alnus and especially birch Betula and poplar Populus. The value of aspen P. tremula stands in helping to conserve the diversity of boreal members of a number of animal and plant groups in Scotland is discussed in Box 6.1. Siberian dwarf pine Pinus pumila covers large areas near the eastern arctic and alpine belts; its North American equivalent is whitebark pine P. albicaulis. Although reasonably rich in tree species, the boreal forest is dominated by relatively few ofthem: 14 in Fennoscandinavia and the former USSR, 15 in North America. Even across the disjunct continents formed by the breakup of Pangaea in the Jurassic Period, the same genera form repeated elements.
Site conditions within these forests range from the extremely cold, dry continental regimes of Siberia and interior Alaska, to the warmer, moist oceanic climates of eastern Canada and Fennoscandinavia. The trees found are closely adapted to the conditions of their particular sites; in terms of soil moisture content the range in North America runs from the dry-tolerant jack pine to the moisture-loving lowland black spruce and tamarack. Taiga (a Russian term) is sometimes taken as being equivalent to the whole of the boreal forest, but is best restricted to its northern edge. The border between taiga and tundra to the north (i.e. the tree line) is a wide zone (see Section 3.5.3), in which the trees gradually become less prominent (Sirois, 1992).
There is no southern hemisphere equivalent ofthe boreal forest owing to the northerly position of the southern continents and the limited geographical range of southern conifers, exemplified by the monkey puzzle (Araucaria araucana) native to comparatively small areas of southern Chile and western Argentina (see below).
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