Rain forests climate soils and variation

2.5.1 Tropical rain forests: the changing archetype

Corner (1964) considered tropical rain forest to be the cradle of flowering plant (angiosperm) evolution and that all other forest types were derived from it - it is the archetype, the earliest common ancestor. Early trees such as those of the Coal Measure forests (see Section 9.1 and Fig. 1.1), as well as the first seed plants and the first flowering plants, do indeed appear to have evolved under conditions of high temperature and constant humidity. Aseasonal tropical rain forests (see Section 1.6.1 for definitions) occur naturally in equatorial regions receiving ample rainfall, averages ranging from 1700 mm to over 10 000 mm annually. Temperatures range between 21 and 34 °C in tropical rain forests, and the daily (diurnal) temperature range is greater than the annual range, in marked contrast to boreal forests. Shelter, freedom from fire, suitable soils and appropriate seed sources are also essential if rain forests are to establish. As with all vegetation everywhere, tropical rain forest has evolved, moved and established in relation to major climatic changes. Both its species composition and its very location on the surface of the Earth have changed: it is far from being a stable archaic relic.

It is impossible to describe the general structure of tropical rain forests without mentioning the work of P. W. Richards (see Willis, 1996), who developed the technique of taking a strip of trees and recording their position, height, diameter and outline drawn to scale to give a profile diagram. Such forests often have three or more tree canopy layers that provide shade, shelter and physical support for smaller plants, as Fig. 2.11 illustrates. This interlocking canopy reduces the rate of airflow, tending to maintain high and constant atmospheric humidity inside the forest. The lack of annual herbs in this 'closed' forest is due to the low amount of light reaching the forest floor; nevertheless the presence of openings facilitates the existence of both light-demanding and strongly shade-tolerant tree seedlings (see Section 3.2.1). These diverse ecosystems with their very many plant and animal species possess a number of specialized features, including those related to reproduction. Parrots and other rain-forest birds are attracted to red, black and blue fruits, whose contained seeds may be transported several kilometres before being deposited in the faeces of the birds. The dust seeds and spores of epiphytic orchids and ferns, which commonly grow high in the tree canopy, are carried long distances by wind and air currents within the forest. Bats assist with both

Hopea Parviflora

Figure 2.11 Profile of lowland evergreen dipterocarp tropical rain forest at Belalong, Brunei, so called because it is dominated by dipterocarp trees (a large family of tropical Asian trees). The two ends of the plot, which measured 60 x 7.5 m, were occupied by mature forest and had younger growth (i.e. building phase) invading a gap between them. All trees over 4.5 m are shown, but there is no attempt to show the seedlings and young saplings which allow gap regeneration to proceed. The canopy top is formed by three giant mature dipterocarps: Shorea laevis (D5), S. parviflora (D6) and Hopea bracteata (D2) up to 45 m high. Other dipterocarp trees below are hatched. Each symbol on a tree outline refers to a particular species. From this, the incredible species richness of the forest can be judged. (From Ashton, 1964. Oxford Forestry Memoirs, 25 in Whitmore, 1998.)

Figure 2.11 Profile of lowland evergreen dipterocarp tropical rain forest at Belalong, Brunei, so called because it is dominated by dipterocarp trees (a large family of tropical Asian trees). The two ends of the plot, which measured 60 x 7.5 m, were occupied by mature forest and had younger growth (i.e. building phase) invading a gap between them. All trees over 4.5 m are shown, but there is no attempt to show the seedlings and young saplings which allow gap regeneration to proceed. The canopy top is formed by three giant mature dipterocarps: Shorea laevis (D5), S. parviflora (D6) and Hopea bracteata (D2) up to 45 m high. Other dipterocarp trees below are hatched. Each symbol on a tree outline refers to a particular species. From this, the incredible species richness of the forest can be judged. (From Ashton, 1964. Oxford Forestry Memoirs, 25 in Whitmore, 1998.)

pollination and seed dispersal; seeds of figs and other rain-forest fruits may be transported as much as 20 km from the parent trees. Winged seeds, such as the shuttlecocks of dipterocarps, glide away from the parent tree for variable distances depending upon the species and the weight of the seed. Butterflies and other insects also act as pollinators and the flowers they visit possess attractive devices of one sort or another. Ants have remarkable symbioses with myrmecophilous plants, which receive protection from predators and epiphytic growths including fungi, and in return provide food (nectar) and galleries to house the insects. Weakened patches of cork provide access to trunk galleries in ant trees, while hollowed leaves, stems, branches and galls of epiphytic ant plants can all provide shelter.

Ants also appear to be responsible for the legendary single-species 'devils' gardens' of the tree Duroia hirsuta in the Amazonian forests of Peru. These trees live for hundreds of years and grow together in quite large groups in a forest famous for its otherwise remarkable diversity of trees, vines, shrubs and forbs. Local legend has it that these areas are maintained by an evil forest spirit. In reality, an ant Myrmelachista schumanni nests in the Duroia trees and poisons all other plants with formic acid, maintaining the monoculture of Duroia. The ants benefit from this pruning because the young Duroia saplings which grow in the cleared sites provide new nesting sites for the expanding ant colony. A colony can have as many as 3 million workers and 15 000 queens and the tree/ant colonies can be over 800 years old (Frederickson et al, 2005).

Many of the trees possess buttresses at the base (Fig. 2.11), which undoubtedly give them mechanical stability and help to stabilize those on unstable substrates. They also enable the tree to absorb adequate amounts of oxygen: roots in the frequently sodden soil have to compete with decomposer organisms for the often limited amount of soil oxygen. Flowers commonly develop directly on the trunk (cauliflory) or on branches (ramiflory). This makes them readily available to pollinators including beetles, and also facilitates the support of large fruits, e.g. jackfruit. Water drains more rapidly from leaves with the drip tips common in rain-forest trees.

Much of the nutrient capital of tropical rain forests is thought to have accumulated when plant roots were in contact with fragmented parent rock, perhaps as far back as the Tertiary (1.6-65 Ma) (Walter, 1973). The above- and below-ground distribution of mineral nutrients in five tropical rainforests is shown in Fig. 2.12, which shows the popular belief that most of the nutrients in a tropical rain forest are in the biomass is seldom true except for certain minerals in certain forests. Moreover, decomposition rates in tropical forests are not necessarily higher than in temperate forests. Tropical soils are, however, often nutrient-poor and acidic (pH 4.5-5.5), while - as we have seen in Section 2.2.1 - the leaching of basic ions and silica, frequently results in the accumulation of iron, manganese and aluminium sesquioxides. Moreover, once the vegetation including roots is removed, leaching of nutrients is rapid; hence the expression mentioned in Section 2.2.1 that rain forests are really 'green deserts' - a rich forest grown over poor soils. Termites and fungi, especially decomposer basidiomycetes, destroy dead wood, while nutrient recycling is accomplished by mineralization and root absorption often assisted by mycorrhizas.

Lowland rain forests

N P K Ca Mg 100 7540 140 550 530 290 kg ha"

20 40 60 80

6870 170 1590 5790 740 kg ha-1

190 3470 26400 2700 kg ha-1

6870 170 1590 5790 740 kg ha-1

190 3470 26400 2700 kg ha-1

20 40 60 80

ÉÜÉÉL

Lower Montane rain forests

Puerto Rico (275 t ha-1)

N P K Ca Mg 11200 110 920 2990 890 kg ha"

N P K Ca Mg 20200 69 1290 5510 957 kg ha-1

¡Ü

Aerial biomass Forest floor

Soil (0-0.3 m except Puerto Rico 0-0.25 m)

Figure 2.12 Distribution of inorganic nutrients above and below ground in five tropical rain forests. Biomass figures are in parentheses. This figure demonstrates that the popular belief that most of the nutrients of a tropical rain forest are in the above-ground biomass, although true for the soluble cations potassium (K), calcium (Ca) and magnesium (Mg) in the Brazilian forest shown here, is usually incorrect. N, nitrogen; P, phosphorus. (Redrawn from Whitmore, 1998. An Introduction to Tropical Rain Forests (2nd edn). Oxford University Press.)

Lower Montane rain forests

Puerto Rico (275 t ha-1)

N P K Ca Mg 11200 110 920 2990 890 kg ha"

N P K Ca Mg 20200 69 1290 5510 957 kg ha-1

Figure 2.12 Distribution of inorganic nutrients above and below ground in five tropical rain forests. Biomass figures are in parentheses. This figure demonstrates that the popular belief that most of the nutrients of a tropical rain forest are in the above-ground biomass, although true for the soluble cations potassium (K), calcium (Ca) and magnesium (Mg) in the Brazilian forest shown here, is usually incorrect. N, nitrogen; P, phosphorus. (Redrawn from Whitmore, 1998. An Introduction to Tropical Rain Forests (2nd edn). Oxford University Press.)

In fact humans have influenced these forests for far longer than is generally supposed. Some soils in the Amazon basin are rich, black and remain nutrient-rich when cleared of vegetation. These terra preta dos Indios (Indian dark earth) soils, which cover perhaps up to 10% of Amazonia (an area the size of France), are thought to be a relict of Amerindians from pre-European contact, dating back several thousand years and may point to early populations being much larger than once thought (Mann, 2002). The soils appear to have been formed by partial burning of the forest ('slash and char' rather than modern 'slash and burn') and integration of the resulting charcoal, which has a high cation exchange capacity (Section 2.2.1), into the top 40-60 cm of the soil. The incorporation of forest organic matter into the soils as charcoal rather than as litter or ash may help to explain the durability of these soils, in comparison with the short-lived increases in fertility of slash and burn. Nutrients were added in the form of faeces and other animal wastes particularly fish, perhaps with an inoculation of microorganisms from an existing terra preta soil which seem important in aiding soil formation. Whether these nutrient-rich anthrosols (anthropogenic soils) were deliberately created or developed inadvertently is open to debate, but current evidence points to the former (Erickson, 2003). The resulting dark soils are higher in phosphorus, calcium, sulphur and nitrogen than surrounding tropical red soils, they retain moisture and nutrients better, and are not as rapidly exhausted by agriculture when well managed. Extensive research is currently looking at how these soils were formed and can be created anew.

Rain forests have in recent centuries suffered severely at the hands of humans, and today even the vast Amazonian rain forests are rapidly losing ground. They form the most complex terrestrial ecosystems that have ever existed and are of potentially immeasurable economic, pharmaceutical and scientific value, besides posing many intriguing questions regarding their biodiversity, variation, stability and former distribution.

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Responses

  • Poppy
    Why are tropical rainforest called archetypal forests?
    3 years ago

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