EQUATORIAL RAIN FOREST The natural distribution of equatorial

Document Sample
EQUATORIAL RAIN FOREST The natural distribution of equatorial Powered By Docstoc
					             EQUATORIAL RAIN FOREST
The natural distribution of equatorial rainforest. The global distribution of equatorial
rainforest is closely tied to the warm, moist climates that occur near to the Equator. Ecologists
recognize this class of forests from a characteristic assemblage of features; the trees tend to
have wide buttress roots that splay out near the ground, and the leaves tend to be large,
evergreen and laurel-like, with an elongated tip (a 'drip tip') on the end of each leaf.

The equatorial rainforest climate. Near to the Equator, the intense energy input from the
sun produces the intertropical convergence zone (the ITCZ), a convection zone of rising air
that loses its moisture as frequent, intense rainstorms. In equatorial areas that are relatively
isolated from the sea winds that carry water vapour inland, there are breaks in the rainforest
belt. Likewise, travelling north and south away from the Equator one generally finds a
decrease in rainfall, as the influence of the ITCZ becomes weaker. In these drier places,
wherever the annual rainfall falls below about 1600mm with an intense dry season during part
of the year, the rainforest gives way to either monsoon (seasonal) forest, open woodland or
grassland. However, the actual limits of rainforest do also vary greatly with soil type, and the
amount of disturbance from humans and fires.

Where the climate remains moist enough to support equatorial rainforest outside the main
tropical belt, what finally puts paid to it are low temperatures and frosts. In a few places in the
world, such as southern China, a belt of moist evergreen forest continues north or southwards
well away from the Equator (as far as 26 degrees North in south-western China), nourished by
moisture-bearing winds from the oceans. The ultimate limits to equatorial rainforest in these
areas seem to be related to the mean temperature of the coldest winter month, with the final
(and rather subjective) boundary between equatorial rainforest and temperate rainforest being
drawn on maps at around the point where there is a significant probability of occasional frosts
occurring on cold winter nights.

A common evolutionary heritage. Despite their wide geographical separation, there are
strong floristic similarities between all the world's rainforest regions that reflect a common
origin. Many of the same families and even genera of plants occur in rainforests on different
continents, separated by thousands of miles of ocean water.

Some of the closest floristic similarities between rainforest regions are probably due to the
exceptional dispersal abilities of certain plants that may have had their seeds carried across
great distances by birds or ocean currents within the last few million years. This mechanism
may explain how, for example, a single species of the highly specialised carnivorous pitcher
plant Nepenthes occurs in the rainforests of Madagascar, whilst around 70 species of this
same genus occur in the Far East.

However, to a large extent the causes of these common floristic links must lie in the more
distant geological past, when several of the world's rainforest regions were in much closer
contact with one another. There is fossil evidence indicating that that many of the world's
widely separated regions of rainforest were once linked together into continuous forest belts
during times of moister and warmer climate. For example, during the early Tertiary Period
around 55 million years ago, equatorial rainforests seem to have formed an almost continous
belt from Africa across southern Europe and southern Asia to the Far East. Though this
rainforest belt was broken up intermittantly by shifting narrow seaways, one can imagine the
African and far Eastern rainforest regions would have exchanged newly evolving types of
plants rapidly and efficiently. This linkage seems to have been broken by about 30 million
years ago, as global climates became cooler and drier, but the closer common heritage
between the African and far Eastern regions still leads to them being classified together as the
palaeotropical (= old world tropics) realm.

A more fundamental set of resemblances between the rainforest regions exists because of the
geological history of plate tectonics, with different land masses that were once linked having
been carried away from one another. During the Cretaceous (around 140-65 myr ago), the
time when the flowering plants (angiosperms) were undergoing their main diversification and
rise to dominance, the land masses of the Southern Hemisphere were still more-or-less in the
form of the ancient southern supercontinent, known as Gondwanaland.

During the key phase of diversification of tropical angiosperms from about 140 and 65 myr
ago, Africa and South America (previously parts of Gondwanaland) were in the process of
rifting apart but were still quite near to one another as shown in this animation by Rowley &
Ziegler from the University of Chicago. Many of the ancestors of what are now tropical
rainforest plant families would have been able to cross between the continents. Sometimes
these newly evolving forms would have spread directly across the last remaining landbridges
or even drifting fragments of continent present during the rifting process. No doubt over the
millions of years there were also many occasional long-distance dispersal events across tens
or hundreds of miles of sea water, aided by shorter distance 'island hopping' across the
scattered groups of volcanic islands that certainly would have existed in such a geologically
active region. As a result of the combination of overland and overwater spread between Africa
and South America, we find many of the same plant families (such as the Euphorbiaceae,
Meliaceae, Moraceae and Sapotaceae) occurring between widely these separated continents,
and indeed all across the world's rainforest flora. Often, the overlay of subsequent evolution
has given an east-west split between separate subgroups shared between Africa and Asia on
one hand, and South and Central America on the other. The sub-family level differences that
have evolved are sufficient for the rainforests of the Americas to be classified separately as
the neotropical ( = new world tropics) realm.

It is difficult to say how much exchange there has been between the two continents of Africa
and South America since they separated off from one another. One can imagine that there
would have been a continuous but decreasing likelihood of plants and animals crossing the
divide as the landmasses drew further apart. In any case, the length of time and the
effectiveness of the separation has allowed only one species of tree (Symphonia globulifera)
to exist in common between these continents.

The Australasian region (including Australia and New Guinea along with other smaller
fragments) separated off from the southern part of Gondwanaland at about the same time as
South America, and evolved its own distinctive flora from the groups of plants that had
managed to reach there. Moist tropical environments were not present in the Australasian
region at the time that it split off from the other continents, but arose afterwards during the
Tertiary as it moved north towards the equator. Consequently the region evolved its own very
distinctive rainforest flora from the plant groups that happened to be present, including an
important role for certain ancient conifer groups that had dominated the Gondwanaland
forests before flowering plants existed. However, many of the same families and even genera
of angiosperms and conifers that evolved to fill the Australian rainforest realm do also occur
elsewhere in the world because of the ancient Gondwanaland link.
The same general patterns are reflected in the faunas of the rainforest regions, but not always
for the same reasons. Thus for example the monkeys of Africa and Asia together form a
linked group (known as the catarrhines) distinct from those of the Americas (the platyrhines),
but it is known that monkeys evolved relatively recently in geological time - after the major
breakup of Gondwanaland - so it is unlikely they managed to get directly across the widening
Atlantic. Instead, they most likely spread across from the old world tropics by forested land
links that existed between northern Eurasia and North America in the mid-Tertiary period.
They would then have spread southwards across the land bridge of central America,
eventually reaching South America. After the Northern Hemisphere link was broken, the new
and old world monkeys evolved into distinct groups. A similar east-west pattern is found for
the bats of the new and old worlds, with the fruit eating niche of the African and Asian 'flying
foxes' of the Macrochiroptera being filled in South America by a distinct and less specialised
group descended from the insect-eating Microchiroptera.

As the belt of warm, moist climate that once linked the African and Asian rainforests is now
long gone, evolution has also taken different directions between these two regions. Thus, the
Asian forests are today dominated by trees of the family Dipterocarpaceae, whilst the handful
of African dipterocarps are savanna and woodland trees, belonging to a quite distinct

During the last 30 million years since the mid-Tertiary, the history of the world's equatorial
forests has been one of further decline. A trend towards aridity in Africa and Australia has left
only small remnants of the much more extensive blocks of rainforest that once existed there.
Often these isolated pockets of rainforest survive hundreds of kilometres away from one
another, in coastal strips or on mountain slopes where the climate remains moist enough.
During each of the cool, dry glacial phases which have occurred repeatedly over the last two-
and-a-half million years, these forest remnants seem to have been even further reduced below
what exists at present, perhaps almost to vanishing point. Likewise, in the Americas, there is
strong evidence for major arid periods having occurred in the rainforest regions during the
past few million years.

As an additional complication to the story, two of the world's main rainforest realms are
currently in the process of merging into one another. As the isolated Australasian plate drew
northwards during the early Tertiary (about 40-30 million years ago), distinctively Asian
species of plants started to appear in its flora as it it approached the edge of the Asian plate.
Following the eventual collision of the Asian and Australasian continental plates beginning
about 20-30 million years ago, dispersal of plants and animals across the relatively narrow
straits that now separate these two continental masses has been a gradual but ongoing process.
A legacy of their previously distinct histories remains in the form of Wallace's Line (named
after A.R. Wallace, the 19th century naturalist who first gave a detailed account of it), the
zone of faunal and floral discontinuity that closely follows the meeting point of the plate
boundaries. Thus, after diverging since the dawn of the angiosperm era, these two rainforest
regions are now engaged in another vast ecological and evolutionary experiment.

Similarities and differences. As a result of this complex history of climatic change and
continental drift, the world's rainforests show many regional floristic and structural
differences that can only be put down to chance. Yet the differences are superimposed upon a
background of striking similarity, produced by the same evolutionary pressures producing
similar growth forms amongst many independent lineages of plants, giving equatorial
rainforests in any part of the world the same characteristic general appearance.
Thus in Asia, the role of gap colonizer in disturbed rainforest sites is filled by the large-leaved
fast growing trees of the genus Macaranga (family Euphorbiaceae), whilst in the Americas it
is filled by the superficially rather similar Cecropia (family Urticaceae). In contrast, whilst
epiphytes growing in the branches of canopy trees occur in all rainforest regions, they are far
more abundant in the American tropics where bromeliads (Bromeliaceae) have successfully
diversified to take up the role.

Large 'emergent' trees which project high above the main canopy occur in all the world's main
rainforest regions, but different tree families predominate in different places. In south-east
Asia, large trees of the family Dipterocarpaceae are particularly important as emergents,
whereas in other regions the commonest emergents belong to a range of other families.

Palms occur as part of the forest canopy in all rainforest regions, but for some reason they are
far more abundant in a range of ecological roles in South American rainforests than

The list of similarities and differences could go on and on. Looking across the range of the
world's rainforests, it is obvious that there must be certain common evolutionary pressures
which are responsible for the striking similarities between unrelated groups of plants growing
in different parts of the world. However, there is also a certain amount of leeway which
allows particular types of plants with distinctive attributes and aptitudes to come to dominate
the structure of the rainforest in a particular region.

The global distribution of species richness in equatorial forests. Equally as striking as the
convergences in plant form within and between rainforest regions are the results of recent
studies on numbers of species present. It has long been known that equatorial rainforests
around the world tend to be exceptionally rich in species of animals and plants, with some
areas being richer than others. It is also evident that within each region, certain groups of
plants have undergone explosive diversification due to the vagaries of history and
opportunity. Thus for example there are 700 species of screwpalms (Pandanus) in Eastern
Asia, but only a handful of species in Africa. However, at the broadest scale there may also be
a suprising amount of order in terms of the patterns of species richness in the rainforest

Recent studies have shown that tree species richness within each tropical forest region tends
to follow the trend of mean annual rainfall. In the drier parts of rainforest regions (around
1000-2000 mm), the number of tree species per 0.1 hectare starts at around 50-100 species,
and climbs to over 250 species in areas of very high rainfall (about 6000mm annually) and
very weak seasonality. Above this level (in areas where as much as 9000 mm of rain can fall
in a year), the curve of rising tree species richness tends to flatten off.

It is striking that despite tens of millions of years of isolation and independent evolution, the
richness of tree species between different tropical regions of similar climate remains fairly
constant. Thus, on the data gathered so far, a hectare of primary rainforest in Africa can be
expected to have roughly the same number of species as one in Asia or South America
existing under similar mean annual levels of rainfall (although the pattern of seasonality in
rainfall does affect this as well). This pattern contradicts the previously accepted notion that
the African rainforests are relatively poor in numbers of species because of their geological
history of arid phases, with these having occurred more severely than in either Asia or South
America. Having said this, the larger area of relatively dry species-poor rainforest in the
African region may account for the lower overall number of species in that region.

It seems that not only is there some ecological driving force which gives rise to more species
of rainforest trees in the wettest tropical climates, but the intensity of this driving force is
modulated to give similar levels of richness between regions that have been separated long
enough for them to have very different floras. There has been a great deal of speculation on
the nature of this mysterious driving force, but as yet no really convincing explanation has
been put forward.

The rainforest regions

African and Madagascan Rain Forests

The forest of the Congo Basin. By far the largest block of rainforest in Africa lies in and
around the Congo Basin, a horseshoe-shaped catchment area formed from sedimentary rocks
which overlie an ancient Precambrian basement layer. Most of the Basin is below 1000m in
elevation, having a flat or gently rolling topography with large swampy areas at it centre. The
Congo River drains the Basin westward into the Gulf of Guinea, across the coastal plain of

The forest of the Congo Basin grows mainly in soils that are of moderate nutrient status (at
least, amongst the range of tropical forest soils, which are notoriously poor in stored mineral
nutrients); there are relatively few areas of the very leached and nutrient-poor white sands that
occur in parts of the far East and the Americas. There are some vast areas of swamp forest and
open reed swamp in the centre of the Basin, (the Cuvette Centrale) with certain of these areas
being virtually uninhabited and largely unexplored.

West African rainforests To the northwest, a fringe of rainforest continues to the Cameroon
Highlands, and along the northern coast of the Gulf of Guinea. After a gap of several hundred
kilometres in the drier region between Togo and Benin, rainforest appears once again and
extends as far as the Atlantic coast. Climates also become drier beyond the east of the Congo
Basin; in the rift valley region the rainforest is mainly confined to localised mountain slopes.
To the south, the rainforest of the Congo breaks up into a mosaic of forest and savanna, with
the forest component becoming less and less significant as the rainfall becomes more

Compared to the other main rainforest regions of the Americas and the Far East, a striking
feature of the whole African rainforest region is its dryness. Only the very wettest parts of the
Cameroon highlands match the annual rainfall of large areas of Amazonia and the Far East,
and even they tend to have quite a strong dry season during part of the year. Indeed, most of
the forest in Africa seems to be existing close to the climatic limits of what we would define
as rainforest vegetation. During the dry seasons that occur once or twice each year, the canopy
trees across much of the Congo Basin show a marked tendency towards deciduousness,
although understory trees remain evergreen.

The deciduous habit is most noticable on well-drained sandy soils, where the dry season
drought might be expected to be more severe upon the trees. However, leaf-shedding is not
automatic but is instead a graded response that depends on the water economy of the
individual tree. In wetter-than-average years, many of the canopy trees will keep more their
leaves during the dry season, often replacing them with new ones just as the next rainy season
gets under way.

In the wettest climates of eastern Zaire and Cameroon, large trees of the leguminous (pea)
family Caesalpiniaceae are abundant and sometimes form pure stands of a single species.
Palms are relatively rare in the African rainforest, and they tend to occur only in particular
situations, such as where there has been disturbance of the forest. In contrast to the Americas
and the Far East, there are far fewer epiphytic plants growing on the branches of the trees in
African forests. This might partly be due to the drier climates in Africa, but is perhaps also a
legacy of the fact that the important epiphyte family Bromeliaceae has failed to reach there, in
contrast to the hundreds of species which occur in the Americas.

There has has been much discussion of the extent to which the present-day flora and fauna of
the African rainforest reflect a history of arid episodes that wiped out the forest in some areas
and allowed it to persist in others. These refugia might then have retained species which have
failed to expand their populations fast enough to spread out and recover their full potential

There are indeed strong signs that forest cover was reduced during the last Ice Age, between
about 25,000 and 12,000 years ago. Whilst forest is known to have persisted in some areas,
such as Cameroon, it was certainly greatly reduced further west. There are indications that
rainforest may have been replaced by savanna and grassland over much of the Congo Basin,
although considerable areas of riverine and swamp forest may have persisted. In general,
some of the fossil and geological evidence suggests that the areas which are richer in species
today (e.g. Cameroon, and the central Congo Basin) were glacial refugia, but then again these
are among the areas of wettest climate at present. It is extremely difficult to disentangle the
effects of glacial history and present-day climate, especially when fossil data on the
distribution of forest during the last Ice Age remain sparse and ambiguous.

Humans and the African rainforests. The distribution of rainforest that one sees today in
Africa is to some extent natural and to some extent the legacy of a long history of human
influence. Modern-day humans seem to have existed longer in Africa than anywhere else, but
it is difficult to gauge what effects they were having on the forests before the spread of
agriculture to the region about 3,000 years ago. The savanna-forest mosaic which exists to the
south of the Congo rainforest, and extends north as a finger almost to the Equator in west
Congo, has been widely thought of as the recent result of denser agricultural populations of
humans burning the savanna and keeping the rainforest at bay. However, fossil evidence
indicates that at least parts of this mosaic have existed continuously since the last Ice Age,
around 20,000 years ago. It seems that the relative proportions of forest and savanna have
fluctuated in line with climatic changes and perhaps in response to changes in the density of
human populations. It may be that preagricultual humans were also burning the savanna often
enough (in order to make hunting easier) to have a significant effect on the extent of forest.

Since the spread of agriculture, the influence of shifting cultivation has been so pervasive that
it has led many authors to wonder if there is any truly untouched forest left in Africa.
Ecological studies of what might appear to be pristine rainforest have often found ancient
traces of cultivation and habitation, such as charcoal, pottery fragments and hut foundations.

During the 20th century, parts of Africa have undergone the same rapid loss of forest cover
that is occurring in other parts of the tropics. West Africa has now lost most of its natural
forest cover to an expanding agricultural population, and Cameroon is starting to suffer the
same problem.

In contrast, most of the central region of Africa remains densely forested, and sparsely
inhabited. There is relatively little sign of the burning activities of cultivators showing up in
satellite imagery, in contrast to the situation in many areas elsewhere in the moist tropics.
However there are now indications from remote sensing of a reticulate network of plantations
and croplands spreading out through the rainforest in Zaire, corresponding to the lines of the
main road system. There are certainly some large areas of cultivated land in coastal Gabon,
and intensive forestry activity in parts of eastern Zaire, although its extent has never been
thoroughly mapped. With the pressure of growing populations, and increasing investment by
foreign timber companies, all the expectations are that the demands on the central African
rainforests will increase over the coming decades.

Madagascar forests. Geologically, Madagascar is a micro-continent which split off from the
eastern side of Africa at some time during the Jurassic Period (more than 140 million years
ago). Its flora and fauna reflect its continuing geographical proximity to Africa, with the
imprint of partial isolation and independent evolution, and also with the addition of a
suprising number of Asian groups of plants and animals. Many of these links to the Asian and
African mainland must be the product of chance dispersal within the last few million years,
carried by ocean currents and winds. However, some of the characteristic features of the
Malagasy biota may in fact be a much older legacy of the former distribution of rainforest
animal and plant groups that were present in the long-vanished Tertiary rainforests of eastern
Africa (or on the northwards moving sub-continent of India) that have managed to reach
Madagascar but have since died out where they came from. Madagascar is perhaps most
famous for its lemurs, descendants of a group of primitive primates that was once much more
widespread in the world. It seems likely that it was their isolation on this island continent
which enabled them to escape competition and extinction at the hands of other more recently
evolved primate groups.

In general, Madagascar is a rather arid place, but along its eastern edge the climate is more
than moist enough to support a long band of rainforest. The high rate of endemism of plant
and animal species (around 85% of the plant species on Madagascar are unique to the island)
reflects the strength of its isolation from the African mainland, so it is perhaps suprising that
the rainforest nevertheless has levels of species richness that are comparable with continental
rainforest areas with similar climate. It is as if evolution has worked overtime on Madagascar
to ensure that it has its full quota of diversity!

However, the distinctiveness of Malagasy plants does not run very deep. Perhaps because
Madagascar was not so isolated during the critical early stages of evolution of the
angiosperms, the process of evolutionary divergence has not been carried very far. In contrast
to the high level of species endemism, there is only one family of flowering plants that is
unique to the Madagascar rainforest, the Humbertiaceae. Many Malagasy plants also belong
to genera that occur elsewhere in the old world tropics, perhaps having reached the island by
dispersing across from the African mainland, although some of these have radiated out into
quite a large number of species on the island. Thus for example, one can find five species of
wild malagasy coffees (Coffea) in only 2 km2 of rainforest. Since these wild coffees are
resistant to the diseases which afflict the African-derived cultivated coffees, one can see that
there may be great potential here for producing commercially important hybrids.
One group of plants which has diversified strongly on Madacascar is the palms (Palmaceae).
There are some 12 endemic genera of palms on the island (compared to only three endemic
palm genera in Africa), but all have strong Asiatic affinities. The remaining 7 non-endemic
palm genera also all occur in eastern Asia, but have close relatives in Africa.

Orchids are also very diverse on the island, with nearly 1000 species having been found (more
than in the whole of continental Africa), many of these being confined to the rainforest.
Perhaps the most remarkable is the white-flowered Agraecum sesquipedale, which has a spur
some 35cm long. The great 19th century naturalist A.R. Wallace - already mentioned above in
the context of Wallace's Line - predicted that there must exist a moth with a tongue long
enough to reach down the spur and drink the nectar at the end, for otherwise the orchid would
not be visited by insects and would not be pollinated. He was laughed at by entomologists for
suggesting this, but sure enough 40 years later a moth was discovered that has a 35cm tongue
capable of reaching down the full length of the spur. This moth, Xanthopan morganii forma
praedicta('predicted form'), only ever fully unfurls its tongue when it nears a flower of the
long-spurred Argraecum, and drinks hovering over the flower while the orchid glues pollen
masses to its proboscis, ready for them to be carried on to pollinate the next flower of the

Sadly, the flora and fauna of Madagascar is suffering like no other from the pressures of
human population. Already, a fantastical array of giant flightless birds and large bear-like
lemurs has been wiped out by hunting and habitat loss since the first humans arrived 1500
years ago. The human population of Madagascar is now growing at an explosive rate, and
fragmentation of the forest is occurring so rapidly that it cannot be long before many more
extinctions follow. Many unique and potentially valuable species are holding on in only a few
scraps of forest, encroached upon year by year as settlers grab more land to feed their

As the original vegetation is lost, artificially introduced species of plants are taking its place
in the disturbed secondary forest and scrub that remains. It seems to be a characteristic of
islands in general that they are susceptible to invasion by garden escapes and weeds brought
over accidentally from other parts of the world, and Madagascar is no exception to this rule.

Rainforests of the Amazon and Orinoco.

Within the linked catchment zones of the great Amazon and Orinoco rivers lies the world's
largest contiguous area of equatorial rainforest. Moist Atlantic air travelling westwards is
thought to supply most of the region's rainfall, but the wettest climates are found on the
opposite side of the continent close to the Andes, where the air is forced to rise and shed its
heavy burden of water vapour. As it travels west from the coast, the prevailing air stream
picks up much of the water vapour evaporated back up from the forest and redeposits it as rain
further on. Despite the recycling, there is also a constant loss of water as it enters the massive
Amazonian river system and flows straight down to the Atlantic Ocean. The role of moist
Pacific airmasses in contributing to the high rainfall in the western Amazon remains
uncertain, although some studies suggest that they are in fact important. Whatever the causes,
the climate becomes drier from West to East across the Amazon Basin, but moister again as
one approaches the Atlantic coast in the north-east.

In general, the Amazonian forests exist under a rather moister climate than their African
counterparts. Although there are huge areas of semi-evergreen rainforest towards the southern
and eastern margins of the Amazon region, there is also a very extensive core area of
evergreen forest. In these wetter parts of the region, with annual rainfall above about 2000mm
and with no strong dry season, the forest is taller and richer in species. Without the
regimentation of strong dry seasons, there is less reason or opportunity for biological
processes to be synchronized, and growth and reproduction of plants and animals occurs more
evenly around the year.

The annual rainfall along the eastern foothills of the Andes is particularly high (reaching
9,000mm in places), and distributed fairly evenly throughout the year. In this ever-wet
climate, species richness of trees, lianas, shrubs and herbaceous plants reaches almost
unbelievable levels. One study found 283 species of trees and lianas with stem diameters over
10cm in a tiny 0.1 ha sample of forest in the Napo region of Peru. Furthermore, many of the
plant species within these forests of the Andean foothills have very localised distributions,
sometimes being found on just one or two hilltops, with their place being taken by other
similarly localised species elsewhere.This adds even further to the species richness on a
broader scale, making these forests a truly remarkable reservoir of biological diversity.

Forest types of the Amazon region.The great extent of the Amazonian forests, together with
the regional peculiarities of the soil types and river systems within the Basin, allows a number
of very distinctive forest types to exist, each with its own specialized flora. By far the largest
area is taken up by tierra firme forest, which is the 'standard' forest type that one usually has
in mind when referring to 'the' Amazon rainforest. Tierra firme forest occupies fairly well-
drained soils that are relatively rich in available nutrients. It is tall (40m or more) and usually
very rich in tree species, with a dense upper canopy and a relatively dark, open interior.

Other forest types occur under conditions on soils that are particulary poor in nutrients, or in
areas subject to fire or flooding. White sand caatinga is a low, scrubby forest with small hard
leaves, that occurs in places where the soil is mainly bleached sand. It is scattered throughout
the Amazon region, but particularly in the Rio Negro catchment area where the soils are
formed on ancient sea beaches and other sandy deposits.

Liana forest (known as cipoal in Brazil) is a relatively open forest in terms of its tree cover,
but is almost smothered by huge numbers of vines growing on, over and between the trees. It
often seems to occur on the intensively weathered latosols which are associated with
important ore deposits of iron (as in the Serra dos Carajas region) and aluminium, but it can
also be found on the more fertile terra roxa soils. The liana forest tends to form a close
patchwork with other types of forest, and is particularly abundant in the area between Maraba
and Itaituba through which the TransAmazon Highway passes.

Another type of more open forest which is widespread in Amazonia is palm forest, dominated
by the babacu palms (Orbignya spp). Palm forest is often partly the product of human
influence, with frequent burning helping to encourage the fire-resistant babacu. In terms of the
indigenous economy of the forested regions, this forest type is important for the Brazil-nut
trees (Bertholletia excelsia) which are also present.

Although there are some areas of permanent swamp forest, this is a much more minor
component than the seasonally inundated forests that line the riverbanks and floodplains of
the Amazon and its tributaries. The highly seasonal nature of the inundation is a result of the
vast extent of the Amazon catchment. The Basin stretches north and south across the equator
and westwards as far as the Andes, so when the rainy season reaches its peak in either the
north or the south, the runoff is sent rushing downstream in a great burst. Hence, the water
levels of rivers even in the relatively non-seasonal areas of the Amazon Basin closest to the
equator can reflect this seasonality occurring in the catchment areas far upstream.

There are various types of seasonally or intermittently inundated forests along the river
systems of the Amazon Basin. Where a river is fed from clay-soil areas, the suspended
sediment in the water tends to be be deposited over the bank tops as it floods. This builds up
broad low ridges, known as levees, along either side of the river, channelling its flow and
limiting the extent of flooding. This levee-top and floodplain forest, known as varzea, is very
similar in structure to the terra firme forest. The trees tend to be heavily buttressed (perhaps
an adaptation for anchorage in the moist clayey soils), and often have seeds with special water
flotation mechanisms that enable them to be dispersed when the river is in flood. The rubber
tree (Hevea brazilensis) is an example of a species which is clearly adapted to the varzea
habitat, having seeds which can float for up to two months and which form an important part
of the diet of certain large fish. Perhaps because of the regular input of nutrient-rich sediment,
the herbaceous understory layer is particularly lush and rich in species of such plants as
gingers (Zingiberaceae) and heliconias (Scitaminae).

On clearwater and blackwater rivers, in areas where the underlying sediment consists of
bleached podsolized sand, a very different type of flood forest exists. This is the igapo of the
Rio Negro and Rio Xingu regions, a relatively short and species-poor forest growing under
conditions of low nutrient supply. Many of the trees that are specialized to grow in the igapo
are members of the family Myrtaceae (for example, Eugenia inundata), and they are zoned
according to slightly different degrees of flooding. Partly because of the lack of any
constricting levees, and also because of the relatively short stature of the trees, the floods
which occur in Igapo forests are often high enough to reach the crowns of the trees. Thus at
the height of the annual flood, it is possible to have the surreal experience of gliding a canoe
through a rainforest canopy! The flooding often lasts for several weeks or even several
months, and the trees need special biochemical adaptations to be able to survive the lack of
oxygen around their roots. During the flood season many of the trees drop their fruit into the
water, where they are eaten by fish. It seems that some of these trees are strongly dependent
on the fish to disperse their seeds through the forest, requiring the seed to have passed through
the gut of a fish before it will germinate.

Diversity centres - real or imaginary? Within the Amazonian rainforest block, there is
undoubtedly a general gradient in tree species richness from west to east, following the trend
in annual rainfall. However, many groups of animals and plants have also been purported to
show scattered centres of high diversity, containing relatively large numbers of endemic
species. It has been suggested that these diversity centres might have been refugia for
rainforest species during dry glacial periods in the past, with many of the species having been
too slow to disperse out of their refugia as the forest expanded again during the wetter
interglacials. Some recent work indicates that many of these apparent diversity centres could
be no more than an illusion, resulting from differences between the amount of fieldwork done
in different areas. If more collecting work has been done in any particular area, as compared
to its neighbouring areas, it is to be expected that more species will have been discovered
there! It seems that more research will be needed to sort out whether these diversity centres
actually do exist.

The Atlantic Rainforest Along the Atlantic coastal strip of south-eastern Brazil, the moist
maritime air gives enough rainfall to sustain a long belt of equatorial rainforest. Towards the
south, this merges imperceptably into cooler moist evergreen temperate forest, losing much of
its species richness as it does so.

Separated from the the main Amazon rainforest block by many hundreds of kilometres of dry
scrub and savanna, the Atlantic forest has a high proportion of endemic species. Nevertheless,
around half of the tree species found in this forest also occur in the Amazon rainforests or
elsewhere in the Americas (e.g. the tree Guarea guldonea), reflecting some sort of common
history in the recent past. Many of these non-endemic trees also occur scattered through the
broad intervening scrub/savanna regions as small populations within strips of 'gallery forest'
along river floodplains. Thus, it is not difficult to imagine how their seeds might have been
dispersed stepwise over much shorter distances between the separate areas of forest. It is also
of course possible that past phases of moister climate could have brought the gallery forests
extending in further towards one another, allowing the gradual exchange of plants and
animals to occur more readily.

The Caribbean rainforests.To the north of the main Amazonian rainforest block, many of the
scattered islands of the Caribbean Basin are partly or wholly covered by rainforest. Although
the climate over much of this region is seasonally quite dry, the steep topography of many of
the islands forces the maritime air to rise and cool, losing its moisture as rainfall as it does so.
Where this happens, rainfall may exceed 2000mm and moist evergreen or semi-evergreen
forest occurs on the lower mountain slopes and hills.

On Trinidad, mora (Morea excelsa) forms extensive stands within the moist forests, but
elsewhere in the Caribbean the forests are generally not dominated by any one particular
species. In the Windward Islands, at the southeastern edge of the Caribbean, typical trees of
the forest canopies include Slonea and Canarium. On the Greater Antilles, comprising such
islands as Jamaica and Cuba, species of Ficus and Psidium are particularly important trees.
These island forests are generally less species-rich than those of the South American
mainland, perhaps partly because of their drier climates at present and their history of greater
aridity during ice ages, and also due to their relative isolation from larger sources of
evolutionary novelty on the mainland. It is a general characteristic of islands everywhere that,
when other things such as climate are equalled out, they tend to be poorer in species than
mainland areas of the same size (a notable exception to this rule is Madagascar, but this has a
long geological history and is in any case quite a large island). This must be partly because
newly evolved species on large landmasses are less likely to reach them, but is also surely
because the smaller populations of plants and animals that exist on these islands are more
likely to be wiped out through chance catastrophes.

Amongst those tree species which do occur on the Caribbean Islands, there is a close affinity
to the South and Central American rainforests. Most of the genera are the same, and quite a
large proportion of the species are also shared with the mainland (e.g. the same Gaurea
guldonia as also occurs in the Amazon, Central American and Atlantic rainforest).

The Pacific rainforests. On the west side of the Andes, through western Ecuador, Peru and
Colombia, there is a thin band of rainforest stretching parallel to the coast. Both floristically
and geographically, it is very close to the Amazonian forests that lie to the east, but it has
probably been completely separated from them for millions of years after the Andes grew up
from beneath. Thus, although one finds many of the same plant genera that are present to the
east, they often belong to distinct species.
Central America. Further north, the west Andean forest connects with the central American
rainforest at Panama, and stretches north to Mexico. Whilst tending to have a rather more
seasonal climate, these central American forests are nevertheless very rich in species. In Costa
Rica, 233 species of plants (of all types taken together) have been found within a sample area
of only 100m2 (0.01 ha) of forest! Generally, the rainforests of central America are very
similar in composition to those of the Amazon and west Andean regions, with local endemic
species mixed together with more widespread ones that occur throughout the region.

Exploitation and loss of equatorial rainforests in the Americas. The central American
rainforests once gave rise to the only ancient civilization ever to develop in a true lowland
rainforest region. The Maya, whose empire was initially centered on the Yucatan Peninsula of
Guatemala, began building stone temples around 400 B.C. and persisted as a powerful
political and cultural force up until 1300 A.D.. They were initially able to support a large
urban population by intensive agriculture, using terracing and manuring to help reduce soil
erosion. Nevertheless, it seems to have been soil erosion and decline in soil fertility -
combined with a sudden severe drought event - that eventually brought about a dramatic
collapse of the population around 800 AD, from which the Maya Empire never truly

Other than the Maya, the only inhabitants of the rainforests of the Americas before European
settlement were native hunter gathers and shifting cultivators. Other native civilizations, such
the Aztecs and Incas, avoided the lowlands and concentrated on montane environments with
their more fertile soils.

Over the last few centuries there has been an increasing drive to utilize the lowland forests
commercially. The process of forest clearance for crop-growing and for ranching has
accelerated greatly over the past few decades, partly as a result of the growing populations in
tropical American countries. Thus for example, in Costa Rica the area of forest has been
reduced from 67% primary forest in 1940 to only 17% in 1983. The Atlantic rainforest on the
south-eastern coast of Brazil has now been almost totally destroyed, with primary forest
occupying less than 2% of its original area.

The process of forest conversion in Amazonia has been increased by planned projects that
have opened up previously inaccessable regions. In the southern Amazonian state of
Rondonia, the Brazilian government built a road (BR-364) which allowed large scale
migration of landless farmers into the region. The result is that much of the primary forest in
Rondonia has now been destroyed. Other government-backed projects in Brazil have resulted
in the large-scale clearance of rainforest for its use as iron smelter fuel in the Carajas region in
the north-east, and substantial areas of forest have been flooded by the building of
hydroelectric dams along the Amazon tributaries.

So far, it seems that somewhere between 5 and 12% of the primary rainforest of Amazonia
has been cleared, although parts of this have now reverted back to relatively species-poor
secondary forest. There are recent signs of a significant shift in Brazilian policy away from
large-scale clearance of rainforest, following the failure of transmigration and ranching
projects to benefit the country's economy.

On the western side of the Andes, the exceptionally species-rich Peruvian and Colombian
forests are now being cleared rapidly, with coca (cocaine) and opium poppy cultivation being
one of the major incentives. There are also plans afoot by foreign timber companies to exploit
these forests on a large scale for chipboard manufacture. To the north, in Panama, the loss of
forest from around the Canal Zone to illegal logging and shifting cultivation has resulted in a
great increase in erosion and a significant decrease in rainfall. As a result, the canal is silting
up and its water level has also fallen, bringing about difficulties for its use by shipping.

The Far Eastern Rain Forests

It has long been known that rainforests of India, Sri Lanka, Malaysia, Australia and New
Guinea form a relatively coherant unit, sharing many floristic elements. The scattering of
lands between Indochina in the northwest and Australia and New Guinea in the east more-or-
less comprises the floristic region known as Malesia, but for present purposes it is too closely
similar to the Indian region to be considered totally separately from it.

The far eastern rainforests seem to be the product of quite separate floras that had evolved on
different crustal plates, which later collided and spilled their animals and plants across onto
one another. Each of these collisions has left its distinctive mark on the biology of the forests,
and on the patterns of distribution of plants and animals which we see today.

All across the far eastern region there exists a strong floristic resemblance to the African
tropics, a reminder of its descent from the biota of the great northern continent of Laurasia
which remained intermittently linked to Africa across a land bridge or separated only by
narrow seaways. Still further to the east, there is the legacy of a long period of separate
evolution on the Australasian plate, which has now collided with the edge of the old
Laurasian plate and received much of its flora.

The west Malesian forests; Malaysia and Indonesia. The equatorial rainforests of west
Malesia have been well described by naturalists and foresters over the past century, and more
recently been they have been mapped in detail using satellite data. Towards the north of the
region, the rainforests fade into deciduous monsoon forests, but bands of moist climate allow
rainforest to continue to its most northerly point (26 Deg.N.) in Southern China.

The canopy of the west Malesian forests is generally between 30 and 40m tall, with large
emergent trees projecting up to 60-70m, and with all the usual assemblage of rainforest
features such as buttress roots and large evergreen leaves. For the most part, the forests of the
region exist under very weakly seasonal climates, with an annual rainfall of over 2000mm
distributed through the year. The wettest climates of all are found in the Kalimantan region of
north-western Borneo, where tree species richness also reaches its highest levels.

Studies of the seasonal behaviour of rainforest trees in equatorial Malaysia have indicated that
even the relatively slight dip in rainstorm frequency that occurs twice each year is enough to
act as a cue for a burst of leaf renewal and flowering in many species. The actual cue that the
trees are sensing may be an increase in drought stress, or perhaps a slight temperature change
within the canopy. Not all trees reproduce every year; many species in the family
Dipterocarpaceae show a pattern of 'mast fruiting' only once every few years, with all of the
tree population across a large area coming into flower simultaneously.

Mast fruiting often seems to be synchronized by the more intense droughts that can occur in
even the wettest regions, once in a while. These occasional droughts tend to be associated
with 'El Nino' events; sudden temporary reversals in the tropical Pacific circulation. An
exceptionally severe El Nino event in 1982-83 led to the drying out and destruction of large
areas of rainforest in Borneo by fire, although these fires were made worse as a result of
forestry activity that had left inflammable debris scattered through the forests. It is also
thought that many of the fires were deliberately started by shifting cultivators to help clear
small patches of forest, and that these quickly went out of control.

It may be suprising to learn that the scattered rainforest islands of west Malesia have a flora
and fauna which is fairly similar throughout. After all, many of these islands are separated
from one another by hundreds of kilometres of seawater, so one might have expected
evolution to have taken different directions due to their relative isolation from one another.
However, as is true at a more general level throughout the tropics, this similarity amongst the
islands is a legacy of land links which once existed but have now vanished. In the case of west
Malesia, these links were last present less than 10,000 years ago, in the period following the
last Ice Age before sea level had risen fully to its present level. In fact, the 'normal' state of
south-east Asia during the past two million years or so has been as a single linked landmass,
with only brief, exceptional warm periods (such as the one we are living in at present) melting
enough polar ice to separate off the islands from one another. As a result of this history, there
has been plenty of opportunity for species to spread overland from one part of west Malesia to
another. Nevertheless, a great many local endemics and races do remain confined to particular
islands or parts of islands, although these differences may have more to do with present-day
variations in climate and soil type than the recent history of isolation by sea level rise.

Variations of forest types. On particular soil conditions, caused by variations in geology or
drainage, various specialized types of forest occur. Each forest type has its own assemblage of
characteristic tree species, together with others that occur more widely.

Heath forest, or kerangas, is ecologically very similar to the white sand caatinga of
Amazonia. It is shorter than other lowland rainforest types, with small leaves clustered on
upturned twigs. As with the Amazonian caatinga, it occurs where the soil has been podsolized
to form a nutrient-poor bleached sand. The abundant phenolics (such as tannins) which occur
within the leaves are thought to be a defense mechanism to prevent insects from eating them
and robbing the plant of precious nutrients. It is these phenolics which seem largely
responsible for staining the water of the blackwater rivers in these areas, giving it the colour
of tea.

Limestone forest occurs in the dramatic karst landscapes of Borneo and the Malaysian
peninsula. These areas have a very rich flora with many endemics, including both trees and
the herbaceous plants that cling to the limestone rock faces. Being so well drained, the forests
growing on limestone areas suffer relatively severe drought stress during dry seasons, and
often develop 'autumn' colours before shedding their leaves for part of the year.

Swamp forests of various sorts occupy considerable areas in western Malesia, particularly in
Borneo. For example, peatswamp forest has a layer of between a few centimetres and several
metres of peat beneath the shallow roots of the trees. In contrast to the forests on dry land,
dipterocarps are seldom dominant, although species such as Shorea platycarpa do sometimes
form dense stands. It was the peatswamp forest of eastern Borneo which fared particularly
badly during the drought and fires of early 1983. At this time, the water table fell far enough
to allow large areas of peat to dry out. The peat then burnt, killing the trees which were rooted
into it.
The Indian rainforests. The equatorial rainforest in India occurs in two separate areas; along
the strip of hills and mountains near to the west coast, the Western Ghats, and in the
northeastern state of Assam close to the border with Myanmar (Burma). In both these areas,
semi-evergreen rainforest is more widespread than evergreen forest, probably due to a long
history of human influence which has degraded the structure of the forest and its soils, and led
to a more precarious water balance.

Typically for the Far Eastern region, dipterocarps are important in both the western and
northeastern rainforest areas of India, but there are none of these species in common between
them. In the Assam valley, isolated individuals of Dipterocarpus macrocarpus and Shorea
assamica tower above the main canopy, reaching a height of up to 50m and a girth of up to
7m. In the Western Ghats, giant buttressed specimens of other Dipterocarpus species
commonly tower to a height of over 30m before reaching their first branch.

In general, the rainforests of the Western Ghats are more diverse than those of Assam, with
over 4,000 plant species occurring in this relatively small area of hill land. Of these, 1,800
species are endemic to the Western Ghats, most of these being confined to the rainforest
rather than the other drier vegetation types that occur mixed in with it. As one would expect
from the general global patterns, the highest species richness is reached in the southernmost
forests of the Western Ghats, where the dry season is shortest.

As with the African forests, there appears to have been a pervasive influence of humans on
the Indian rainforests going back thousands of years. There are still a few of the ancient
hunter gatherer groups which must have existed in this region before the arrival of agriculture.
Other indigenous groups and more recent settlers still utilize the traditional system of shifting
cultivation, growing crops in a clearing for a few years before abandoning the land to the
forest. It is thought to be the history of disturbance through shifting agriculture that has led to
clumps of bamboo being widespread within the forests along streams and in poorly drained
hollows. As India's population expands, the intensity of shifting cultivation is becoming more
intense, with shorter fallow periods giving less time for the forest to recolonize and recover.

Sri Lanka. In Sri Lanka, lowland rainforest would naturally occur over much of the south-
western half of the island, where annual rainfall is between about 3500 and 5000mm,
although over the past few decades it has been greatly depleted. There are many plant genera
of Malesian affinities, such as Mesua and Vitex. Dipterocarpaceae are also locally important,
with such species as Dipterocarpus hispidus and Shorea spp occurring on moist, fertile
floodplain soils.

The Dipterocarps. There are some interesting features of the far Eastern rainforests that seem
to be a legacy of chance evolutionary events in the region's history. Most striking of all, the
west Malesian and Indian/Sri Lankan rainforests are dominated beyond all proportion by
members of a single family of trees, the Dipterocarpaceae. Dipterocarp pollen first appears in
the fossil record of west Malesia as an explosive burst about 30 million years ago. The timing
of this event follows the early Tertiary collision of the northwards moving India-Ceylon plate
with southern Asia, and it is possible that the group may originally have come from there. A
few species of dipterocarps have managed to reach across the second collision zone to the east
of the island of Sulawesi, so the eastward range of the family extends across Wallace's Line
into New Guinea. Although several dipterocarps do occur in Africa, they are trees of savanna
and woodland and not of the rainforest. A couple of species of what may be dipterocarps have
recently been found in the Guiana Shield rainforests of South America, but their real
evolutionary affinities are still in question. What is not in dispute is that the dipterocarps of
the Far East belong to a quite distinct subfamily, the Dipterocarpoideae, and that they have
diversified disproportionately into tens of genera and hundreds of species growing in a range
of habitats throughout the rainforest.

Dipterocarps of the far east are not just abundant in terms of species; they also make their
presence felt by their size and sheer weight of numbers. Flying low in an aircraft over the
primary forest of Borneo or the Malayan peninsula, one would notice that the canopy is
punctuated by clumps of huge dipterocarp trees projecting to a height of perhaps 60m. These
emergent groups may consist of a single species (often of the genus Shorea), breaking the
general rule that rainforest trees grow only as isolated individuals scattered in amongst other
species. The surrounding forest is also likely to contain a large variety of different dipterocarp
species, some of them scattered as rare individuals and others more abundant and gregarious.

The very abundance of dipterocarps, and the high quality of their timber, has been their
downfall in the 20th century. They are a lucrative target for timber companies and as a result
they have been virtually logged out from large areas of the Philippines, Sabah and elsewhere.

Loss of rainforest in the far East

The destruction of the far eastern rainforests has been exceptionally rapid during the last 50
years. As mentioned above, the most of the primary forest has been disturbed to some extent
by timber extraction. In many areas, the forest cover has been lost altogether. For example
Vietnam was about 45% forested in 1943, but this figure had fallen to about 20% by the mid-
1980s. Population pressure for agriculural land seems to have been the main factor behind the
loss of forest in Vietnam, but deliberate bombing and spraying for defoliation were also
significant during the war of the 1960s and 70s. Many of the bombed and sprayed areas have
so far shown little sign of natural recovery of forest vegetation, although replanting efforts are
now underway.

In Indonesia, a government-sponsored transmigration programme in the 1980's shifted some
three million people from densely populated and heavily deforested islands such as Java, to
less densely populated islands. This resulted in more rainforest being cleared for cropland and
towns, but due to various problems such as the poor suitability of soils for agriculture, the
programme has recently been scaled down.

In contrast, Borneo still retains a great deal of primary forest, although the extent of forestry
and agriculture is on the increase, especially in the northern state of Sabah. Timber companies
are now investing heavily in the region as the Malaysian forests become exhausted of useful
timber. Population pressure is also leading to an increase in the area of forest cleared for
slash-and-burn agriculture. As mentioned above, it seems that both agriculture and forestry
practices combined to help produce the extensive fires that occurred in northern and eastern
Borneo during the drought of 1982-83.

The Indian and Sri Lankan rainforests have suffered badly as a result of increasing human
populations and poorly controlled logging during the past few decades. The longer-term result
of unsustainable exploitation is a degradation in the quality of the forest resources for the
people that benefit from them, and a loss of biological diversity. In Sri Lanka, uncontrolled
deforestation occurred partly as a result of the breakdown of order resulting from civil war.
Various endemic tree species with localised distributions on the island are thought to have
gone extinct now, since the only areas they occurred in have been completely deforested.

In Myanmar (Burma), there is an increasing need for agricultural land from the growing
population, with shifting cultivation occurring at intensities that cannot sustain allow the
forest to recover fully. Timber poaching to feed markets in neighbouring countries, and
excessive felling rates in production forests, is also depleting the forests on a large scale.

East Malesia; the New Guinean and Australian Rain Forests

The East Malesian region basically consists of the Australasian plate, a fragment of the
ancient southern continent of Gondwanaland. Presently separated from one another by only a
shallow continental sea, Australia and New Guinea were repeatedly linked by a land bridge
(known as the Sahul shelf) in the past, particularly during the many low sea level phases that
have occurred during the last two million years. The last land linkage between the two
landmasses seems to have broken less than 10,000 years ago when sea levels rose after the
last ice age. Although equatorial or sub-equatorial rainforest climates were apparently present
over much of the northern half of Australia during the mid-Tertiary, the rainforest is now
confined to a few fragments along the Queensland shore. In contrast, New Guinea remains
largely covered in rainforest, with a few areas of monsoon forest at its southern and eastern

For the most part, the Australian and New Guinea rainforests are now dominated by west
Malesian groups of plants that have managed to disperse across the plate collision boundary.
Many are isolated species belonging to genera which are much more diverse on the other side
of Wallace's line in west Malesia. However, there are also many endemic or nearly-endemic
genera of canopy trees present (e.g. Doryphora). These include a strong element of
indigenous groups of plants that once formed an ancient rainforest flora in the east Malesian
region, as part of the Australasian crustal plate. As a legacy of this past, the conifer genera
Auracaria, Dacrydium, Podocarpus and Agathis still occur in the lowland rainforest of the
region, sometimes as extensive stands. The only rainforest eucalypt, Eucalyptus deglupta,
forms pure stands of trees in the New Guinea heathforests, where it is of considerable
economic importance as a timber tree.

The east Malesian rainforests remain remarkably intact compared to those of most other
regions. Very little of the forest of New Guinea has been touched by forestry or large-scale
agriculture, although it has a long history of habitation by shifting cultivators. The Australian
rainforest covers about 90% of the area it occupied when the first Europeans arrived, and
much of what remains is now protected.

However, the native fauna and flora are of the Australian rainforest to some extent being
menaced by introduced species, for example the American cane toad (Bufo marinus) which, if
eaten, poisons the native marsupial cat (Dasyurus maculatus). Once they have been
introduced, pest species such as the cane toad are often virtually impossible to eradicate from
the forest; it seems that they will simply remain there forever. By bring such species with
them, humans have managed to break geographical barriers between regions that had
previously stood firm for many millions of years. It is the very fact that these barriers were
once so effective that has allowed evolution to take so many different but parallel paths,
giving rise to the fascinating range of rainforest biotas that exists across the world. It seems
possible that from from this age forward, humans and the introduced animals and plants they
leave behind will turn out to have a profound influence on the evolution of rainforest biotas.

Rainforests of the Amazon and Orinoco.

Within the linked catchment zones of the great Amazon and Orinoco rivers lies the world's
largest contiguous area of equatorial rainforest. Moist Atlantic air travelling westwards is
thought to supply most of the region's rainfall, but the wettest climates are found on the
opposite side of the continent close to the Andes, where the air is forced to rise and shed its
heavy burden of water vapour. As it travels west from the coast, the prevailing air stream
picks up much of the water vapour evaporated back up from the forest and redeposits it as rain
further on. Despite the recycling, there is also a constant loss of water as it enters the massive
Amazonian river system and flows straight down to the Atlantic Ocean. The role of moist
Pacific airmasses in contributing to the high rainfall in the western Amazon remains
uncertain, although some studies suggest that they are in fact important. Whatever the causes,
the climate becomes drier from West to East across the Amazon Basin, but moister again as
one approaches the Atlantic coast in the north-east.

In general, the Amazonian forests exist under a rather moister climate than their African
counterparts. Although there are huge areas of semi-evergreen rainforest towards the southern
and eastern margins of the Amazon region, there is also a very extensive core area of
evergreen forest. In these wetter parts of the region, with annual rainfall above about 2000mm
and with no strong dry season, the forest is taller and richer in species. Without the
regimentation of strong dry seasons, there is less reason or opportunity for biological
processes to be synchronized, and growth and reproduction of plants and animals occurs more
evenly around the year.

The annual rainfall along the eastern foothills of the Andes is particularly high (reaching
9,000mm in places), and distributed fairly evenly throughout the year. In this ever-wet
climate, species richness of trees, lianas, shrubs and herbaceous plants reaches almost
unbelievable levels. One study found 283 species of trees and lianas with stem diameters over
10cm in a tiny 0.1 ha sample of forest in the Napo region of Peru. Furthermore, many of the
plant species within these forests of the Andean foothills have very localised distributions,
sometimes being found on just one or two hilltops, with their place being taken by other
similarly localised species elsewhere.This adds even further to the species richness on a
broader scale, making these forests a truly remarkable reservoir of biological diversity.

Shared By: