rainforests and ice ages

rainforests and ice ages Rainforests that are rich in plant and animal species occur today in tropical lowlands of the Earth, up to about 1300 m above sea level. Here temperatures are high with little daily or seasonal variation and precipitation is abundant throughout the year. At higher elevations, and up to about 3000 m in the tropics, montane rainforest is found. This is less diverse taxonomically but contains some species in common with its lowland counterpart.

Until about thirty-five years ago these rainforests were thought to have remained unchanged for a long period of geological time. Their diverse flora and fauna were considered mainly a response to a stable and an equable climatic regime. In particular, the series of Quaternary ice ages, whose harsh climates have lead to marked changes in the composition and distribution of biota (animal and plant life in a region) in temperate regions over the past two million years, were regarded as having had little affect on life in low latitudes, where they caused wetter (pluvial) episodes. However, as John Flenley has stated in his seminal study of the equatorial rainforest, later geological and biological research has shown these ideas to be wrong: it has demonstrated that environments in low latitudes have undergone significant modifications. These were especially marked during Quaternary time, the cold stages of which, while giving rise to extensive glaciations in high and middle latitudes, mainly caused aridity (rather than wetness) in low latitudes. In addition it must not be overlooked that the majority of tropical mountains higher than about 3800 m above sea level were glaciated in the Quaternary. It is in these localities that the environmental perturbations brought about by the last major cold episode, whose ice disappeared at various times between about 15 000 and 9000 years before the present (bp), remain most noticeable.

Climate models and evidence from fossils indicate temperatures 2 to 4 degrees C below those of today in tropical lowlands during Quaternary glacial episodes; mountains at these latitudes experienced a reduction of between 5 and 15 degrees C. It has been predicted that mean annual precipitation amounts fell by 20 per cent during glacial cycles, and that rainfall incidence was more seasonal than at present. As Paul Colinvaux has noted though, a one-fifth reduction in annual precipitation in tropical lowland forest areas (between 2 and 7 m in Amazonia today) would still mean substantial rainfall, and render it possible that the ecological tolerances of a large number of rainforest species were not exceeded. Moreover, as Colinvaux has emphasized, arid conditions were unlikely to have occurred everywhere at low latitudes. Modifications in atmospheric and oceanic circulation patterns would have caused local effects, raising some levels of precipitation and reducing others.

Information concerning the past vegetation of current rainforest areas has come from plant microfossils (pollen and spores) and macrofossils (seeds, leaves, wood) preserved in lake sediments and peat bogs. Most data are from uplands, where climatic cooling lead to the contraction and lowering of vegetation belts, the latter by up to 1500 m. Tropical lowlands have produced fewer fossil records. These have made it possible to infer alternating episodes of forest, within cold stages of the Quaternary, savannah, and shifts in the latitudinal boundaries of these vegetation types by about 5° (see tropical refugia).

South America

In South America there is currently a vast expanse of rainforest, mainly in the Amazon Basin and across the Andes on their Pacific side in Columbia and Equador. Vegetation changes in this region over the past few million years have been elucidated largely by T. van der Hammen and his co-workers. A sequence of lake sediments, about 200 m in thickness, from Sabana de Bogotá, 2600 m above sea level in the Colombian Andes and now within montane forest, has produced plant remains which show that Pliocene (Late Tertiary) vegetation in its environs was akin to present-day lowland tropical forest. By the Early Quaternary, Paramo (grassland) vegetation, currently distributed above the montane (mountain) forest, had developed, denoting a cooling of climate. Subsequent changes in the pollen record illustrate shifts between grassland and montane forest as vegetation zones moved up and down in response to climatic fluctuations. Amazonia has yielded little palynological data pertinent to Quaternary vegetation history, but that which exists has been interpreted as depicting the replacement of rainforest by savannah a number of times over this timespan. However, Colinvaux has pointed out that Amazonian ecosystems are currently characterized by a high level of physical disturbance—because of flooding and burning, for example. Such factors would have been operational throughout the Quaternary, and he has suggested that the effects of ice ages have been given undue importance in the explanation of vegetation changes in this region. Pollen evidence gathered by Mark Bush and Paul Colinvaux from lowland Panama probably reflects vegetation changes over the past 150 000 years in this part of Central America. They were unable to demonstrate conclusively the replacement of rainforest by savanna during glacial episodes.

Non-biological evidence of former climatic regimes in tropical Latin America has come from lower Amazonia, north-eastern Brazil, and eastern Colombia, where tropical rainforest now grows in soil developed within unconsolidated rock material—the regolith—that was deposited under arid conditions, purportedly during one or more cold periods of the Quaternary.

Africa

African rainforest is today less diverse than that of other tropical regions. This is perhaps the result of biotic extinctions during arid episodes according to Paul Richards. Compared with other regions, it also lacks major areal extent and is moreover located in two parts, one in the west and one in the east of the continent, both with montane elements.

The Quaternary vegetational history of East Africa is best known. It has been synthesized by a major contributor to its elucidation, A. C. Hamilton. During an ice-age climate about 26 000 to 14 000 years BP, the plant cover of areas now with rainforest resembled that currently growing in areas either with lower precipitation, or with precipitation less evenly distributed seasonally. Between about 14 000 and 8000 years bp, there were botanical changes which culminated in a flora and vegetation analogous to those of the present. This flora and vegetation have remained in existence throughout the last eight millennia.

Non-biological evidence of palaeoclimates in Africa includes sand over a large part of the terrain beneath present rainforest in the Congo. This sand is thought to have been redistributed from the Kalahari Desert during colder and drier conditions in the Middle Quaternary, and to have caused a substantial reduction in forest cover.

Asia and the Pacific

Indo-Malesia is currently well endowed with tropical rainforest, within which montane types are abundant. The floristic composition is similar in Malaya, Indonesia, the Philippines, and Papua New Guinea, but differs in southern India and Sri Lanka. The tropical Pacific islands posses an impoverished flora of Malesian character. The lowering of montane vegetation belts in New Guinea during the last ice age was broadly synchronous with that of South America and Africa. The vegetational data from upland New Guinea suggest that between about 18 000 and 16 000 years BP, temperatures there were up to 11 degrees C below those of today, and that the latter were restored 9000–6000 years ago. Sites at lower elevations in Sumatra confirm these trends and imply that 11 000–8000 years BP temperatures were 2 to 4 degrees C below those of the present.

Australia

The tropical rainforest seen today in northern Australia has floristic affinity with that of South-East Asia. The greatest continuous extent of this rainforest is in north-east Queensland. At Lynch's Crater, within rainforest on the Atherton Tablelands in this region (17 °S), the palynological record from a swamp studied by Peter Kershaw has provided a vegetation history spanning approximately the past 200 000 years. Over this time-span, rainforest with a complex composition has dominated the area around Lynch's Crater during interglacial stages, when moisture has been plentiful. Between about 165 000 and 126 000 years BP, Araucarian rainforest (of which the gymnosperms Araucaria and Podocarpus were significant constituents) developed in a drier climate. The palynological evidence suggests that the transition from wetter to drier conditions was abrupt. From about 63 000 to 26 000 years ago (within the last cold stage), Araucarian forest was paramount in the vegetation once again. This forest was gradually replaced by sclerophyll woodland composed of trees with leathery leaves (in which Casuarina and Eucalyptus were of prime importance) from about 38 000 years bp. Such woodland dominated until about 9000 years bp, when complex rainforest reappeared. A change to sclerophyll woodland from Araucarian forest did not occur within the first period of drier conditions. This fact, together with the gradual nature of the modification, lead Kershaw to postulate that climate alone did not cause it. Fire was thought to have played an important role in the change.

Reduced rainfall in north-eastern Queensland was probably the result of diminished evaporation from the ocean consequent upon lower global temperatures and eustatic changes (in sea level) during ice ages. A fall (in sea level) associated with ice accumulation would have exposed the Sahul Shelf north-west of Australia. The moisture which currently sustains the rainforest is mainly provided by north-west winds, the efficacy of which would have been reduced as they traversed land rather than water. The same effect would have been experienced by the south-east trade winds, which contribute to the climate of the eastern seaboard of Australia.

R. L. Jones

Bibliography

Flenley, J. R. (1979) The equatorial rain forest: a geological history. Butterworth, London.
Hamilton, A. C. (1982) Environmental history of East Africa: a study of the Quaternary. Academic Press, London.
Williams, M. A. J.,, Dunkerley, D. L.,, De Dekker, P.,, Kershaw, A. P.,, and and Stokes, T. (1993) Quaternary environments. Edward Arnold, London.