The origins of diversity in tropical rain forests

The origins of diversity in tropical rain forests

TREE vol. 8, no. 4, April THERE ARE MANY THEORIES to account for the fact that tropical rain forests are vastly more diverse in species than tempera...

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TREE vol. 8, no. 4, April

THERE ARE MANY THEORIES to

account for the fact that tropical rain forests are vastly more diverse in species than temperate forests. A particularly popular one in recent years has been the refuge theorylw4. This postulates that during major adverse climatic phases of the Pleistocene, populations of plants and animals were divided into subpopulations in refugia, thus permitting allopatric Reunited by climatic speciation. amelioration, the new species remained genetically isolated. Repetition of this process by repeated climatic oscillation led to a ‘species pump’ and hence to the present high diversity. As a corollary to the theory, areas of present exceptionally high diversity are thought to indicate the locations of the Pleistocene refugia. Attractive as it is, the theory and its corollary have nevertheless received some criticism. It has not yet been clearly demonstrated that the climatic oscillations in the tropics restrict rain forests in the way forest popurequired 5-g. Temperate lations were also restricted to refugia in adverse climatic phases”. Why, therefore, does the species pump concept not apply equally in temperate regions? Some of the areas of present high diversity have been shown to be at-tefacts of collection density ‘I. Some of the proposed refugia have been shown by palynology not to have borne forest during the last adverse climatic phase’2f’3. A new paper by Bush et a1.14 throws further light on this and other matters concerning tropical rain forests. These authors have investigated the palaeoecology of an area of lowland rain forest at 650 m altitude in Panama. The main technique used was palynology of the deposits of Lake La Yeguada, which yielded a continuous record covering the last 14 000 years. Diatoms, phytoliths and sediment mineralogy and chemistry were also studied. These records document changes in lowland vegetation communities through a major climatic shift and through the onset of human disturbance. The pollen record demonstrates that between 14 300 BP and 11000 BP the land around La Yeguada supported a forest rich in Quercus, llex and Myrtaceae. These taxa do not occur together at present below c. 1500 m altitude. Modern pollen rain data are quoted to demonstrate that the pollen record is not an arteJohn Flenley is at the Geography Dept. Massey University, Palmerston North, New Zealand.

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TheOrigins ofDiversity inTropical Rain Forests John Flenley fact of long distance pollen transport, and Bush et al. conclude that montane taxa formerly co-occurred >800 m lower than now. Using a temperature lapse rate of 6°C per 1000 m altitude, this translates into a cooling of c. 5°C in the Late Pleistocene. The period from 11 000 to IO 500 BP was transitional. Montane elements continued their high pollen influx, but pollen of lowland taxa such as Cecropia, Bocconia, Cek, Byrsonima, Bursera, Pilea, Mortoniodendron, Proteaceae and Trema became increasingly important. After 10500 BP, pollen (and phytoliths) of an assortment of lowland forest taxa dominated the record, although Quercus persisted in low amounts until 4000 BP. The record of human occupance and land use around La Yeguada is also of exceptional interest. Particulate carbon (probably charcoal) suddenly increases by more than an order of magnitude at c. 11 000 BP. This is during a period when diatom and sedimentological evidence shows that lake level was rising and climate becoming moister, so the possibility that the charcoal resulted from natural fires rather than human activity is extremely remote. There was a rise in Gramineae pollen from IO 500 BP, suggesting forest clearance. At c. 4000 BP there was a sudden decline in arboreal pollen taxa, and the first occurrence of Zea phytoliths, suggesting intensified human activity. How does all this affect refuge theory? The main point is that, according to the theory, the upper part of the lowland forest zone (say, around 650m, the altitude of Lake La Yeguada) was supposed to be a refugium for lowland rainforest elements during the Pleistocene, when the lower altitudes became too dry for rain forests. According to the new pollen record, the space around 650 m was unavailable as a refugium, because it was occupied by lower montane elements (Ouercus, Hex, Myrtaceae) at that time, and was therefore probably too cool for lowland elements. This does not necessarily mean that the theory is wrong: there may have been refugia but not where we first thought. Perhaps the refugia were along the coast. Sea level was lowered eustatically by c. 100 m in the Pleistocene cool phases, so the evidence for refugia may now be offshore. Alternatively,

if the lower part of the lowland zone (say O-500 m altitude) was warm enough for lowland forest, but mostly too dry for it, forest elements could have survived in linear refugia along river banks. This is how rain forest survives at present on the edge of its range in Australia. The new evidence for prolonged human activity in the rain forest could give a boost to another aspect of diversity theory. It has been argued15,16 that the highest diversity is produced by a disturbance regime that is intermediate between stability and complete instability. Could the 7000 years of human disturbance (between the start at 11 000 BP and the deforestation from 4000s~) have provided the intermediate disturbance that this theory demands? New pollen evidence from New Zealand (A. Cole, pers. commun.) suggests that, in the temperate rain forest there, the intermediate disturbance levels just before deforestation may lead to a peak of diversity. In conclusion, the paper by Bush et al. has given a new stimulus to empirical palaeoecology of lowland rainforest areas. Clearly, their finding of cooling in the tropical lowlands needs to be substantiated at other Latin American sites or in other rainforest regions of the world. If supported (and there is already some support from Indonesian work17), then general circulation models for climatic change will have to be rewritten. Also, the empirical search for the tropical lowland rain forest’s Pleistocene refugia must continue. Until they are located, or are shown to have been unnecessary because the forest survived virtually unchanged, ecological understanding of the modern rain forest will lack a firm foundation.

References 1 Haffer, J. (1969) Science 165,131-137 2 Vanzolini, P.E. and Williams, E.E. (1970) Arch. Zoo/. Estado Sao Paul0 19, 1-124 3 Brown, K.W., Jr (1972) Zoologica 57, 41-69 4 Prance, G.T. (1973) Acta Amazonica 3, 5-28 5 Livingstone, D.A. (1982) in Biological Diversification in the Tropics: Proceedings of the Fifth International Symposium of the Association for Tropical Biology (Prance, G.T., ed.), PP. 523-536, Columbia Universitv Press k.Connor, E.F. (1986) Trends EC&. Evol. 1,165-168

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7 Liu, K. and Colinvaux, P.A. (1985) Nature 318,558-557 8 Colinvaux, P.A. (1989) Nature 340, 188-189 9 Markgraf, V. (1989) 0. Sci. Rev. 8, l-24 10 Huntley, B. and Birks, H.J.B. (1983) An Atlas of Past and Present Pollen Maps for Europe: O-13 000 Years Ago (vols 1,2), Cambridge University Press

11 Nelson, B.W., Ferreira, C.A.C., da Silva, M.F. and Kawasaki, M.L. (1990) Nature 345,714-716 12 Salgado-Labouriau, M.L. (1980) Rev. Palaeobot Palynol. 30,297-312 13 Bradbury, J.P. eta/. (1981) Science 214,1299-1305 14 Bush, M.B., Piperno, D.R., Colinvaux, C.A., De Oliviera, P.E., Krissek, L.A.,

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Miller, MC and Rowe, W.L. (1993) Ecol. Monogr. 62,251-275 15 Connell, J.H. (1978) Science 199, 1302-1309 16 Huston, M. (1979) Am. Nat 113,81-101 17 Barmawidjaja, D.M., de Jong, A.F.M., van der Borg, K., van der Kaars, W.A. and Zachariasse, W.J. (1989) Neth. J. Sea Res. 24,591-605

Woodland WaterBalance Todd E. Dawson FOREST AND WOODLAND ECOSYSTEMS cover approximately 55 000 000 km* (c. 37%) of the earth’s terrestrial surface’. The trees in these ecosystems are a primary route or ‘conduit’ by which water in soils and groundwater aquifers re-enters the hydrologic cycle2r3. In recent years, and especially since human activities have begun dramatically to reduce tree cover over the surface of the globe, ecologists and hydrologists have begun to work on improving our understanding of the role that individual tree species and tree stands play in the water balance of forest and woodland ecosystems. Such an understanding is important to improving predictions concerning the influences that vegetated surfaces play in regional and global ecological and hydrological processes, as well as how these systems may respond to increased deforestation, fragmentation and climatic changes. Several research groups have been engaged in constructing the water balance for specific forest types4g5 and tree species”. Results from these and other’-” investigations have demonstrated that broadleaf and coniferous forests with continuous cover (‘closed’ forests) may behave similarly with respect to water balance. Of particular interest, however, is what can be learned in forest ecosys‘open’ transitional tems, such as woodlands or savannas where the trees are imbedded in a matrix of grassland or other nonwoody vegetation. In this respect, Mediterranean environments provide an ideal setting. Recently, two research groups have begun to provide information about the water flux and water balance of ‘open’ Mediterranean forests and Todd Dawson is at the Section of Ecology and Systematics, Cornell University,Corson Hall, Ithaca, NY 14853-2701,USA. 120

woodlands. Richard0 Valentini and his colleagues5 have used an eddy correlation technique (Box I) to investigate both water vapor and carbon dioxide flux in oak and juniper woodlands along the Tirrenian coastal plain of Italy. Their objectives were to make accurate measurements of tree canopy flux rates as an avenue for eventual scaling-up to entire ecosystems. Comparing the results of the eddy correlation findings with those obtained from Bowen ratiog*“‘, canopy resistance/ aerodynamic resistance3r4 and omega factor”*8 analyses (Box I), they found general agreement among the different analyses about the behavior of plant canopies in this ecosystem and also discovered that the woodlands were intermediate in water balance to coniferous and deciduous forests. important These results are because they demonstrate that the eddy correlation technique can be used as a reliable tool for evaluating water flux at the canopy scale. Moreover, this technique can be used to test ecophysiological models of canopy gas exchange behavior or validate ecosystem-level water balance estimates. Such estimates are being provided by a second team of investigators working in Spain. Richard Joffre and Serge Rambal”~‘* have taken an approach that focuses on the entire ecosystem. Their work is providing evidence that for Mediterranean ‘dehasa’ ecosystems, which cover a large portion of south-western Spain and parts of Portugal, water balance can vary dramatically, and this variation can be directly linked to whichever vegetation component is being studied: the oak-grass woodland or grassland savanna. Joffre and Rambal used a systemlevel water balance approach (Box I), which incorporates having to

account for changes in precipitation, water storage of soils, evapotranspiration (the sum of soil evaporation and plant transpiration), surface run-off and deep drainage (flow below plant roots). They demonstrate that the water balance of grass and tree-grass components can differ by as much as 50%. This difference is caused by the spatial heterogeneity of water resources among the different vegetation components. The authors quantified these differences in a number of ways. For example, even though tree cover in the tree-grass component averaged between 15 and 20% of the total cover (40-50 trees/ha), evapotranspiration 45-60% was greater than for grasses alone. At the same time, soil water storage was much greater for the tree-grass component of the ecosystem. The authors suggest that this finding is likely to be due to greater soil development beneath the trees (greater porosity, higher percentage of soil organic matter, etc.), which would lead to a greater capacity for soil water storage. Poorer soil development and lower total plant cover in the relatively more open grass component of the ecosystem lead to much greater annual surface run-off and deep drainage. As expected, transpiration by trees had a very strong influence on site water balance, mostly because trees have access to a greater proportion of the soil volume than grasses and thus represent the dominant component of evapotranspirational water flux from the system. Interestingly, Joffre and Rambal found a 20-50% disagreement between transpiration actually measured on trees and that computed from the water balance equation. They advance two possible explanations for the discrepancy: (1) failure to account for roots that may extract water from below the zone where their soil moisture determinations were made and (2) their not having accounted for roots involved

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