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Three pollen diagrams of forest disturbance in the western amazon basin
Review of Palaeobotany and Palynology, 55 (1988): 73-81
73
Elsevier Science Publishers B.V., Amsterdam - - Printed in The Netherlands
THREE POLLEN DIAGRAMS OF FOREST DISTURBANCE IN THE WESTERN AMAZON BASIN P.A. COLINVAUX z, M. FROST 1, I. FROST z, KAM-BIU LIU 2 and M. STEINITZ-KANNAN 3 1Department of Zoology, The Ohio State University, Columbus, OH 43210 (U.S.A.) 2Department of Geography and Anthropology, Louisiana State University, Baton Rouge, LA 70803 (U.S.A.) 3Department of Biology, Northern Kentucky University, Highland Heights, K Y 41076 (U.S.A.) (Received September 22, 1987)
Abstract Colinvaux, P.A., Frost, M., Frost, I., Liu, K.-B. and Steinitz-Kannan, M., 1988. Three pollen diagrams of forest disturbance in the western Amazon Basin. Rev. Palaeobot. Palynol., 55: 73-81. Pollen profiles have been obtained from sediment cores t a k e n in three old river channel lakes in the western Amazon basin of Ecuador. Pollen was recovered from flood deposits of the 1300-800 yr B.P. interval and of the last 800 or so years of continuous gyttja deposition. Pollen spectra show t h a t a detailed pollen record of Amazonian forests is preserved. The disturbed vegetation of the flooding episode is recorded, together with subsequent changes in vegetation which were synchronous at sites up to 100 km apart. Pollen data suggest t h a t forest changes following the 800-yr old disturbance are still in progress. Different pollen assemblages from those of the central Amazon region show t h a t different forest associations occupy areas within the lowland forest of Amazonia as a whole. It is suggested t h a t forest heterogeneity, r a t h e r t h a n the print of postulated ancient refugia, accounts for disjunct distributions of Amazonian biota.
Introduction We report three pollen diagrams from riverine lakes of the western Amazon basin. The records are short, dating from the local prolonged inundation that lasted between about 1300 and 800 yr B.P. (Colinvaux et al., 1985). They reveal, however, the response of local vegetation to t h a t inundation and the long developmental process which resulted in the present riverain forests of the region. Lake Santa Cecilia (0°4'N 77°1'W), Lake Agrio (0°7'N, 76°55'W), and Limoncocha (the Quechua suffix cocha means "lake") (0°24'S, 76°38'W), occupy deep sections of abandoned channels of the Aguarico and Napo rivers (Figs.l-3). Water chemistry and other limnic properties are given in Steinitz-Kannan et al.
(1983) and Colinvaux et al. (1985). Lake Santa Cecilia and Lake Agrio lie close to the Andean foothills, but are well down in the lowland rain forest at 330 m elevation. Limoncocha is further east, about 120 km from the foothills and at about 230 m elevation. All three lakes have accumulated gyttja continuously for the last 700 to 800 radiocarbon years, although at different rates, with 6.0 m of gyttja deposited in Lake Santa Cecilia but only 1.5 m in Lake Agrio and Limoncocha. Beneath the gyttja in all three lakes are deposits of riverine silt and clay of varying thickness (Santa Cecilia 4 m, Limoncocha 2.5 m, Agrio 0.5 m), under which is riverine sand. In a fourth lake, Afiangucocha, a similar sequence is underlain by older peat and gyttja. The parallel sedimentary sequence in all four lakes has been interpreted
74
A COLOMBIA
I .E.u
o"
\~
Fig.1. Map of Ecuador, showing study sites. S C = L a k e Santa Cecilia, L A = Lake Agrio, L M = Limoncocha. Rectangular areas are enlarged in Figs.2 and 3.
Fig.3. Location and bathymetry of Limoncocha. Depth contours in meters. Coring sites are indicated by a cross.
as showing that all four basins are sections of ancient river channels that were reinvaded by t h e p a r e n t r i v e r s a r o u n d 1300 y r B.P., o c c u p i e d b y f l o o d w a t e r u n t i l a b o u t 800 y r B.P., t h e n
abandoned again by the rivers to exist in i s o l a t i o n e v e r s i n c e ( F r o s t , 1984; C o l i n v a u x e t al., 1985). T h e e x t r a - w e t r e g i o n a l c l i m a t e o f t h e f l o o d i n t e r v a l (1300-800 y r B.P.) a p p e a r s t o
Fig.2. Location and bathymetry of Lake Santa Cecilia and Lake Agrio. Depth contours in meters. Coring sites are indicated by a cross.
75 have had effects not only in the western Amazon but also on the eastern side of the Cordillera Oriental (Colinvaux et al., 1988). Even now the region is one of the wettest in the Amazon basin. Ecuadorian climate maps show the area as receiving in excess of 5 m of precipitation annually, though local measurements at Limoncocha recorded only 3259 mm in a recent year, much of it falling in short intense storms depositing 200 mm in 24 hours. Instead of the marked dry season of the central Brazilian Amazon, rainfall slackens only about 40~/o between November and February and effective precipitation still exceeds effective evaporation by an order of magnitude even then (Landlivar, 1977; Naranjo, 1981). Thus the region has a wet rain forest climate year-round and the typical vegetation is a closed, evergreen forest. There are none of the open vegetation types characteristic of flood plains in central Brazil. The Napo, Aguarico, and other rivers are highenergy streams of large size, driven both by their headwaters in the high Andes and by locally heavy rains. In this landscape, the three ancient riverine lakes are not comparable to the varzea lakes of the central Amazon, which flood and drain annually, and the pollen histories of which are written largely in fluctuations of pollen of grasses and other colonizing herbs (Absy, 1979). Instead, in the western Amazon we can report pollen histories of facies of true rain forest vegetation.
Regional vegetation No local vegetation surveys of the region exist, though one has now been started (J.E. Lawesson, pers. comm., 1985). Published accounts of the western Amazon rely heavily on comparison with central Amazonia, the rather different vegetation of which is better known. Vegetation descriptions are given by Grubb et al. (1963), Moore (1973), and by Brandbyge and Azanza (1982). Three main vegetation types are generally
recognized in the Amazon: igapo forest on continuously flooded or marshy sites, varzea forest on periodically flooded areas, and terra firme forest above seasonal high water marks (Hueck, 1966). Of these only the concept of the very general forest type of terra firme translates well to our region. Where there is no seasonal drying, identifying any forest as varzea seems unsafe. Igapo may also be a technical term to be used only with caution in our region, though comparable stands of palms in wet bottom lands are common. Undoubtedly a long continuum of forest communities can be found along moisture gradients between the habitats of palms in bottom lands and the forests of terra firme. For the purpose of this analysis we recognise in place of the three general Amazonian types: local terra firme, wet palm forest, and the floating aquatic community. Wet palm forest is so common that large areas can be seen from low-flying aircraft, all with a glint of water between the fronds. Stands of Mauritia flexuosa characterize this community, but numerous non-palm genera are also present. Taxon diversity is low by the standards of other Amazon forests. Diverse rain forests on terra firme have large species lists in such genera as Bombax, Ficus, and Gustavia. Numerous seral communities of gap colonizers occur and such early colonists as Cecropia are always in the area. Patches of this forest reach the higher banks of all three of the studied lakes, where the tree branches overhang the water (Fig.4). Extensive and varied communities of emergent or floating aquatic plants occur in the lakes, particularly in Limoncocha. These may range from floating mats, which may exist with or without bottom rooting, and a series of emergent communities along transects from open water to marshland on more gradual shorelines (Fig.5). Major components of the floating mats are grasses such as Paspalum and Echinochloa, together with sedges (Junk, 1970), suggesting t h a t grass and sedge pollen in the region might be from this aquatic vegetation.
76
Fig.4. Terra firme forest on the bank of Lake Agrio.
Fig.5. Aquatic vegetation and shoreline at Limoncocha.
77 Methods Five parallel cores were raised from near the center of Lake Agrio, together with a single core from Lake Santa Cecilia in February 1980, and two parallel cores from Limoncocha in J u l y 1982. All cores were taken from a raft of rubber boats with a modified Livingstone sampler, and the cores were returned to the Columbus laboratory unopened in the aluminum alloy sample tubes in which they were taken. This coring procedure, together with subsequent sampling and treatment, including X-radiography, radiocarbon dating, and pollen analysis, is described in Colinvaux et al. (1988). Wherever possible, a count of 250 pollen grains was made for each sample. Pollen identification was based on a pollen reference collection of about 3000 neotropical taxa and on published plates for this and adjacent regions (e.g. Absy, 1979; Van der Hammen and Gonzalez, 1960). R e s u l t s and d i s c u s s i o n Gross stratigraphy and radiocarbon chronology of the sediments are given at the left of Figs.6-8 and are as in Colinvaux et al. (1985). Although pollen concentration was measured in all samples, radiocarbon control was deemed inadequate for the calculation of pollen influx so we present only pollen percentage diagrams (Figs.6-8). Pollen concentrations were in the range of 60,000-80,000 grains per cm 3, which, for sedimentation rates approximating 2 m per millennium in the gyttja bodies of Lago Agrio and Limoncocha, yields typical total pollen influx of 15,000 grains cm- 2 y r - 1. The taxon composition of the pollen spectra at all three lakes is similar, as well as strikingly different from the taxon composition of pollen spectra in central Amazon varzea lakes such as Lake Surara and Lake do Caj (Absy, 1979). Not only are the details of the taxon lists different in the western and central parts of the Amazon, but the western sites do not have the Gramineae peaks, with associated herbs, t h a t Absy interpreted as evidence of dry conditions. Furthermore, the Cecropia pollen
that is so notable a constituent of the western Amazon diagrams appears lacking in the central Amazon samples. Another difference t h a t may be more apparent than real is the absence of myrtaceous pollen in our diagrams, though the family is prominent in the central Amazon; we did not count pollen of this family as we had used Eucalyptus as our exotic pollen marker for the pollen concentration counts, but we do not think many myrtaceous grains were actually overlooked because we found very few in surface samples t h a t were not prepared with eucalypt pollen. This first comparison of pollen from central and western parts of the Amazon basin suggests t h a t the species composition of the forests of the two areas is very different, as would be expected from the different climates of the two regions. In Figs.6-8 the pollen percentage diagrams for one core from each of the three lakes are divided into three zones, though the bottom zone is missing from the Lake Agrio profile (Fig.8). The three zones are listed below.
Zone 1 (Mauritia-fern spore zone) All pollen grains and spores of this zone come from clastic riverine sediments of the flooding event of about 1300-800 yr B.P. This may also be called the "low Cecropia" zone in which Cecropia pollen remains below or near 20~/o. Mauritia palm pollen approaches 10°/0 in all samples, but is present in only trace amounts in the other zones. Fern spores and short-spined grains of the Tubuliflorae are at maxima.
Zone 2 (Cecropia-Urticaceae-Moraceae zone) Pollen in this zone comes from the base of the organic gyttja, or from the transition between allochthonous riverine sediment and autochthonous gyttja. Cecropia increases to more than 50% of the total pollen in all three profiles. Most other taxa in Urticaceae and Moraceae increase. (The majority of the grains
78
LIMONCOCHA
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~-,-~,. o.
! II
2
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) ;
~
I I
0 10 203040 ~060
0
102030
%
Fig.6. Pollen percentage diagram from Limoncocha. Pollen sum includes all pollen taxa counted but e x c l u d e s fern spores.
LAGO SANTA CECILIA
.2
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,~"
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/
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~# t,-~'L4":.4"~d',.~;./;,~"., ;.fT
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Fig.7. Pollen percentage diagram from L a k e S a n t a Cecilia. Pollen sum includes all pollen taxa counted but excludes fern spores.
in our undifferentiated Urticaceae-Moraceae group are diporate (P2) or triporate (P3), but we also include in this category some 4-porate (P4) and 5-porate (P5) grains with a psilate to granulate exine suggestive of pollen of these two families.) Mauritia pollen declines to a trace.
Zone 3 (Iriartia-Urticaceae-Moraceae zone) This is the upper pollen zone, which is also characteristic of the surface, mud and water interface samples. Cecropia declines from the high of zone 2 and Iriartia (Palmae) becomes prominent reaching up to 20% in two profiles.
79
LAGOAGR~
"
"-
~
/I./////L,A?¢
. HA
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%
Fig.8. Pollen percentage diagram from Lake Agrio. Pollen sum includes all pollen taxa counted but excludes fern spores.
The forest taxa of Urticaceae and Moraceae are collectively at a maximum.
Interpretation of the pollen record The three-zone sequence apparently describes the recovery of the riverain forest of the western Amazon basin from gross disturbance caused by regional flooding. When the valleys of both the Aguarico and Napo rivers were inundated (zone 1), pollen taxa characteristic of the modern terra firme forests were not prominent, which is to be expected after destructive flooding. The wet palm forest typified by stands of Mauritia became more extensive, as what are now terra firme became marshy or waterlogged. The increase in fern spores, and the maxima of sedge and grass pollen at Limoncocha (Fig.6) suggest increased aquatic vegetation, perhaps of the floating mat type. Zone 2 shows the response of western Amazonian vegetation to the gradual creation of terra tirme with the coming of a drier climate. The most immediate response was a large increase in abundance of pollen, and presumably in population, of the forest-colonizing genus Cecropia. Cecropia, an anemophilous taxon, seems to be regularly overrepresented in western Amazonian and Andean pollen diagrams and its pollen may be dispersed well outside the limits of the parent populations (Hansen et al., 1984). But the very large increase at the bottom of zone 2 at sites
spanning more than 100 km suggests a large increase in population. This suggests that forest successions were beginning everywhere on land that had dried as climate changed. The reduction of the wet palm forest is reflected in a precipitous decline in Mauritia pollen, synchronous with the rise in Cecropia. Zone 3 indicates continuing forest succession, because numerous forest taxa increase in importance as Cecropia declines. Probably, part of the apparent increase in forest taxa reflects no more t h a n a real decline in Cecropia influx in zone 3; yet the details of fresh taxa appearing, and of the changing percentages between taxa, suggest an actual increase in influx of forest taxa over and above the statistical effect of the Cecropia decline. By the time of sampling, this successional process had led to the present vegetation of the region: diverse rain forest on all sites above high water, restricted wet palm forest on bottom lands, and bands of aquatic plants floating at the margins of the lakes. The rise of Iriartia pollen in zone 3 appears to be associated with the general development of the present vegetation. The sequence of Mauritia to Iriartia from zone 3 to zone 1 is not a simple replacement but seems to be separated by an interval, suggesting t h a t the population histories of the two palm genera are independent. Probably the significance of the Iriartia rise cannot be understood without further field studies on the species. Vegetation at the three sites has been
80
disturbed by human activities at different times in the recent past. Land around Lake Agrio (Fig.4) and Lake Santa Cecilia may have had minimum disturbance until very recently, except for the pattern of diffuse swidden agriculture to which all the western Amazon may have been subject (Meggers, 1966; Duellman, 1978). But clearing is extensive around Limoncocha, suggesting long occupancy; and the distinctive properties of the upper part of zone 3 at Limoncocha (Fig.6) may reflect this, particularly perhaps in the appearance of such disturbance indicators as Celtis and Trema (Ulmaceae), and possibly the lesser expression of the Iriartia rise.
equilibrium processes operating at intermediate time scales (ConneI1, 1978; Campbell and Frailey, 1984; Colinvaux et al., 1985; Saldarriaga and West, 1986; Salo et al., 1986). The second rests on the difference between pollen spectra from the western Amazon and those reported from parts of the Central Amazon by Absy (1979, 1985)-. : C l ~ l y , significantly different forest communities are represented in the pollen records of different parts of Amazonia, leading to the suggestion that areal differences in modern Amazonian vegetation, rather than supposed patterns of refugia in the past, account for disjunct distributions of Amazonian biota.
Concluding note
Acknowledgements
Possibly the most interesting thing to be said about these pollen diagrams from the true rain forest of the Amazon basin is that they demonstrate how well a complex record of forest history is preserved by pollen. Despite the fact that the vegetation is hugely diverse, with most of its important plants pollinated by insects, birds, or even bats, an interesting array of pollen types yet collects in the lake sediments. The pollen rain is not drowned out by pollen of grasses or of the other colonist herbs. Before the record can be read in more detail we must learn more of the pollen morphology of many types and perhaps devise better methods of extraction and analysis. Because of low recoveries we had to become resigned to spending a week or more to count enough microspores for a single pollen sum. Much also will have to await better ecological knowledge of the plant populations represented by the pollen, but the data show that a record of fairly detailed changes in the forest is preservedl Two last ecological observations seem warranted. The first is that the pollen diagrams cannot be taken to suggest that the succession following the flooding that ended 800 years ago is over: rather they suggest change still in progress. Our data are consistent with the view that Amazonian diversity is a function of non-
Like the inter-Andean operations described in Colinvaux e t al. (this issue) this work depended on the hospitality of H. and T. Steinitz and our many helpers in Ecuador. In addition we thank J. Vela for providing our base at Limoncocha, M. Eggart for drawing the diagrams, and C. Vasquez, K. Olson, S. Longenbaker and B. Bethel for technical assistance. The research was funded by grants from NSF to The Ohio State University.
References Absy, M.L., 1979. A palynological study of Holocene sediments in the Amazon. Thesis. Univ. Amsterdam, 86 pp. (unpublished). Absy, M.L., 1985. The palynology of Amazonia: The history of the forests as revealed by the palynological record. In: G.T. Prance and T.E. Lovejoy (Editors), Amazonia. Pergamon Press, Oxford, pp.72-82. Brandbyge, J. and Azanza, E., 1982. Report on the Fifth and Seventh Danish-Ecuadorian Botanical Expedition. Univ. Aarhus, Risskov, 138 pp. Campbell, K.E., Jr. and Frailey, D., 1984. Holocene flooding and species diversity in southwestern Amazonia. Quat. Res., 21: 369-375. Colinvaux, P.A., Miller, M.C., Liu, K.-B., Steinitz-Kannan, M. and Frost, I., 1985. Discovery of permanent Amazon lakes and hydraulic disturbance in the upper Amazon Basin. Nature, 313: 42-45. Colinvaux, P.A., Olson, K. and Liu, K.-B., 1988. Lateglacial and Holocene pollen diagrams from two endorheic lakes of the Inter-Andean Plateau of Ecuador. Rev. Palaeobot. Palynol., 55: 83-99.
81 Connell, J.H., 1978. Diversity in tropical rainforests and coral reefs. Science, 199: 1302-1310. Duellman, W.E., 1978. The biology of an equatorial herpetofauna in Amazonian Ecuador. Univ. Kans. Mus. Nat. Hist., Misc. Publ., 65, 352 pp. Frost, I., 1984. A paleolimnological and palynological investigation in the Ecuadorian rain forest: evidence of regional flooding and paleohydrological disturbance in the Amazon rainforest. Thesis. Ohio State Univ., Columbus, Ohio (unpublished). Grubb, P.J., Lloyd, J.R., Pennington, T.D. and Whitmore, T.C., 1963. A comparison of montane and lowland forests in Ecuador 1: the forest structure, physiognomy, and floristics. J. Ecol., 51: 576-601. Hansen, B.C.S., Wright, H.E. and Bradbury, J.P., 1984. Pollen studies in the Junin area, central Peruvian Andes. Geol. Soc. Am. Bull., 95: 1454-1465. Hueck, K., 1966. Die W~ilder Siidamerikas. Stuttgart. Junk, W., 1970. Investigations of the ecology and production-biology of the "Floating Meadows" on the middle Amazon. Amazoniana, 4: 449-495. Landlivar, C.B., 1977. E1 Clima y sus Caracteristicas en El Ecuador. In: XI Asamblea General y Reuniones Panamericanas de Cunculta Conexas, Quito.
Meggers, B.J., 1966. Ecuador. Praeger, New York, N.Y., 220 pp. Moore, H.E., 1973. Palms in the tropical forest ecosystems of Africa and South America. In: B.J. Meggers, E.S. Ayensu and W.D. Duckworth (Editors), Tropical Forest Ecosystems of Africa and South America: a Comparative Review. Smithson. Inst. Press, Washington, D.C., pp.63-88. Naranjo, P., 1981. E1 Clima del Ecuador. Casa de la Cultura Ecuatoriana, Quito. Salo, J., Kalliola, R., H/ikkinen, I., M~ikinen, Y., Niemel~i, P., Puhakka, M. and Coley, P.D., 1986. River dynamics and the diversity of Amazon lowland forest. Nature, 322: 254-258. Saldarriaga, J.G. and West, D.C., 1986. Holocene fires in the northern Amazon Basin. Quat. Res., 26: 358-366. Steinitz-Kannan, M., Colinvaux, P.A. and Kannan, R., 1983. Limnological studies in Ecuador: 1. A survey of chemical and physical properties of Ecuadorian lakes. Arch. Hydrobiol., Suppl., 65: 61-105. Van der Hammen, T. and Gonzales, E., 1960. Upper Pleistocene and Holocene climate and vegetation of the "Sabana de Bogota" (Colombia, South America). Leidse Geol. Meded., 25: 262-315.
73
Elsevier Science Publishers B.V., Amsterdam - - Printed in The Netherlands
THREE POLLEN DIAGRAMS OF FOREST DISTURBANCE IN THE WESTERN AMAZON BASIN P.A. COLINVAUX z, M. FROST 1, I. FROST z, KAM-BIU LIU 2 and M. STEINITZ-KANNAN 3 1Department of Zoology, The Ohio State University, Columbus, OH 43210 (U.S.A.) 2Department of Geography and Anthropology, Louisiana State University, Baton Rouge, LA 70803 (U.S.A.) 3Department of Biology, Northern Kentucky University, Highland Heights, K Y 41076 (U.S.A.) (Received September 22, 1987)
Abstract Colinvaux, P.A., Frost, M., Frost, I., Liu, K.-B. and Steinitz-Kannan, M., 1988. Three pollen diagrams of forest disturbance in the western Amazon Basin. Rev. Palaeobot. Palynol., 55: 73-81. Pollen profiles have been obtained from sediment cores t a k e n in three old river channel lakes in the western Amazon basin of Ecuador. Pollen was recovered from flood deposits of the 1300-800 yr B.P. interval and of the last 800 or so years of continuous gyttja deposition. Pollen spectra show t h a t a detailed pollen record of Amazonian forests is preserved. The disturbed vegetation of the flooding episode is recorded, together with subsequent changes in vegetation which were synchronous at sites up to 100 km apart. Pollen data suggest t h a t forest changes following the 800-yr old disturbance are still in progress. Different pollen assemblages from those of the central Amazon region show t h a t different forest associations occupy areas within the lowland forest of Amazonia as a whole. It is suggested t h a t forest heterogeneity, r a t h e r t h a n the print of postulated ancient refugia, accounts for disjunct distributions of Amazonian biota.
Introduction We report three pollen diagrams from riverine lakes of the western Amazon basin. The records are short, dating from the local prolonged inundation that lasted between about 1300 and 800 yr B.P. (Colinvaux et al., 1985). They reveal, however, the response of local vegetation to t h a t inundation and the long developmental process which resulted in the present riverain forests of the region. Lake Santa Cecilia (0°4'N 77°1'W), Lake Agrio (0°7'N, 76°55'W), and Limoncocha (the Quechua suffix cocha means "lake") (0°24'S, 76°38'W), occupy deep sections of abandoned channels of the Aguarico and Napo rivers (Figs.l-3). Water chemistry and other limnic properties are given in Steinitz-Kannan et al.
(1983) and Colinvaux et al. (1985). Lake Santa Cecilia and Lake Agrio lie close to the Andean foothills, but are well down in the lowland rain forest at 330 m elevation. Limoncocha is further east, about 120 km from the foothills and at about 230 m elevation. All three lakes have accumulated gyttja continuously for the last 700 to 800 radiocarbon years, although at different rates, with 6.0 m of gyttja deposited in Lake Santa Cecilia but only 1.5 m in Lake Agrio and Limoncocha. Beneath the gyttja in all three lakes are deposits of riverine silt and clay of varying thickness (Santa Cecilia 4 m, Limoncocha 2.5 m, Agrio 0.5 m), under which is riverine sand. In a fourth lake, Afiangucocha, a similar sequence is underlain by older peat and gyttja. The parallel sedimentary sequence in all four lakes has been interpreted
74
A COLOMBIA
I .E.u
o"
\~
Fig.1. Map of Ecuador, showing study sites. S C = L a k e Santa Cecilia, L A = Lake Agrio, L M = Limoncocha. Rectangular areas are enlarged in Figs.2 and 3.
Fig.3. Location and bathymetry of Limoncocha. Depth contours in meters. Coring sites are indicated by a cross.
as showing that all four basins are sections of ancient river channels that were reinvaded by t h e p a r e n t r i v e r s a r o u n d 1300 y r B.P., o c c u p i e d b y f l o o d w a t e r u n t i l a b o u t 800 y r B.P., t h e n
abandoned again by the rivers to exist in i s o l a t i o n e v e r s i n c e ( F r o s t , 1984; C o l i n v a u x e t al., 1985). T h e e x t r a - w e t r e g i o n a l c l i m a t e o f t h e f l o o d i n t e r v a l (1300-800 y r B.P.) a p p e a r s t o
Fig.2. Location and bathymetry of Lake Santa Cecilia and Lake Agrio. Depth contours in meters. Coring sites are indicated by a cross.
75 have had effects not only in the western Amazon but also on the eastern side of the Cordillera Oriental (Colinvaux et al., 1988). Even now the region is one of the wettest in the Amazon basin. Ecuadorian climate maps show the area as receiving in excess of 5 m of precipitation annually, though local measurements at Limoncocha recorded only 3259 mm in a recent year, much of it falling in short intense storms depositing 200 mm in 24 hours. Instead of the marked dry season of the central Brazilian Amazon, rainfall slackens only about 40~/o between November and February and effective precipitation still exceeds effective evaporation by an order of magnitude even then (Landlivar, 1977; Naranjo, 1981). Thus the region has a wet rain forest climate year-round and the typical vegetation is a closed, evergreen forest. There are none of the open vegetation types characteristic of flood plains in central Brazil. The Napo, Aguarico, and other rivers are highenergy streams of large size, driven both by their headwaters in the high Andes and by locally heavy rains. In this landscape, the three ancient riverine lakes are not comparable to the varzea lakes of the central Amazon, which flood and drain annually, and the pollen histories of which are written largely in fluctuations of pollen of grasses and other colonizing herbs (Absy, 1979). Instead, in the western Amazon we can report pollen histories of facies of true rain forest vegetation.
Regional vegetation No local vegetation surveys of the region exist, though one has now been started (J.E. Lawesson, pers. comm., 1985). Published accounts of the western Amazon rely heavily on comparison with central Amazonia, the rather different vegetation of which is better known. Vegetation descriptions are given by Grubb et al. (1963), Moore (1973), and by Brandbyge and Azanza (1982). Three main vegetation types are generally
recognized in the Amazon: igapo forest on continuously flooded or marshy sites, varzea forest on periodically flooded areas, and terra firme forest above seasonal high water marks (Hueck, 1966). Of these only the concept of the very general forest type of terra firme translates well to our region. Where there is no seasonal drying, identifying any forest as varzea seems unsafe. Igapo may also be a technical term to be used only with caution in our region, though comparable stands of palms in wet bottom lands are common. Undoubtedly a long continuum of forest communities can be found along moisture gradients between the habitats of palms in bottom lands and the forests of terra firme. For the purpose of this analysis we recognise in place of the three general Amazonian types: local terra firme, wet palm forest, and the floating aquatic community. Wet palm forest is so common that large areas can be seen from low-flying aircraft, all with a glint of water between the fronds. Stands of Mauritia flexuosa characterize this community, but numerous non-palm genera are also present. Taxon diversity is low by the standards of other Amazon forests. Diverse rain forests on terra firme have large species lists in such genera as Bombax, Ficus, and Gustavia. Numerous seral communities of gap colonizers occur and such early colonists as Cecropia are always in the area. Patches of this forest reach the higher banks of all three of the studied lakes, where the tree branches overhang the water (Fig.4). Extensive and varied communities of emergent or floating aquatic plants occur in the lakes, particularly in Limoncocha. These may range from floating mats, which may exist with or without bottom rooting, and a series of emergent communities along transects from open water to marshland on more gradual shorelines (Fig.5). Major components of the floating mats are grasses such as Paspalum and Echinochloa, together with sedges (Junk, 1970), suggesting t h a t grass and sedge pollen in the region might be from this aquatic vegetation.
76
Fig.4. Terra firme forest on the bank of Lake Agrio.
Fig.5. Aquatic vegetation and shoreline at Limoncocha.
77 Methods Five parallel cores were raised from near the center of Lake Agrio, together with a single core from Lake Santa Cecilia in February 1980, and two parallel cores from Limoncocha in J u l y 1982. All cores were taken from a raft of rubber boats with a modified Livingstone sampler, and the cores were returned to the Columbus laboratory unopened in the aluminum alloy sample tubes in which they were taken. This coring procedure, together with subsequent sampling and treatment, including X-radiography, radiocarbon dating, and pollen analysis, is described in Colinvaux et al. (1988). Wherever possible, a count of 250 pollen grains was made for each sample. Pollen identification was based on a pollen reference collection of about 3000 neotropical taxa and on published plates for this and adjacent regions (e.g. Absy, 1979; Van der Hammen and Gonzalez, 1960). R e s u l t s and d i s c u s s i o n Gross stratigraphy and radiocarbon chronology of the sediments are given at the left of Figs.6-8 and are as in Colinvaux et al. (1985). Although pollen concentration was measured in all samples, radiocarbon control was deemed inadequate for the calculation of pollen influx so we present only pollen percentage diagrams (Figs.6-8). Pollen concentrations were in the range of 60,000-80,000 grains per cm 3, which, for sedimentation rates approximating 2 m per millennium in the gyttja bodies of Lago Agrio and Limoncocha, yields typical total pollen influx of 15,000 grains cm- 2 y r - 1. The taxon composition of the pollen spectra at all three lakes is similar, as well as strikingly different from the taxon composition of pollen spectra in central Amazon varzea lakes such as Lake Surara and Lake do Caj (Absy, 1979). Not only are the details of the taxon lists different in the western and central parts of the Amazon, but the western sites do not have the Gramineae peaks, with associated herbs, t h a t Absy interpreted as evidence of dry conditions. Furthermore, the Cecropia pollen
that is so notable a constituent of the western Amazon diagrams appears lacking in the central Amazon samples. Another difference t h a t may be more apparent than real is the absence of myrtaceous pollen in our diagrams, though the family is prominent in the central Amazon; we did not count pollen of this family as we had used Eucalyptus as our exotic pollen marker for the pollen concentration counts, but we do not think many myrtaceous grains were actually overlooked because we found very few in surface samples t h a t were not prepared with eucalypt pollen. This first comparison of pollen from central and western parts of the Amazon basin suggests t h a t the species composition of the forests of the two areas is very different, as would be expected from the different climates of the two regions. In Figs.6-8 the pollen percentage diagrams for one core from each of the three lakes are divided into three zones, though the bottom zone is missing from the Lake Agrio profile (Fig.8). The three zones are listed below.
Zone 1 (Mauritia-fern spore zone) All pollen grains and spores of this zone come from clastic riverine sediments of the flooding event of about 1300-800 yr B.P. This may also be called the "low Cecropia" zone in which Cecropia pollen remains below or near 20~/o. Mauritia palm pollen approaches 10°/0 in all samples, but is present in only trace amounts in the other zones. Fern spores and short-spined grains of the Tubuliflorae are at maxima.
Zone 2 (Cecropia-Urticaceae-Moraceae zone) Pollen in this zone comes from the base of the organic gyttja, or from the transition between allochthonous riverine sediment and autochthonous gyttja. Cecropia increases to more than 50% of the total pollen in all three profiles. Most other taxa in Urticaceae and Moraceae increase. (The majority of the grains
78
LIMONCOCHA
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3
t i
~-,-~,. o.
! II
2
--
) ;
~
I I
0 10 203040 ~060
0
102030
%
Fig.6. Pollen percentage diagram from Limoncocha. Pollen sum includes all pollen taxa counted but e x c l u d e s fern spores.
LAGO SANTA CECILIA
.2
.o~- j
,~"
d'
,/,/,/'
/
•
,,o o~
" ~ - ~ I - -
~# t,-~'L4":.4"~d',.~;./;,~"., ;.fT
~
//.~I
. -
/
r
m
-
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i
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mm
It
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~,--,,---),-,~,-,,--,~m~,~, .,,
,, .,'.,',
,
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Fig.7. Pollen percentage diagram from L a k e S a n t a Cecilia. Pollen sum includes all pollen taxa counted but excludes fern spores.
in our undifferentiated Urticaceae-Moraceae group are diporate (P2) or triporate (P3), but we also include in this category some 4-porate (P4) and 5-porate (P5) grains with a psilate to granulate exine suggestive of pollen of these two families.) Mauritia pollen declines to a trace.
Zone 3 (Iriartia-Urticaceae-Moraceae zone) This is the upper pollen zone, which is also characteristic of the surface, mud and water interface samples. Cecropia declines from the high of zone 2 and Iriartia (Palmae) becomes prominent reaching up to 20% in two profiles.
79
LAGOAGR~
"
"-
~
/I./////L,A?¢
. HA
,h'/ /
%
Fig.8. Pollen percentage diagram from Lake Agrio. Pollen sum includes all pollen taxa counted but excludes fern spores.
The forest taxa of Urticaceae and Moraceae are collectively at a maximum.
Interpretation of the pollen record The three-zone sequence apparently describes the recovery of the riverain forest of the western Amazon basin from gross disturbance caused by regional flooding. When the valleys of both the Aguarico and Napo rivers were inundated (zone 1), pollen taxa characteristic of the modern terra firme forests were not prominent, which is to be expected after destructive flooding. The wet palm forest typified by stands of Mauritia became more extensive, as what are now terra firme became marshy or waterlogged. The increase in fern spores, and the maxima of sedge and grass pollen at Limoncocha (Fig.6) suggest increased aquatic vegetation, perhaps of the floating mat type. Zone 2 shows the response of western Amazonian vegetation to the gradual creation of terra tirme with the coming of a drier climate. The most immediate response was a large increase in abundance of pollen, and presumably in population, of the forest-colonizing genus Cecropia. Cecropia, an anemophilous taxon, seems to be regularly overrepresented in western Amazonian and Andean pollen diagrams and its pollen may be dispersed well outside the limits of the parent populations (Hansen et al., 1984). But the very large increase at the bottom of zone 2 at sites
spanning more than 100 km suggests a large increase in population. This suggests that forest successions were beginning everywhere on land that had dried as climate changed. The reduction of the wet palm forest is reflected in a precipitous decline in Mauritia pollen, synchronous with the rise in Cecropia. Zone 3 indicates continuing forest succession, because numerous forest taxa increase in importance as Cecropia declines. Probably, part of the apparent increase in forest taxa reflects no more t h a n a real decline in Cecropia influx in zone 3; yet the details of fresh taxa appearing, and of the changing percentages between taxa, suggest an actual increase in influx of forest taxa over and above the statistical effect of the Cecropia decline. By the time of sampling, this successional process had led to the present vegetation of the region: diverse rain forest on all sites above high water, restricted wet palm forest on bottom lands, and bands of aquatic plants floating at the margins of the lakes. The rise of Iriartia pollen in zone 3 appears to be associated with the general development of the present vegetation. The sequence of Mauritia to Iriartia from zone 3 to zone 1 is not a simple replacement but seems to be separated by an interval, suggesting t h a t the population histories of the two palm genera are independent. Probably the significance of the Iriartia rise cannot be understood without further field studies on the species. Vegetation at the three sites has been
80
disturbed by human activities at different times in the recent past. Land around Lake Agrio (Fig.4) and Lake Santa Cecilia may have had minimum disturbance until very recently, except for the pattern of diffuse swidden agriculture to which all the western Amazon may have been subject (Meggers, 1966; Duellman, 1978). But clearing is extensive around Limoncocha, suggesting long occupancy; and the distinctive properties of the upper part of zone 3 at Limoncocha (Fig.6) may reflect this, particularly perhaps in the appearance of such disturbance indicators as Celtis and Trema (Ulmaceae), and possibly the lesser expression of the Iriartia rise.
equilibrium processes operating at intermediate time scales (ConneI1, 1978; Campbell and Frailey, 1984; Colinvaux et al., 1985; Saldarriaga and West, 1986; Salo et al., 1986). The second rests on the difference between pollen spectra from the western Amazon and those reported from parts of the Central Amazon by Absy (1979, 1985)-. : C l ~ l y , significantly different forest communities are represented in the pollen records of different parts of Amazonia, leading to the suggestion that areal differences in modern Amazonian vegetation, rather than supposed patterns of refugia in the past, account for disjunct distributions of Amazonian biota.
Concluding note
Acknowledgements
Possibly the most interesting thing to be said about these pollen diagrams from the true rain forest of the Amazon basin is that they demonstrate how well a complex record of forest history is preserved by pollen. Despite the fact that the vegetation is hugely diverse, with most of its important plants pollinated by insects, birds, or even bats, an interesting array of pollen types yet collects in the lake sediments. The pollen rain is not drowned out by pollen of grasses or of the other colonist herbs. Before the record can be read in more detail we must learn more of the pollen morphology of many types and perhaps devise better methods of extraction and analysis. Because of low recoveries we had to become resigned to spending a week or more to count enough microspores for a single pollen sum. Much also will have to await better ecological knowledge of the plant populations represented by the pollen, but the data show that a record of fairly detailed changes in the forest is preservedl Two last ecological observations seem warranted. The first is that the pollen diagrams cannot be taken to suggest that the succession following the flooding that ended 800 years ago is over: rather they suggest change still in progress. Our data are consistent with the view that Amazonian diversity is a function of non-
Like the inter-Andean operations described in Colinvaux e t al. (this issue) this work depended on the hospitality of H. and T. Steinitz and our many helpers in Ecuador. In addition we thank J. Vela for providing our base at Limoncocha, M. Eggart for drawing the diagrams, and C. Vasquez, K. Olson, S. Longenbaker and B. Bethel for technical assistance. The research was funded by grants from NSF to The Ohio State University.
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