Paulo Eduardo De Oliveira

A 14 300-Yr Paleoecological Profile of a Lowland Tropical Lake in Panama

The first paleoecological analysis of a complete sedimentary record spanning the period from the late Pleistocene to the present from lowland Panama, documents changes in lowland vegetation communities through major climatic change and the onset of human disturbance. Past sympatry is found among presently allopatric species, suggesting that tropical forest communities are not species-stable through time. Late Pleistocene floras at Lake La Yeguada (elevation 650 m), Panama, had high relative abundance of montane forest elements, e.g., Quercus and Magnolia, existing some 900 m below their present range, suggesting a climatic cooling of - 50C below present. This descent of montane forest taxa onto lowland hilltops denied the ground to postulated lowland rain forest refugia. The late Pleistocene (14 350-11 050 yr BP) was not uniformly cool and was interrupted by brief phases of near present-day warming. The onset of the Holocene was abrupt, taking < 100 yr, and was almost coincidental with the start of human forest disturbance. Changes in climate at La Yeguada were found to be largely synchronous with those documented at Lake Valencia, Venezuela, but no fine-scale climatic synchrony was apparent with South American or European sites, and significant departures from the predictions of published climatic circulation models are found.

Temperature depression in the Lowland Tropics in glacial times

Equatorial air temperatures at low elevations in the New World tropics are shown by pollen and other data to have been significantly lowered in long intervals of the last glaciation. These new data show that long recognized evidence for cooling at high elevations in the tropics were symptomatic of general tropical cooling and that they did not require appeal to altered lapse rates or other special mechanisms to be made to conform with conclusions that equatorial sea surface temperatures (SSTs) were scarcely changed in glacial times. The new data should be read in conjunction with recent findings that Caribbean (SSTs) were lowered in the order of 5 ° C, contrary to previous interpretations. Thus these accumulating data show that low latitudes as well as high were cooled in glaciations. In part the earlier failure to find evidence of low elevation cooling in the lowland tropics resulted from the data being masked by strong signals for aridity given by old lake levels in parts of Africa and elsewhere. Global circulation models used to predict future effects of greenhouse warming must also be able to simulate the significant cooling of the large tropical land masses at glacial times with reduced greenhouse gas concentrations. Plants and animals of the Amazon forest and similar ecosystems are able to survive in wide ranges of temperatures, CO2 concentrations, and disturbance, though associations change constantly.

Holocene fire and occupation in Amazonia: records from two lake districts

While large-scale pre-Columbian human occupation and ecological disturbance have been demonstrated close to major Amazonian waterways, less is known of sites in terra firme settings. Palaeoecological analyses of two lake districts in central and western Amazonia reveal long histories of occupation and land use. At both locations, human activity was centred on one of the lakes, while the others were either lightly used or unused. These analyses indicate that the scale of human impacts in these terra firme settings is localized and probably strongly influenced by the presence of a permanent open-water body. Evidence is found of forest clearance and cultivation of maize and manioc. These data are directly relevant to the resilience of Amazonian conservation, as they do not support the contention that all of Amazonia is a ‘built landscape’ and therefore a product of past human land use.

The rise and fall of the Refugial Hypothesis of Amazonian speciation: a paleoecological perspective

The refugial hypothesis is treated as the definitive history of Amazonian forests in many texts. Surprisingly, this important theoretical framework has not been based on paleoecological data. Consequently, a model of Amazonian aridity during the northern hemispheric glaciation has been accepted uncritically. Ironically, the Refuge Hypothesis has not been tested by paleobotanical data. We present a revision of the concept of Neotropical Pleistocene Forest Refuges and test it in the light of paleocological studies derived from pollen analysis of Amazonian lake sediments deposited during the last 20,000 years. Our analysis is based primarily on paleoenvironmental data obtained from sites in Brazil and Ecuador. These data are contrasted with those that favor the hypothesis of fragmented tropical forests in a landscape dominated mainly by tropical savannas under an arid climate. The Ecuadorian data set strongly suggests a 5oC cooling and presence of humid forests at the foot of the Andes, during the last Ice Age. The same climatic and vegetational scenario was found in the western Brazilian Amazon. On the other hand, somewhat drier conditions were observed in the central Amazon, but the landscape remained a forested landscape during the supposedly arid phases of the Late Quaternary. Data obtained from the Amazon Fan sediments containing pollen derived from extensive sections of the Amazon Basin, were fundamental to the conclusion that the predominance of savannas in this region is not supported by botanical data. Our revision of the assumptions derived from the Refuge Hypothesis indicates that it has succumbed to the test now permitted by a larger paleocological data set, which were not available during the golden age of this paradigm, when indirect evidence was considered satisfactory to support it.

Two histories of environmental change and human disturbance in eastern lowland Amazonia

Two long palaeoecological records (pollen, diatoms, 14C, chemistry and charcoal) provide a detailed record of Holocene environmental conditions in the lowlands of the lower Amazon. Changes in the composition of the terra firme forest are evident but a forest cover is maintained throughout. Fluctuations in lake level and changes in seasonality are tracked in both sedimentary sequences. Rising early-Holocene sea levels and the consequent rise of local water tables probably caused the flooding of one of the sites. A stepped increase in precipitation is suggested with increases evident at 8800 yr BP, 7400 BP and between 6100 and 5800 yr BP. Increased storminess is suggested between|7400 yr BP and|6100 yr BP. Human disturbance is manifested in the palaeoecological records at one study site as early as 5500 yr BP (c. 6500 years ago), and an intensification, or onset, of agriculture is evident at c. 3350 yr BP. The terminating phase of the long record of disturbance is a period of forest regrowth that may correlate to the depopulation of Amazonia following Euro pean contact.

Palaeoecology and climate of the Amazon basin during the last glacial cycle

Our interpretation of the available palynological data from the Amazon basin is that they show a long history of continuous occupation by forest. We have thoroughly reviewed the accumulated data leading to this conclusion elsewhere, but briefly recapitulate the arguments here. One implication of this botanical reconstruction, if correct, is that interpretations of geomorphological features requiring an arid land surface for glacial age Amazonia require reassessment. Copyright

The influence of biogeographic and ecological heterogeneity on Amazonian pollen spectra

The influence of gamma- (y) and beta- (p) diversity on modern pollen rain is assessed using data from three Amazonian forests. Pollen rain of 79 forest locations was collected in modified Oldfield pollen traps between 1991 and 1993. Pollen diversity in the traps was high with > 280 palynomorph types recognized. Gamma diversity was assessed by comparing lowland terra firme forests in Cuyab- eno, Ecuador, with two terra firme forests near Manaus, Brazil. The influence of P-diversity on local pollen rain was investigated using samples collected from neighbouring terra firme forests, seasonally flooded forests, and Mauritia-rich for- ests at Cuyabeno, Ecuador. Multivariate analyses revealed that y-diversity pro- duces a stronger signal in the pollen rain than ,-diversity. However, 5-diversity is accurately reflected in the pollen rain when the diversity is an expression of strong environmental gradients.

Microrefugia, Climate Change, and Conservation of Cedrus atlantica in the Rif Mountains, Morocco

This study reconstructs and interprets the changing range of Atlas cedar in northern Morocco over the last 9,000 years. A synthesis of fossil pollen records indicated that Atlas cedars occupied a wider range at lower elevations during the mid-Holocene than today. The mid-Holocene geographical expansion reflected low winter temperatures and higher water availability over the whole range of the Rif Mountains relative to modern conditions. A trend of increasing aridity observed after 6000 years BP progressively reduced the range of Atlas cedar and prompted its migration towards elevations above 1400 masl. To assess the impact of climate change on cedar populations over the last decades, we performed a transient model simulation for the period between 1960 and 2010. Our simulation showed that the range of Atlas cedar decreased by about 75% over the last 50 years and that the eastern populations of the range in the Rif Mountains were even more threatened by the overall lack of water availability than the western ones. Today, Atlas cedar populations in the Rif Mountains are persisting in restricted and isolated areas (Jbel Kelti, Talassemtane, Jbel Tiziren, Oursane, Tidighine) that we consider to be modern microrefugia. Conservation of these isolated populations is essential for the future survival of the species, preserving polymorphisms and the potential for population recovery under different climatic conditions.

Direct archaeological evidence for Southwestern Amazonia as an early plant domestication and food production centre

Southwestern Amazonia is considered an early centre of plant domestication in the New World, but most of the evidence for this hypothesis comes from genetic data since systematic archaeological fieldwork in the area is recent. This paper provides first-hand archaeobotanical evidence of food production from early and middle Holocene (ca. 9,000–5000 cal. BP) deposits at Teotonio, an open-air site located on a 40 m-high bluff on the south bank of the Madeira river. Such evidence includes the presence of local and exotic domesticates such as manioc (Manihot esculenta), squash (Cucurbita sp.) and beans (Phaseolus sp.), alongside edible fruits such as pequiá (Caryocar sp.) and guava (Psidium sp.) that point to the beginnings of landscape domestication. The results contribute to an ever-growing number of studies that posit southwest Amazonia as an important centre for early crop domestication and experimentation, and which highlight the longue-durée of human impacts on tropical forest biodiversity around the world.

New Holocene pollen records from the Brazilian Caatinga

We present two pollen diagrams from the semi-arid Caatinga of the Catimbau National Park, in Pernambuco and from a Mauritia palm forest in the Caatinga/Cerrado ecotone of southern Piauí, NE Brazil, spanning the last 10,000 cal. yrs BP and the last 1,750 cal yrs BP, respectively. These two records contain a signature of the local vegetation and permit the correlation of the pollen signal with regional climatic changes. The Catimbau record shows Zizyphus sp., a typical Caatinga taxon, in all three pollen zones indicating regional Caatinga vegetation and the predominance of local arboreal taxa adapted to high humidity from 10,000 to ca. 6,000 cal. yrs BP with a gradual tendency towards drier conditions revealed by a deposition hiatus between 6,000 to ca. 2,000 cal. yrs BP. This abrupt loss of sediments in both localities is interpreted as a consequence of the establishment of modern semi-arid climates. The subsequent return of humidity is signaled by increased sedimentation rates and 14C date inversions in agreement with high precipitation, revealed by σ18O ratios in speleothems from NE Brazil. Modern sediments deposited in the last 500 years reflect local conditions with the maintenance of humidity by geological faulting and surfacing water tables.