Seasonally dry tropical forests have been largely ignored in discussions of vegetation changes during the Quaternary. We distinguish dry forests, which are essentially tree-dominated ecosystems, from open savannas that have a xeromorphic fire-tolerant, grass layer and grow on dystrophic, acid soils. Seasonally dry tropical forests grow on fertile soils, usually have a closed canopy, have woody floras dominated by the Leguminosae and Bignoniaceae and a sparse ground flora with few grasses. They occur in disjunct areas throughout the Neotropics. The Chaco forests of central South America experience regular annual frosts, and are considered a subtropical extension of temperate vegetation formations. At least 104 plant species from a wide range of families are each found in two or more of the isolated areas of seasonally dry tropical forest scattered across the Neotropics, and these repeated patterns of distribution suggest a more widespread expanse of this vegetation, presumably in drier and cooler periods of the Pleistocene. We propose a new vegetation model for some areas of the Ice-Age Amazon: a type of seasonally dry tropical forest, with rain forest and montane taxa largely confined to gallery forest. This model is consistent with the distributions of contemporary seasonally dry tropical forest species in Amazonia and existing palynological data. The hypothesis of vicariance of a wider historical area of seasonally dry tropical forests could be tested using a cladistic biogeographic approach focusing on plant genera that have species showing high levels of endemicity in the different areas of these forests.

Neotropical seasonally dry forests and Quaternary vegetation changes

Amazonian rain forest-savanna boundaries are highly sensitive to climatic change and may also play an important role in rain forest speciation. However, their dynamics over millennial time scales are poorly understood. Here, we present late Quaternary pollen records from the southern margin of Amazonia, which show that the humid evergreen rain forests of eastern Bolivia have been expanding southward over the past 3000 years and that their present-day limit represents the southernmost extent of Amazonian rain forest over at least the past 50,000 years. This rain forest expansion is attributed to increased seasonal latitudinal migration of the Intertropical Convergence Zone, which can in turn be explained by Milankovitch astronomic forcing.

/pdf/millennial-scale-dynamics-of-southern-amazonian-rain-forests-trnpulvo9m.pdf

Millennial-Scale Dynamics of Southern Amazonian Rain Forests

Summary

1. The South and Middle American tropics contain the world's largest area of moist savanna. Despite an apparent uniformity in appearance, floristic groupings can be detected and four zones are provisionally outlined with a number of character- istic plants.2. Although current knowledge of their bio- geography and palaeoecology is scattered and incomplete, it is evident that there have been significant changes throughout the neotropics since the glacial maximum.3. The largest and most significant savanna formations are the Brazilian cerrados, which occupy nearly 23% of that country, an area equivalent to that of western Europe. The cerrados range from savanna woodlands (cerradao), through progres- sively less arboreal formations (cerrado), to nearly treeless grasslands (campo). A number of adjoining vegetation formations are closely related, including evergreen and deciduous forests, scrub woodlands and wetlands.4. Until the last decade, the biological diversity of the neotropical savannas was thought to be limited since, structurally, they appeared to form a relatively simple mosaic of repeated physiognomic units. New evidence reveals a rich diversity of tree, shrub and herbaceous plants in a continuously changing spatial pattern. Such diversity reflects the great area of the biome, together with local differences in topography and drainage, soil conditions and fire frequency.5. Research on the dynamic processes affecting savannas is of increasing importance and urgency given the potential value of the area that is still relatively undisturbed (about 50% of the original, but with less than 2% having federal reserve status) and the pace of current conversion to different forms of land use.

The nature and diversity of neotropical savanna vegetation with particular reference to the Brazilian cerrados

The aims of this paper are to review previously published palaeovegetation and independent palaeoclimatic datasets together with new results we present from dynamic vegetation model simulations and modern pollen rain studies to: (i) determine the responses of Amazonian ecosystems to changes in temperature, precipitation and atmospheric CO2 concentrations that occurred since the last glacial maximum (LGM), ca. 21 000 years ago; and (ii) use this long-term perspective to predict the likely vegetation responses to future climate change. Amazonia remained predominantly forested at the LGM, although the combination of reduced temperatures, precipitation and atmospheric CO2 concentrations resulted in forests structurally and floristically quite different from those of today. Cold-adapted Andean taxa mixed with rainforest taxa in central areas, while dry forest species and lianas probably became important in the more seasonal southern Amazon forests and savannahs expanded at forest-savannah ecotones. Net primary productivity (NPP) and canopy density were significantly lower than today. Evergreen rainforest distribution and NPP increased during the glacial-Holocene transition owing to ameliorating climatic and CO2 conditions. However, reduced precipitation in the Early-Mid-Holocene (ca. 8000-3600 years ago) caused widespread, frequent fires in seasonal southern Amazonia, causing increased abundance of drought-tolerant dry forest taxa and savannahs in ecotonal areas. Rainforests expanded once more in the Late Holocene owing to increased precipitation caused by greater austral summer insolation, although some of this forest expansion (e.g. in parts of the Bolivian Beni) is clearly caused by palaeo Indian landscape modification. The plant communities that existed during the Early-Mid-Holocene may provide insights into the kinds of vegetation response expected from similar increases in temperature and aridity predicted for the twenty-first century. We infer that ecotonal areas near the margins of the Amazon Basin are liable to be most sensitive to future environmental change and should therefore be targeted with conservation strategies that allow 'natural' species movements and plant community re-assortments to occur.

/pdf/responses-of-amazonian-ecosystems-to-climatic-and-15o2u4wmre.pdf

Responses of Amazonian ecosystems to climatic and atmospheric carbon dioxide changes since the last glacial maximum

/pdf/paleo-environmental-change-in-amazonian-and-african-keagclfl81.pdf

Paleo-environmental change in Amazonian and African rainforest during the LGM

This paper presents carbon isotope data on soil organic matter (SOM) collected along an ecosystem transect that includes a wooded savannah (cerrado), a tropical semideciduous forest (cerradao), a forest tran- sition type and a tropical forest. The study area is located in the Rondonia state, southwestern Brazilian Amazon region. I4C data of total soil organic matter and charcoal indicate that the organic matter in these soils is at least Holocene in age. The forest and forest transition sites are characterized by 6I3C soil depth profiles gener- ated typically by C3 plants, indicating no major changes in plant communities have occurred in this region during the time period represented by the isotope data. In contrast, the cerrado and cerradao have experienced significant vegetation changes during the Holocene. The st3C data (-3o%o to -27%0) obtained in the deepest part of the profile at the cerradao site show the expansion of the C3 forest vegetation into this region during early Holocene. A vegetation change consisting of increased C, plant influence is reflected in the "C-enriched values (-19%0 to -16.0%) at both sites during the middle Holocene. The recent part of the I3C record shows a clear expansion of C, vegetation, particularly at the cerrad20 site. The regressionkxpansion of the forest and savannah vegetation documented at the cerradao and cerrado sites is probably related to changes from a humid to a drier climate and a return to more humid conditions and is in agreement with palaeoclimatic information reported for Brazil and the Bolivian Altiplano. This study suggests that large areas in the Amazon basin have been affected by vegetation changes during the Holocene and that soil organic matter in the transition areas ,between savannah and forest ecotones contains a valuable palaeorecord of vegetation changes in the Ama- ,'zen region.

/pdf/the-carbon-isotope-record-in-soils-along-a-forest-cerrado-2oouhakpga.pdf

The carbon isotope record in soils along a forest-cerrado ecosystem transect: implications for vegetation changes in the Rondonia state, southwestern Brazilian Amazon region:

This study, which was carried out in the southern Brazilian Amazon region (Rondonia state and Humaita, Southern Amazon state), presents and discusses the significance of carbon isotope data measured in soil profiles collected across natural boundaries of forest to savanna vegetation. The main objective of this study was to evaluate the expansion-regression dynamics of these vegetation units in relation to climate changes during the Holocene. 14C data from charcoal, soil organic matter (SOM) and its component humin fraction indicate that the organic matter in the studied soils is essentially Holocene in origin. 13C data indicate that C3 type plants were the dominant vegetation at all study areas in the early Holocene, and during the entire Holocene, in the forest sites of Central Rondonia state and in the forest site 50 km from the city of Humaita. 13C data also indicate that C4 plants have influenced significantly the vegetation at the transitional forest and the Cerrado (wooded savanna) sites of Southern Rondonia state and the forest ecosystem located 20 km from the Humaita city. These typical C4 type isotopic signatures probably reflect a drier climate during the mid-Holocene. The 13C records representing probably the last 3000 yr show an expansion of the forest, due to a climatic improvement, in areas previously occupied by savanna vegetation. These results and other published data for the Amazon region indicate that the areas representing today's forest-savanna boundaries have been determined by significant vegetation changes during the Holocene. The boundary between forest and savanna vegetation seems to be quite sensitive to climatic change and should be the focus of more extensive research to correlate climate and past vegetation dynamics in the Amazon region.

/pdf/14c-dating-and-stable-carbon-isotopes-of-soil-organic-matter-35sodl9v8g.pdf

14c dating and stable carbon isotopes of soil organic matter in forest-savanna boundary areas in the southern Brazilian Amazon region

Palaeovegetation dynamics of an ecotone forest-savannah in the southern Brazilian Amazon during the Late Pleistocene and Holocene based on carbon isotopes of soil organic matter

B M Gomes

Papers

The carbon isotope record in soils along a forest-cerrado ecosystem transect: implications for vegetation changes in the Rondonia state, southwestern Brazilian Amazon region:

14c dating and stable carbon isotopes of soil organic matter in forest-savanna boundary areas in the southern Brazilian Amazon region

Palaeovegetation dynamics of an ecotone forest-savannah in the southern Brazilian Amazon during the Late Pleistocene and Holocene based on carbon isotopes of soil organic matter