Characterisation of Bolivian savanna ecosystems by their modern pollen rain and implications for fossil pollen records

Abstract

The majority of vegetation reconstructions from the Neotropics are derived from fossil pollen records extracted from lake sediments. However, the interpretation of these records is restricted by limited knowledge of the contemporary relationships between the vegetation and pollen rain of Neotropical ecosystems, especially for more open vegetation such as savannas. This research aims to improve the interpretation of these records by investigating the vegetation and modern pollen rain of different savanna ecosystems in Bolivia using vegetation inventories, artificial pollen traps and surface lake sediments. Two types of savanna were studied, upland savannas (cerrado), occurring on well drained soils, and seasonally-inundated savannas occurring on seasonally water-logged soils. Quantitative vegetation data are used to identify taxa that are floristically important in the different savanna types and to allow modern pollen/vegetation ratios to be calculated. Artificial pollen traps from the upland savanna site are dominated by Moraceae (35%), Poaceae (30%), Alchornea (6%) and Cecropia (4%). The two seasonally-inundated savanna sites are dominated by Moraceae (37%), Poaceae (20%), Alchornea (8%) and Cecropia (7%), and Moraceae (25%), Cyperaceae (22%), Poaceae (19%) and Cecropia (9%), respectively. The modern pollen rain of seasonally-inundated savannas from surface lake sediments is dominated by Cyperaceae (35%), Poaceae (33%), Moraceae (9%) and Asteraceae (5%). Upland and seasonally-flooded savannas were found to be only subtly distinct from each other palynologically. All sites have a high proportion of Moraceae pollen due to effective wind dispersal of this pollen type from areas of evergreen forest close to the study sites. Modern pollen/vegetation ratios show that many key woody plant taxa are absent/under-represented in the modern pollen rain (e.g., Caryocar and Tabebuia). The lower-than-expected percentages of Poaceae pollen, and the scarcity of savanna indicators, in the modern pollen rain of these ecosystems mean that savannas could potentially be overlooked in fossil pollen records without consideration of the full pollen spectrum available.

Introduction

Modern pollen rain studies have shown great potential for improving the quality of fossil pollen based reconstructions of past vegetation change in the lowland Neotropics (Bush, 1991, Gosling et al., 2009, Burn et al., 2010). However, there is still uncertainty over the relative extent of evergreen rainforest, seasonally dry forest and savanna during the Pleistocene and Holocene (Haffer, 1969, Colinvaux et al., 1996, Pennington et al., 2000, Mayle, 2006, Mayle and Power, 2008). This uncertainty largely results from a poor understanding of the modern pollen/vegetation relationships for contemporary ecosystems. The recognition of savannas is especially problematic as their presence, or absence, is often based solely on the percentage of grass pollen, which is a method fraught with uncertainties (Bush, 2002), notwithstanding recent advances in grass pollen taxonomy (Schüler and Behling, 2010). The accuracy of these reconstructions is vitally important to improve our understanding of long-term Neotropical ecosystem dynamics, which can provide important insights into ecosystem responses to past natural (e.g., climatic) and anthropogenic (e.g., burning) disturbance and implications for changes in biodiversity and carbon storage in the Neotropics (Adams and Faure, 1998, Behling, 2002, Mayle and Beerling, 2004).

Correct identification of savannas in fossil pollen records is vitally important for testing the validity of different theories of Neotropical vegetation change during the Late Quaternary. For example, enhanced recognition of savanna ecosystems in fossil pollen records from Amazonia would enable more rigorous testing of the rainforest refugia hypothesis, which states that Amazonia was dominated by savanna rather than rainforest during drier periods of the Quaternary (Haffer, 1969). Additionally, it would help determine the extent to which cerrado savannas in the southern Neotropics were replaced by other ecosystems during the Pleistocene (e.g., Ratter et al., 1988, Prado and Gibbs, 1993).

Savanna ecosystems are estimated to cover over 3 million square kilometres of the Neotropics (Huber, 1987). They can be divided into those with seasonal flooding during the wet season (seasonally-inundated savannas), and those with a soil profile that remains well drained throughout the year (upland cerrado savannas). This differentiation is important as seasonally-inundated savannas can change their distribution due to factors that are unrelated to climatic change, such as river migration (Killeen, 1998), whereas the replacement of an ecosystem by upland savannas can give a clear indication of environmental changes, such as a change in precipitation or fire frequency. Therefore, this research aims to characterise these two savanna types by their modern pollen rain and test the degree to which they can be differentiated.

In this paper, a comparison of the modern pollen rain of these savanna ecosystems with vegetation inventories available through the SALVIAS database (2002) allows the contemporary relationship between the savanna vegetation and its pollen rain to be studied. In addition, savanna pollen rain will be captured by both natural (surface lake sediment) and artificial (plastic funnel) pollen traps and compared with each other to test the hypothesis that these two types of pollen trap sample pollen rain from distinctly different source areas (Birks and Birks, 1980). A comparison between the modern pollen rain spectra from these two methods allows the applicability of the results to fossil pollen records obtained from lake sediments to be assessed. Finally, by sampling several different artificial pollen traps from the same site, over a period of several years, the temporal and spatial variability in the modern pollen rain can be investigated.