Differentiation between Neotropical rainforest, dry forest, and savannah ecosystems by their modern pollen spectra and implications for the fossil pollen record

Accurate differentiation between tropical forest and savannah ecosystems in the fossil pollen record is hampered by the combination of: i) poor taxonomic resolution in pollen identification, and ii) the high species diversity of many lowland tropical families, i.e. with many different growth forms living in numerous environmental settings. These barriers to interpreting the fossil record hinder our understanding of the past distributions of different Neotropical ecosystems and consequently cloud our knowledge of past climatic, biodiversity and carbon storage patterns. Modern pollen studies facilitate an improved understanding of how ecosystems are represented by the pollen their plants produce and therefore aid interpretation of fossil pollen records. To understand how to differentiate ecosystems palynologically, it is essential that a consistent sampling method is used across ecosystems. However, to date, modern pollen studies from tropical South America have employed a variety of methodologies (e.g. pollen traps, moss polsters, soil samples). In this paper, we present the first modern pollen study from the Neotropics to examine the modern pollen rain from moist evergreen tropical forest (METF), semi-deciduous dry tropical forest (SDTF) and wooded savannah (cerradão) using a consistent sampling methodology (pollen traps). Pollen rain was sampled annually in September for the years 1999–2001 from within permanent vegetation study plots in, or near, the Noel Kempff Mercado National Park (NKMNP), Bolivia. Comparison of the modern pollen rain within these plots with detailed floristic inventories allowed estimates of the relative pollen productivity and dispersal for individual taxa to be made (% pollen/% vegetation or ‘p/v’). The applicability of these data to interpreting fossil records from lake sediments was then explored by comparison with pollen assemblages obtained from five lake surface samples.Pollen productivity is demonstrated to vary inter-annually and conforms to a consistent hierarchy for any given year: METF > SDTF > cerradão. This suggests an association between pollen productivity and basic structural characteristics of the ecosystem, i.e. closed canopy vs. open canopy vs. savannah. Comparison of modern pollen and vegetation revealed that some important floristic elements were completely absent from the pollen: Qualea and Erisma (METF), Bauhinia, Simira and Guazuma (SDTF), and Pouteria and Caryocar (cerradão). Anadenanthera was found to be abundant in both the pollen and flora of SDTF (p/v = 3.6), while Poaceae was relatively poorly represented in cerradão (0.2). Moraceae, Cecropia and Schefflera were found to be over-represented palynologically in all ecosystems. Overall, the data demonstrated that no one taxon could be used as a definitive indicator of any of the ecosystems. Instead, associations of taxa were found to be important: METF = Moraceae (> 40%), Cecropia, Hyeronima, Celtis; SDTF = Anadenanthera, Apuleia, Ferdinandusa and non-arboreal Asteraceae, Bromeliaceae, Piper and fern spores; cerradão = Poaceae, Myrtaceae, Borreria, Solanum plus Asteraceae and fern spores. Interpretation of Poaceae pollen was highlighted as problematic, with relatively low abundance in the cerradão (< 20%) in comparison to high abundance in lake environments (c. 30–50%). Re-examination of fossil pollen records from NKMNP revealed that modern vegetation associations were only established in the last few thousand years.