Differentiation of Neotropical Ecosystems by Modern Soil Phytolith Assemblages and Implications for Palaeoenvironmental and Archaeological Reconstruction José Iriarte a, Ruth Dickau a, Bronwen Whitney b, & Francis Mayle b

Introduction The accurate reconstruction of vegetation history is key to addressing current debates regarding the nature and scale of pre-Columbian human impacts on Amazonian ecosystems 1-6. The interpretation of palaeoenvironmental records, including fossil phytolith assemblages, relies on the existence of appropriate modern reference analogues. As part of a project integrating archaeology and palaeoecology to investigate human land-use in the Bolivian Amazon, we analyzed modern phytolith assemblages from the soils of ten distinctive Neotropical vegetation formations, ranging from humid evergreen forest to permanently inundated savannah.

Sample Locations and Methods Sediment samples were taken from 1 ha vegetation study plots in and around Noel Kempff Mercado National Park (NKMNP)7,8, and two additional locations in the Beni basin savannah eco-region (Fig. 1). They were divided in to silt (2-50 µm, A-fraction) and sand (>50 µm, C-fraction), and phytoliths were extracted using standard procedures9.

Results The ratio of grass (Poaceae) to arboreal phytoliths distinguishes savannah and forest ecosystems (Fig. 2). Evergreen forests have higher frequencies of palms (Arecaceae), Olyreae, Phenakospermum-type, and Annonaceae-type, and lower frequencies of Bambusoideae and irregular stipulate tracheids, than seasonally-dry forests (Fig. 2A). Wooded and upland savannahs are distinguished from inundated savannahs by more arboreal phytoliths, and less Oryzoideae grasses, Cyperaceae sedges (including Scirpus), Asteraceae, and Heliconia herbs (Fig. 2B). Marantaceae phytoliths, usually a marker of evergreen forests, dominate permanently inundated savanna soils due to the widespread occurrence of wet savannah species Thalia genticulata4.

Principal Component Analysis Principal Component Analysis (PCA) confirms trends observed in the frequency diagrams. C-fraction phytoliths show separation between broad ecosystem classes, particularly evergreen and seasonally dry forest (Fig. 3A). There is some overlap between seasonally dry forest and savannah formations; however, many savannah formations could not be included in the C-fraction PCA because they yielded few or no large diagnostic phytoliths. The A-fraction PCA of all samples shows significant overlap between formations (except permanently inundated savanna), but a clear trend along axis PCA1 from savannah to evergreen forest (Fig. 3B). Separate PCAs on savannah and forest ecosystems more clearly distinguish their constituent vegetation formations (Fig. 3 C,D).

Conclusions Broad ecosystem classes are clearly differentiated by phytolith assemblages. Differences between vegetation formations within each class are more subtle, but still observable. Some phytolith taxa are under- or over-represented based on standing vegetation8,10. However, by using an assemblage-based approach and looking at associations of taxa, different vegetation formations can be distinguished. Moreover, when phytolith data are combined with pollen rain11-15 and stable carbon isotope16 analysis from the same plots, taxonomic and ecosystem resolution is improved. The results of this study will aid in interpreting palaeoenvironmental records from the Bolivian Amazon, necessary for investigating the nature and level of landscape alteration by people in the past.

References 1 Denevan, W. M. The pristine myth: the landscape of the Americas in 1492. Ann. Assoc. Am. Geogr. 82, 369-385 (1992). 2 Iriarte, J., et al. Fire-free land use in pre-1492 Amazonian savannas. P. Natl. Acad. Sci. USA (2012). 3 Heckenberger, M. J. & Neves, A. Amazonian Archaeology. Annual Review of Anthropology 38, 251-266 (2009). 4 McKey, D. et al. Pre-Columbian agricultural landscapes, ecosystem engineers, and self-organized patchiness in Amazonia. P. Natl. Acad. Sci. USA 107, 7823-7828 (2010). 5 Meggers, B. J. Amazonia: Real or Counterfeit Paradise? The Review of Archaeology 13, 25-40 (1992). 6 Woods, W. I. et al. (eds.) Amazonian Dark Earths: Wim Sombroek's Vision (Springer, 2009). 7 Killeen, T. J. in A Biological Assessment of Parque Nacional Noel Kempff Mercado, Bolivia RAP Working Papers 10 (eds T. J. Killeen & T. S. Schulenberg) 61-88 (Conservation International, 1998). 8 SALVIAS. http://www.salvias.net. (2004). 9 Piperno, D. R. Phytoliths: A Comprehensive Guide for Archaeologists and Paleoecologists. (Altamira Press, 2006). 10 Soto, J. D. Diagnostico de la vegetación en lagunas y camellones en los Llanos de Moxos del Beni. (Herbario Regional del Oriente Boliviano, Museo de Historia Natural Noel Kempff Mercado, Santa Cruz, Bolivia, 2010). 11 Burn, M. J., et al. Pollen-based differentiation of Amazonian rainforest communities and implications for lowland palaeoecology in tropical South America. Palaeogeogr. Palaeocl. 295, 1-18 (2010). 12 Gosling, W. D., et al. Modern pollen-rain characteristics of tall terra fume moist evergreen forest, southern Amazonia. Quaternary Res. 64, 284-297 (2005). 13 Gosling, W. D., et al. Differentiation between Neotropical rainforest, dry forest, and savannah ecosystems by their modern pollen spectra and implications for the fossil pollen record. Rev. Palaeobot. Palyno. 153, 70-85 (2009). 14 Jones, H. T., et al. Characterisation of Bolivian savanna ecosystems by their modern pollen rain and implications for fossil pollen records. Rev. Palaeobot. Palyno. 164, 223-237 (2011). 15 Jones, H. T., et al. Characterisation of neotropical dry forest ecosystem by their modern pollen rain and implications for fossil pollen records. Rev. Palaeobot. Palyno. (in prep.). 16 Metcalfe, P. The late quaternary history of the rainforest-savanna boundary of Southwest Amazonia Ph.D. thesis, University of Wales, (2004).

Fig. 1: Map of sample plot locations and major vegetation formations. Terra firme palm forest (TFPF, represented by samples Cusi1 and 2) is part of the Deciduous dry forest vegetation formation.

Fig. 2: Percentage diagrams of phytolith taxa in each vegetation formation. Solid bars = A-fraction (2-50 µm) Outline bars = C-fraction (>50 µm) + = frequency lower than 2% x = presence of taxon in a C-fraction with <50 total phytoliths counted

Acknowledgments • Funded by Leverhulme Trust Research Grant F/00 158/CH • Comparative material provided by El Herbario Regional del Oriente Boliviano, Museo

de Historia Natural ‘Noel Kempff Mercado’, Santa Cruz, Bolivia • Lab assistance by Adam Wainwright • Figure preparation by Seán Goddard

Fig. 3: PCAs of phytolith assemblages.

a University of Exeter, UK, b University of Edinburgh, UK

A) Forest phytolith frequencies

B) Savanna phytolith frequencies

C) Phytolith category totals Vegetation Formations Selected vegetation formations and associated phytoliths

Terra firme humid evergreen liana forest. Mendoncia and Olyraeae phytoliths.

Terra firme semi-deciduous dry forest. Bambusoideae and irregular stipulate tracheid phytoliths.

Terra firme wooded savanna (cerradão). Panicoideae and tall rondel (Poaceae) phytoliths.

Permanently inundated savanna. Cyperus/ Kyllinga and Marantaceae phytoliths. Noel Kempff Mercado National Park, Bolivia. Photo: Jon Hornbuckle.

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