Phytoliths are microscopic silica bodies that occur in stems, leaves, roots, and inflorescences of plants. Silica that forms phytoliths is carried up from ground water as monosilicic acid, and is deposited in epidermal and other cells of the growing plant, eventually forming bodies composed of opaline silica. Following the decay or burning of organic tissues, phytoliths are released back into the soil, preserving a record of vegetation and plants used by people.
Research on phytolith production and silicification patterns in a wide array of plants has shown that a strong genetic influence governs phytolith formation. Families and orders of plants show strong tendencies to silicify or not silicify their tissues. Production of many kinds of phytoliths is consistent within the same taxon growing under different environmental conditions. Distinctive phytoliths are produced in many plant families; genus-level diagnostics are common; species-level identification is possible for a number of taxa. It is thus no exaggeration to say that our knowledge of occurrence of phytoliths in the plant world has increased manyfold in recent decades.
Paleoenvironmental reconstruction is an important contribution of phytolith analysis to archaeology and paleoecology. Phytoliths extracted from natural soil accumulations potentially provide a detailed record of vegetation, and the impact on that vegetation by human activities such as clearance for agriculture. Successful tapping of the wealth of information from phytolith analysis depends on two things: (1) establishing diagnostic types for key indicator species, and (2) developing phytolith vegetation analogs, or signatures, for modern plant communities.
The goal of the Phytoliths in the Flora of Ecuador (PFE) project was to establish diagnostic phytolith types and phytolith vegetation signatures for the region to enhance archaeological and paleoenvironmental phytolith applications. Specifically, phytolith production in vegetation dominants, largely arboreal, was studied for plant communities along two idealized transects that sampled a significant part of the floristic diversity of this tropical country: (1) coastal to Amazon sample, encompassing the outer coastal plain, inner coastal plain, western slopes, interandean zone, eastern slopes, and eastern lowlands between approximately 1o north and 1o south latitude, and (2) coastal plain sample, encompassing the north to south shift from evergreen forest to open woodland/steppe from 1o north to 4o south latitude, at roughly 80o longitude.
Dr. Robin Kennedy, Director of the University of Missouri Herbarium, provided a list of vegetation dominants in the transects. From among those we tested plant groups (families, genera, and species) not previously studied for phytolith production, examined Ecuadorian representatives of previously studied groups, and examined replicate samples of important economic and indicator species. Data on phytolith production were compiled from the literature in 1997 to aid in this selection (see MATERIALS, phytolith production tables). The specimens studied were provided in part by the University of Missouri Herbarium, the Missouri Botanical Garden, and the National Herbarium, Quito, Ecuador. Phytolith assemblages of soil samples from four distinctive vegetation formations (xerophytic coastal forest, mixed deciduous/evergreen forest, coastal evergreen forest, and Amazonian lowland forest) were also characterized. See Pearsall (2015b) for a detailed overview of the PFE project.
(accessed under IMAGE DATABASE)
A diagnostic phytolith is one that can be used to distinguish among plant taxa in a given flora (Pearsall 2015a). The most useful diagnostics are those that can be used to distinguish among taxa at the level of family or below. As more plant taxa are studied, there are increasing numbers of one-to-one matches between phytoliths and plant taxa (Piperno 2006). Distinctive phytoliths are produced in the inflorescences of many plants, for example, permitting genus- and species-level identifications.
Because it can take years to compile and publish the results of a project like PFE, Pearsall decided to establish an on-line image database to display potentially diagnostic phytoliths as they were discovered. The database was set up in 2000 by then graduate student Neil A. Duncan, and expanded in 2005, with further additions planned in the near future. Potential diagnostics were evaluated on the basis of: (1) distinctiveness, (2) estimated abundance in tissues, and (3) distribution within the study population. Since diverse morphological features are useful for describing and distinguishing among phytoliths (e.g., see features discussed in Bowdery et al. 2001 and Madella et al. 2005), our approach was to describe the diagnostic using the appropriate characteristics in one field in the database. Our evaluation of the diagnostic level of the type was entered in the Comments field. Each record also includes the MU classification number and the family, genus and species of the specimen. Lab specimen numbers and sources of specimens are found in the Flora of Ecuador Summary Table (see MATERIALS, order tables). This table also shows what other kinds of phytoliths occur in the plant, and how common the diagnostic is.
Albert, R. M. and Madella, M. (editors). 2009. Perspectives on Phytolith Research: 6th International Meeting on Phytolith Research (Quaternary International, Vol. 193). Elsevier, Amsterdam.
Bowdery, D., Hart, D.M., Lentfer, C., Wallis, L.A. 2001. A universal phytolith key, in: Meunier, J.D., Collin, F. (Eds.), Phytoliths: Applications in Earth Sciences and Human History. A. A. Balkema, Lisse, pp. 267-278.
Hart, D. M. and Wallis, L. A. (editors). 2003. Phytolith and Starch Research in the Austalian-Pacific-Asian Regions: The State of the Art. Terra Australis 19. Pandanus Books, Research School of Pacific and Asian Studies, The Australian National University, Canberra.
Madella, M., Alexandre, A., Ball, T. 2005. International code for phytolith nomenclature 1.0. Annals of Botany 96, 253-260.
Meunier, J. D. and Colin, F. (editors). 2001. Phytoliths: Applications in Earth Sciences and Human History. A. A. Balkema, Lisse.
Pearsall, D. M. 2015a. Paleoethnobotany: A Handbook of Procedures, Third Edition. Left Coast Press, Walnut Creek, CA.
Pearsall, D. M. 2015b. The Phytoliths in the Flora of Ecuador Project: Perspectives on Phytolith Classification, Identification, and Establishing Regional Phytolith Databases. Journal of Archaeological Science http://dx.doi.org/10.1016/j.jas.2015.06.014
Pearsall, D. M. and Piperno, D. R. (editors). 1993. Current Research in Phytolith Analysis: Applications in Archaeology and Paleoecology. MASCA, University of Pennsylvania Museum, Philadelphia.
Pinilla, A., Juan-Tresserras, J. and Machado, M. J. (editors). 1997. The State-of-the-Art of Phytoliths in Soils and Plants. Centro de Ciéncias Medioambientales del Consejo Superior de Investigaciones Científicas, Madrid.
Piperno, D. R. 1988. Phytolith Analysis: An Archaeological and Geological Perspective. San Diego, Academic Press.
Piperno, D. R. 2006. Phytoliths: A Comprehensive Guide for Archaeologists and Paleoecologists. AltaMira Press, Lanham, MD.
Rapp, G., Jr. and Mulholland, S. (editors). 1992. Phytolith Systematics: Emerging Issues. Plenum Press, New York.
