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Methodological development of phytolith analysis for studying non-dietary crop products in prehistoric agricultural societies

Periodic Report Summary 1 - PHYTORES (Methodological development of phytolith analysis for studying non-dietary crop products in prehistoric agricultural societies.)

-- Introduction --

In former societies, non-dietary, secondary crop products of cereals such as straw, hay and chaff derived from the culms, leaves and inflorescence were of substantial economical importance. Indeed, they were used, amongst others, as fuel, temper and construction material. In archaeology, better detection and identification of these non-dietary, secondary crop products of cereals is essential for a better understanding of the importance of crop plants, the interpretation of archaeological features, the crop processing activities and the related socio-economic aspects of ancient and historical societies. Non-dietary crop products are however often overlooked in the archaeobotanical record, and even when they are detected, their identification can problematic . In order to contribute to solving this problem, the project PhytoRes aimed at developing phytolith-based criteria for:
1) Identifying the different anatomical parts of Eurasian cereals crops by using morphometric and assemblage approach to phytoliths, and
2) Identifying the diverse crop plants at taxonomic level by using the culm and leaf evidence (and partially for the inflorescence of understudied species).

-- Methodology --

To achieve the projects aims, the following steps were carried out:
1) A substantial reference collection was assembled from grown plant and herbarium specimens. This includes plants of an assortment of populations of 15 European, African and Asian cereals.
2) In-situ phytolith samples (tissue samples) from the specimens’ plant parts were extracted by a systematic sampling strategy that uses bleaching (sodium hypochlorite). Samples were prepared from:
a) leaves and culms of Panicum miliaceum, Panicum sumatrense, Setaria italica, Seteria verticillata, Pennisetum glaucum and Sorghum bicolor,
b) leaves of Triticum aestivum, Triticum dicoccon, Hordeum vulgare ssp. vulgare,
c) inflorescences of Eleusine coracana, Pennisetum glaucum and Sorghum bicolor.
3) From each in-situ sample, microphotographs were taken from 50 phytoliths. Photographic record has been collected for Eleusine coracana, Panicum miliaceum, Panicum sumatrense, Setaria italica, Pennisetum glaucum and Sorghum bicolor.
4) After the selection of an open-source software program (ImageJ), an innovative tool has been developed for the semi-automatic collection of morphometric measurements of phytoliths. This tool has a strong potential to ameliorate the uniformity in morphometric measurement of phytoliths, which is until now carried out by means of different methods and softwares. Moreover, the tool can also be applied to acquire morphometric measurements from images of objects other than phytoliths.
5) Measurements were collected from phytoliths of Panicum miliaceum, Panicum sumatrense, Setaria italica, Pennisetum glaucum and Sorghum bicolor.
6) Statistical analysis was carried out to compare the size and shape of the short cells bilobates phytoliths from the leaves of Pennisetum glaucum and Sorghum bicolor, Panicum miliaceum and Setaria italica.
7) Furthermore, following the same systematic sampling strategy as described above, samples from Panicum miliaceum and Setaria italica were dry-ashed for further analysis aimed at image-grabbing and morphometric measurements from non in situ material (not in the tissues but released phytoliths).
8) Finally, archaeological phytolith samples from a Neolithic site in Sudan were analyzed in cooperation with an international team to assess the strength and validity of the developed methodology.

-- Results --

The major result of the study is that, in contrast to what is generally assumed, also short cells from leaves can be statistically separated to identify different taxa. Further important results arising from the PHYTORES research project are:

1) Culms, leaves and inflorescences of Eleusine coracana, Pennisetum glaucum and Sorghum bicolor produce different phytolith morphotypes and in different ratios. As expected, leaves of Eleusine coracana can easily be distinguished from the other two taxa by the presence of saddle short cell phytoliths.
2) The bilobate phytoliths from leaves of Pennisetum glaucum and Sorghum bicolor significantly differ in size and shape. This offers possibilities for taxonomic identification of non-dietary crop plants of these taxa at archaeological sites.
3) The bilobate phytoliths from leaves of Panicum miliaceum and Setaria italica show subtle differences. Further research on the possibilities of these closely related taxa has been initiated and will be carried out during the new phase of research to be developed at Kiel (Germany).
4) The systematic sampling strategy allowed to produce statistically sound samples that gave the possibility to generate strong and significant conclusions on phytolith production and diversity in cereals, and to explore the variability of phytolith morphometry within species, populations, plants and organs (leaf).
5) The archaeological pilot project using phytolith samples from two Sudan early Neolithic cemeteries gave the possibility to apply the results from the experimental part of PHYTORES. The results of this pilot made apparent the exploitation of wild grasses and cultivated cereals during the Sudanese Neolithic, something that was never achieved before using traditional archaeobotanical evidence (e.g. macro-remains). The two important strands of results in here were to first check the validity of our experimental study and to expand our knowledge on plant exploitation in a period of important changes such as the passage from foraging to cultivation. Both aims were positively fulfilled.
6) The project developed a new, user-friendly method based on open-source software (FIJI) that is proposed as a step towards the introduction of a standard methodology for micro-morphometry. The method works by obtaining a mask of a phytolith by making a digital drawing and measure 27 commonly used variables of size shape. This method is not only useful for phytolith analysis but also for other fields of morphometric research. Users can furthermore customize the software tool when additional variables are required.

-- Impact --

The project PHYTORES has furthered the development of an archaeobotanical methodology for plant microremains (phytoliths), deepened the understanding of economic importance of secondary products such as straw and processing leftover, and further developed the taxonomic identification of phytolith for certain taxa (millets) and plant parts (leaves) that have until now received only little attention. These results are relevant for the fields of Archaeobotany, Archaeology, Palaeoecology and Taxonomy, and can be applied to studies in Europe, Asia and Africa. Highly significant results include an innovative tool for morphometric measurements, the possibility to distinguish two major millet crops (Pennisetum glaucum and Sorghum bicolor) by phytolith analysis, and finally new groundbreaking archaeobotanical data from Neolithic Sudan.

As a result from training and research developments, the Fellow gained substantial new experience in methods and theory of phytolith analysis, morphometry, software understanding and basic development, statistical analysis, leadership skills, multicultural negotiation skills, and project management. The offered possibilities for career exploration were fully exploited, resulting in a wide dissemination of the produced research as well as additional innovative archaeobotanical research in cooperation with international groups (e.g. British Museum - UK). It has been demonstrated that the high standard of training, the experience gained and the active dissemination has helped the researcher to return from the industry and to continue her scientific career in an academic environment. Indeed, she has been appointed at a new postdoctoral position in Archaeobotany for at least three years at the University of Kiel (Germany). This also positively affects the number of women in science, the mobility of researchers in Europe, and the intra-European transfer of knowledge and cooperation.

The reference collection of the host institute has been expanded with plant material, prepared samples of plant material and photographs of phytoliths, which will be available for future research by the team of the host institute. Part of the reference collection will be transferred to the new researcher’s host institute while the remaining collection will still be available to her for future work.