Preservation of fungi in archaeological charcoal

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Abstract

During the analysis of wood charcoal remains from archaeological sites, it is common to find different microorganisms and different forms of degradation present in the plant tissue. However, one may encounter difficulties when attempting to identify these microorganisms and determine when their attack occurred. This paper focuses on preservation aspects related to the microorganisms in wood and demonstrates the structural changes that take place in different types of decayed wood after it was converted into charcoal. The study seeks to determine whether the microbial attack found in archaeological woods took place before the burning of the wood or after. Burning experiments were conducted using wood that had been decayed by various types of fungi including white-rot, brown-rot, and soft-rot. The laboratory burnt wood samples showed decay patterns that were comparable to those observed in archaeological charcoal samples, indicating that signs of fungal infestation and features of decay can be preserved after burning with micromorphological details of mycelium and cell wall attack evident. This indication may provide important information related to the gathering of deadwood as fuelwood. In addition, examples of decayed wood preserved in archaeological charcoal assemblages are described. Their relationship to the archaeological context and environmental conditions may suggest different interpretative models concerning wood management strategies applied by past societies.

Introduction

Anthracology is a discipline that investigates the remains of wood charcoals from archaeological excavations and natural deposits (Chabal et al., 1999). The charcoal is a final product of the carbonization process as well as a result of incomplete combustion during the charring process (Braadbaart and Poole, 2008, Fengel and Weneger, 1983, Smart and Hoffman, 1988). The charcoal retains the anatomical structure of the wood and may permit its botanical identification. Their taxonomic identification depends on anatomical characteristics of the species, the size of the charcoal fragments, and their state of preservation (Chabal et al., 1999, Schweingruber, 1982). In the field of anthracology, a special methodology that advances both palaeoethnographical and palaeoenvironmental information exists that has demonstrated the importance of charcoal analysis in archaeobotany and archaeology (Asouti and Austin, 2005, Badal García, 1992, Chabal, 1997, Chabal et al., 1999, Carrión Marco, 2005, Figueiral and Mosbrugger, 2000, Heinz and Thièbault, 1998, Lityńska-Zając and Wasylikowa, 2005, Marguerie and Hunot, 2007, Ntinou, 2002, Smart and Hoffman, 1988).

One of the purposes of anthracological analysis is to gain palaeoethnographic information about wood used by humans. For example, in the case of fuelwood, there is an assumption that firewood gathering derives from simple necessity based on availability and effort required rather than intentional selection of a particular species of wood, which is called “Principle of Least Effort” (Shackleton and Prins, 1992). Ethnographic studies have shown that this important human activity depends on the wood's abundance, the ease of collecting, and the ability to transport the wood. Deadwood recovered from a forest in the proximity to the habitation site fulfills these requirements, and the documentation of its gathering may lead to the formulation of new hypotheses that permit understanding the management of forest communities by past societies (Asouti, 2005, Asouti and Austin, 2005, Salisbury and Jane, 1940, Théry-Parisot, 2001). Palaeoethnographic information concerning the use of deadwood recovered from forest floor or attached to standing trees may be obtained after observing characteristic features of the decayed wood such as changes in anatomical structure and the presence of microorganisms, and determining if the microbial attack took place before or after burning.

The main objective of the present study is to confirm that hyphae and other structures produced by microorganisms can be preserved within the wood charcoal after the burning process. In previous work, Schweingruber (1982, p. 204) had indicated that in charcoal, fungal hyphae may be preserved in addition to structural features of the wood. Also, Théry-Parisot (2001) conducted significant experiments related to the detection of rotten wood and driftwood in charcoalified samples. Théry-Parisot had demonstrated that hyphae may be preserved in the charcoal fragments after the burning process. This statement later served as a basis for an assessment of the deadwood present in the anthracological assemblages (Badal, 2004, Carrión Marco, 2005). Reflectance microscopy is a promising method that may help to prove the microbial preservation within the charcoal structure and may also serve as a technique that can distinguish between burnt and uncharred material. In particular, the incidences of microbial and animal organs along with the remnants of the arthropod's fecal pellets were documented in the plant material. The analysis showed that both the aforementioned materials and the charcoals exhibited high reflectance, confirming their charred conditions (Scott et al., 2000). Nonetheless, the way the microbes are preserved in the charcoals still remains unclear. Heiss and Oeggl (2008) recently performed studies on a fuelwood used in prehistoric copper mines in which special attention was paid to fungus-infected charcoal fragments. However, the evaluation of the pre-burning attack was based on the supposition that “wood-decaying fungi are destroyed during the carbonization” (Heiss and Oeggl, 2008, p. 214). Therefore, only the material in which hyphal imprints are observed was taken into account in the gathered–stored wood interpretation while charcoals with fungal hyphae were considered to result from modern contamination.

Also, it was important to examine the charcoal fragments to determine if characteristic patterns were preserved in the anatomical structure of decayed wood. Many studies have demonstrated special features related to the different types of wood degradation observed in xylem (Blanchette et al., 1990, Blanchette, 2000, Blanchette, 2003, Eriksson et al., 1990) and it would be valuable to verify if the same pattern could be documented in the wood structure after charcoalification. It is important to observe whether the changes in microstructure of the cell walls (erosion or cavities) may also be evident after burning, since homogenization of all cell layers happens during this process. This objective may in turn confirm fungal actions in the wood before the burning process and thereby indicate the use of infested wood by the humans in the past.

Finally, some examples of archaeological charcoals that exhibit the characteristics of decayed wood are presented and discussed in relation to their contexts.

Section snippets

Background

During the analysis of charcoal fragments, different groups of organisms are usually observed within the anatomical structure of the wood; however, their identifications and mode of attack are difficult to establish. These groups include mainly fungi, bacteria, and animals such as insects or nematodes which are responsible for degradation processes of living and dead plants in almost all environments (Blanchette, 2000, Blanchette, 2003, Caneva et al., 2008, Eriksson et al., 1990, Koestler

Wood samples

Six wood samples that represent the major fungal degradation groups were prepared for conversion to charcoal. The first group included two samples, a coniferous wood Tsuga sp. (Fig. 1) and a hardwood (Populus sp.), infected with brown-rot. The second group of hardwoods was infected with white-rot fungi and included one sample from a deciduous oak (Quercus sp.; Fig. 2) and two poplar samples (Populus sp.; Fig. 3). All of these woods were collected from downed trees on the ground in a natural

Experimental results

Observations of wood and charcoal samples under the microscope show similarities in decay patterns, and hyphae were identified in both types of samples, the foil-wrapped and unwrapped samples (Fig. 1, Fig. 2, Fig. 3, Fig. 4).

Conclusions

The biodeteriorated structure of wood and the fungi responsible for its decay exhibit similar characteristics in both wood and charcoal samples. Charcoal produced using a limited or unlimited oxygen supply seems to have little influence on the preservation of fungi because in both kinds of samples hyphae were documented as individual hypha, mass of mycelia or as hyphal imprints. The patterns of decayed wood attacked by the three major wood-rot groups may be a valuable source of information when

Acknowledgements

The authors would like to express their gratitude to Dr. R.J. Koestler and Dr. P. DePriest from the Smithsonian Museum Conservation Institute and Prof. Dr. E. Badal from the University of Valencia for their valuable comments. We would like to thank Prof. Dr. P. Raczky, Dr. A. Anders (Eötvös Loránd University, Budapest), A. Matoga (Archaeological Museum, Kraków), and the Kraków Archaeological Team for Excavation of the Motorways for providing archaeological charcoals. Special thanks to J. Watson

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