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Methodology
Introduction
Palaeoecologists and archaeologists have developed an important body of work relating to the reflexive long-term changes that occur in humans and the plant environments they live in. Much of this research has relied on the analysis of pollen, spores, and charcoal concentrations in sediment cores, but studying human-plant interactions at regional scales are challenging because many plants cannot be detected in these records. Carbonised botanical materials from archaeological contexts, on the other hand, provide direct low-level information about human use of plants in the past. In combination, these data sets inform broadly on plant environments and human activity in the distant past, and are useful to address big-picture research questions (e.g., Frawley and O’Connor 2010; Nelle, Dreibrodt, and Dannath 2010; Touflan, Talon, and Walsh 2010). Archaeological wood charcoal analysis is an important component of this research.
In this chapter, I first review methods generally used to analyse charcoal from archaeological contexts and then describe the methods specifically used in the present study. Methods archaeologists use to collect and analyse fossil wood charcoal are reviewed in the first section. This review begins with a technical description of wood structure and the charcoalification process. A general background of the practice of archaeological wood charcoal analysis is then presented, and some important Polynesian studies are reviewed. Current methods used to study archaeological wood charcoal assemblages are then explained. Recovery and analysis methodologies have been developed by palaeoethnobotanists to study all types of plant materials from archaeological contexts (recently reviewed by Wright 2010), and methods of analysing charcoal have been further refined by anthracologists to perform vegetation reconstructions and, more recently, to study past human activities (reviewed by Théry-Parisot, Chabal, and Chrzavzez 2010). While these two bodies of literature are not often cross-referenced, it was noted that they share many similar themes and both are incorporated into the following review. In this review, I consider various parameters that affect archaeobotanical assemblages including cultural, natural, and analytical influences, and evaluate the current state of research within the discipline. In the last part of this chapter, methods used in the present study are described, including details of the field methodology, procedures used to assemble the reference collection, and laboratory methods. The rationale used in sample selection and a summary of the sub-sampling procedures are also described.
Several terms are defined before delving into a methodological review. First, studies of ancient plant materials are alternately referred to as palaeoethnobotany and archaeobotany, though these two terms have somewhat different meanings. Archaeobotany is the study of plant materials preserved in archaeological contexts, and it encompasses the technical and scientific analyses of these remains. It is considered fundamental to palaeoethnobotany, which is a more interpretive approach focusing on the study of past human and plant interactions (e.g., Ford 1979; Hastorf and Popper 1988; Pearsall 2000). The sub-field of wood charcoal analysis is sometimes referred to as anthracology, a term used primarily by researchers affiliated with the University of Montpellier 2 in France (e.g., Chabal et al. 1999).
Wood structure
Wood is a rigid plant tissue used for the transport of water and nutrients. These tissues are incrusted with lignin, and organic compound, which provides structural support for plants to grow into large forms such as shrubs and trees. Many parts of trees including the trunk, branches, roots, and seeds contain woody tissues (Evert and Esau 2006). Many gymnosperms and angiosperms, and some ferns, produce woody tissues. They include the conifers (softwoods) as well as several other gymnosperms, the dicotyledons (hardwoods), and woody monocotyledons (most notably, palms and bamboos). The internal structure of hardwoods, softwoods, and woody palms are different.
Both hardwoods and softwoods have similar xylem structures composed of cells that support two systems: an axial system, which primarily transports water and nutrients up and down the stem, and a radial system that conveys these materials from the outside to the centre of the stem. Woody monocotyledons have a much simpler structure composed of vascular bundles that include xylem and phloem, and rigid fibres surrounded by a mass of parenchyma tissues. Transverse sections of these materials are illustrated in Figure 5.1.
The axial system of a hardwood tree is composed of various types of cells that have different functions including vessels, axial (longitudinal) parenchyma, and fibres. The radial system is entirely composed of parenchyma cells. Anatomists describe wood structure in a standardised way based on features of these two systems, which are observed at both macro-and microscopic levels. Wood identification typically involves the examination of three planes (Figure 5.2) that are carefully exposed for study by cutting small square blocks with a microtome or razor blade. The transverse plane (TS) displays a cross section of vessels, axial parenchyma, and fibres, and a lateral view of ray cells. The tangential longitudinal plane (TLS) reveals a cross section of ray parenchyma cells, and a longitudinal view of the axial cells. The radial plane (RLS or RS) shows a distinctive longitudinal view of ray cells, and an additional longitudinal view of the axial system cells.
Vessels arrangements in hardwoods are classified into several high-level categories based on vessel porosity that are described as ring-porous, semi-ring porous, or diffuse porous (Wheeler, Baas, and Gasson 1989). Vessel size and grouping arrangements are also distinctive features. Rays are classified by a combination of attributes including width, which can range from one cell wide (uniseriate) to many cells wide (multiseriate), and by cell orientations (procumbent, square, and/or upright) (Kribs 1935). Axial parenchyma patterns are also important for wood classification; they are grouped by degree of association with vessels, by shape, and/or by width when in continuous bands. Many other features are important to wood classification, including minute attributes of cellular anatomy, the presence of mineral inclusions, and presence of various secretory elements.
Comparative wood anatomists have demonstrated that closely related taxa share many anatomical features (e.g., Carlquist 2001, 335–350). These distinctions should, in theory, provide a means to identify wood from trees that are closely related, but in practice there are a number of internal and external factors that also influence anatomical variation. Some are briefly reviewed to illustrate the challenges to and limitations of wood identification.
Firstly, the rate of wood formation within a tree is influenced by numerous environmental factors. Structural changes may be affected by variations in the local environment including available moisture, temperature, and soil conditions (Metcalfe and Chalk 1950, 2:152–6). For example, vessel diameter and length has been shown to vary in some woods based on the elevation, latitude, or amount of rainfall the tree has been exposed to (e.g., van der Graaff and Baas 1974). Variations in local climate can also influence ring porosity and vessel size, and these differences can be notable in trees that have a wide biogeographic range. For example, those growing in temperate zones tend to produce larger and more frequent vessels early in the growing season.
Secondly, sapwood anatomy can vary from heartwood. While sapwood is living tissue, heartwood is not. Oftentimes it darkens in colour as deposits accumulate in the tissues, and ultimately heartwood becomes less permeable (Soerianegara and Lemmens 1993, 26).
Wood structure is also influenced by when and where within the tree it was formed. A typical growth pattern is illustrated in Figure 5.3. New wood is formed by the cambium, a thin layer of cells that lie beneath the inner bark. While mature cambium produces outerwood as parts of the tree continues to grow in girth, young cambium produces juvenile wood (sometimes called corewood). This wood is typically formed at the crown and the tips of branches, and is found in the inner 5 to 20 (or more) rings throughout the tree, with hardwoods falling at the lower end of this range. Juvenile wood has notable differences in cell size and arrangement including smaller, shorter vessels that tend to be more frequent, narrow rays with fewer cells than outerwood, and shorter fibres with thinner walls (Bendtsen 1978; Evert and Esau 2006; Lachenbruch, Moore, and Evans 2011; Lev-Yadun 2007; Metcalfe and Chalk 1950, 2:42-43; Rendle 1960). Aside from the aforementioned studies, which are generalised descriptions of wood formation, little specific information could be located regarding juvenile wood.
Another aspect of wood variability to consider are tree rings. This feature is of particular interest in a study of tropical woods, as they are often assumed to be absent, but research has demonstrated this is not always the case. Tree ring porosity is often used to classify woods from temperate regions as patterning can be very distinctive. Rings are formed when the cambium becomes dormant; tree growth slows considerably and a fine line of thin-walled marginal parenchyma is created (Metcalfe and Chalk 1950, 2:44). Dormancy generally occurs when leaves fall from the tree. In temperate climates, this typically occurs as temperatures drop, though it can also be triggered by very dry or wet conditions. In tropical climates the growth of most trees also slows, though researchers have not been able to precisely identify external triggers (Soerianegara and Lemmens 1993, 28–31). Wood anatomists have recently confirmed that tropical trees can produce rings at varying (i.e., not necessarily annual) rates and though they may be visible in some species, in many they are faint and sometimes invisible (Worbes and Fichtler 2010). Therefore, while ring presence may assist in some tropical wood identifications, they are ultimately not reliable features.
Finally, tree growth patterns can also react to fungus and insect pests, which can produce complicated modifications to the typical anatomy. Insects can, for example, defoliate trees to the extent that growth slows considerably. Wood that forms around damage, knots, under pressure, or under the influence of strong winds can contain cells that are atypically aligned (e.g., Schweingruber 2007).
Overall, certain environmental influences that affect wood anatomy have been more extensively studied than others, and causes of variation may not be well-understood. Though variability is broadly described in textbooks, woods from temperate regions are the most commonly cited examples, with woods of interest to the timber and paper industries being the most extensively documented types.
Charcoal
The term charcoal can refer to many types of carbonised plant materials including wood, seeds, nuts, fruits, and parenchymous (storage) tissues. Charcoal is an inert material largely composed of carbon, though it does have varying chemical and physical properties that relate to both the composition of the originating material and the heat source that was applied. These variables include differences in anatomy, degree of and direction of shrinkage, reflectance values, and elemental composition (Braadbaart and Poole 2008).
Wood becomes charcoalified when it is exposed to sufficient heat (usually between 300 to 400 degrees Celsius, per Braadbaart and Poole 2008) over time. Charcoal is formed when wood smoulders in the absence of oxygen; it can also form when wood exposed to open flame is incompletely combusted. During the charcoalification process, wood shrinks. As temperatures increase, wood shrinks to a greater extent, it becomes more reflective, and the carbon content increases. With enough heat and time, wood structure disintegrates and charcoal becomes ash (Figure 5.4).
The anatomical structure of wood is generally preserved when charcoalified, making taxonomic identifications possible. The structure of woody monocotyledon tissues are less distinctive, though it is straightforward to distinguish between different types such as coconut, Pandanus, and Cordyline. Nut shells, which can be abundant in Pacific archaeological contexts, are often well-preserved (e.g., Yen 1974b, 1982, 1991) , and fracture in predictable ways. Fragments of charred and carbonised parenchymous tissue, which can derive from underground storage tissues and a variety of other plant parts, are sometimes frequent but can be indistinct (but see Hather 2000a). On occasion, the external morphology of underground storage organs or fruit endocarp are retained and can be recognisable (e.g., Coil 2004; Coil and Kirch 2005; Hather and Kirch 1991; Kahn and Ragone 2013; Ladefoged, Graves, and Coil 2005; Rosendahl and Yen 1971; Yen and Head 1993), though these materials are rare.
Overall, charcoal tends to be durable and it is not subject to the biological processes that would otherwise destroy woody plant parts, but it can disintegrate into very small fragments when trampled, abraded, or subject to repeated wetting and drying cycles. Wood charcoal is the most common material recovered from archaeological contexts by either sieving or flotation (Pearsall 2000, 144). In the Pacific region, charred wood and nut shells form the bulk of archaeobotanical material recovered in excavation (Hather 1992) as few other botanical materials survive.
Brief history of the discipline
Archaeological wood charcoal analysis was one of the earliest forms of archaeobotany to be practised, dating to the mid-19th and early 20th centuries (e.g., Fietz 1926; Heer 1865, 38–41). However, the work of Salisbury and Jane (1940) is generally considered the inception of wood charcoal analysis as a specialised discipline. Their study was one of the first to attempt to reconstruct vegetation histories by examining archaeological wood charcoal on a large scale. The authors hypothesised that the proportions of charcoal taxa recovered from domestic fires should equate with the proportions of tree species in the local area in the past.
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