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Archaeobotanical Analysis: Principles and Methods

by Karen R. Adams

Introduction
Sample Types
General Principles and Conventions
   Charred vs. Noncharred Remains
   Wood Charcoal vs. Charred Nonwood Specimens
   Naming Conventions
Methods
   Macrofossils
      Wood Charcoal
      Charred Nonwood Specimens
      Modified Vegetal Items
   Microfossils: Flotation Samples
      Processing
      Subsampling for Wood Charcoal
      Subsampling for Charred Nonwood Specimens
      Examination and Identification
      Indices
Additional Resources
References Cited
To Cite This Publication


Introduction

1
Archaeologists collect ancient plant remains in an effort to understand prehistoric plant use by the inhabitants of a site or locale and to reconstruct the nature and composition of vegetation communities that existed in the past. Plant remains collected from archaeological contexts provide information about both wild and domesticated plant resources used for food, fuel, construction, and other purposes. Because people generally used resources that were available locally, plant assemblages can also provide important clues about the characteristics of the natural vegetation that formerly grew in the vicinity of sites. Under certain circumstances, data derived from ancient plant assemblages can also provide insights into seasonality of site use and abandonment, paleoenvironmental conditions, and human nutrition and health; this is especially true when the plant data are evaluated in conjunction with other kinds of evidence, including pollen, tree-ring, faunal, and human osteological data.

2
The purpose of this publication is to describe for researchers and other interested readers the types of archaeobotanical samples collected by the Crow Canyon Archaeological Center and the methods used to process and analyze them.1 This information should allow others to evaluate not only the methods employed, but also the data obtained as a result—data which are reported in detail in The Crow Canyon Archaeological Center Research Database and in individual chapters in Crow Canyon's series of on-line site reports.2 Although this publication is designed primarily as a companion piece to the site reports, researchers and students might find the information contained herein useful in guiding their own studies as well.

3
This publication deals with plant remains collected as individual vegetal specimens and plant remains recovered in flotation samples. It does not deal with pieces of wood collected as tree-ring samples, which are processed and analyzed by the Laboratory of Tree-Ring Research in Tucson, Arizona, or with pollen samples, which are processed and analyzed by various independent contractors. For a description of how tree-ring and pollen samples are collected in the field, refer to Crow Canyon's on-line field manual.

Sample Types

4
The plant remains collected from sites excavated by the Crow Canyon Archaeological Center fall into one of two broad categories depending on their size and the method used to collect them. Macrofossils are pieces of plants that are large enough to be seen with the unaided eye and that are retrieved by hand during the excavation and screening of sediment in the field. The majority of macrofossils are submitted to the laboratory as vegetal specimens (data-entry code: VEG), and most often they consist of pieces of wood charcoal, although corn cobs, squash seeds, and other large, distinctive plant remains are occasionally collected as well. Also included in this category are plant materials that were intentionally modified into tools, containers, or other serviceable items; these are submitted to the laboratory as basketry (BAS), textile (TEX), or "other modified vegetal" (OMV) specimens. Macrofossil remains constitute a subjective sample of the contents of a given deposit, because collection depends on what an individual excavator decides to retrieve. For example, all modified vegetal specimens recognized in the field are collected, but it is impractical for excavators to retrieve every piece of wood charcoal observed in the course of digging a burned structure.

5
Microfossils are tiny pieces of plant material recovered specifically in flotation samples (data-entry code: FLO). Small seeds, fruits, and other reproductive parts make up the majority of items in this category, but tiny fragments of wood and other plant parts may be recovered as well.3 Flotation samples consist of a standard volume of sediment (usually 1 liter), and they are routinely collected from contexts where plant remains are expected or perceived to be plentiful. These contexts include primary refuse (for example, ash in thermal features), secondary refuse (middens and trash fill in structures), collapsed structure roofs ("roof fall"), and other cultural deposits. Because they are collected and processed systematically, flotation samples provide relatively unbiased data about prehistoric plant use at a site. For a more detailed discussion of when and how flotation samples are collected at sites excavated by Crow Canyon, refer to the field manual.

General Principles and Conventions

Charred vs. Noncharred Remains

6
Both charred and noncharred plant remains are recovered from sites excavated by the Crow Canyon Archaeological Center. Charring is usually considered to be the result of prehistoric cultural activities; noncharred remains are more likely to be modern or to have been introduced through natural processes such as rodent activity and wind (Minnis 1981*1:147). Therefore, most often only charred (including partly charred) materials are considered in analyses of Crow Canyon archaeobotanical assemblages; the presence of noncharred specimens is noted, but these items are not usually included in assessments of prehistoric plant use. There are exceptions to this rule, however, and every instance is considered on a case-by-case basis. For example, the recovery of a single charred domesticated wheat (Triticum) grain from Troy's Tower (Site 5MT3951), a Pueblo III site tested by Crow Canyon (Varien 1999*10), can be explained by the fact that in modern times wheat stubble had been burned in the agricultural field in which the site is located (Adams 1999*3). On the other hand, the recovery of a noncharred gourd container from Sand Canyon Pueblo (Site 5MT765), another Pueblo III site excavated by Crow Canyon, clearly is a case of extraordinary preservation of a prehistoric artifact (Adams 2000*1).

Wood Charcoal vs. Charred Nonwood Specimens

7
Charcoal is defined as any "dark or black porous carbon prepared from vegetable or animal substances" (Merriam-Webster 1996*1:192). Most archaeobotanists, however, restrict the use of the word "charcoal" to describe burned wood specifically—that is, charred pieces of trunks, branches, and twigs from both trees and shrubs. The charred remains of the nonwoody parts of trees and shrubs (for example, fruits and seeds) and the charred remains of nonwoody plants (herbaceous perennials and grasses) are generally not described as charcoal, even though they, too, clearly fall under the strict definition. Instead, such specimens are usually referred to in more-specific descriptive terms, such as "reproductive" or "vegetative" parts, without continued reference to their charred condition. Because of the potential for confusion, especially among readers who are not archaeobotanists, I have begun referring to the two categories as "wood charcoal" and "charred nonwood specimens." This convention should make clear that plant remains in both categories are burned, but it also preserves the custom with regard to the use of the word "charcoal" that has established itself in the archaeobotanical literature.

Naming Conventions

8
Because many plants are known by multiple common names, which vary according to geographic region and local custom, the only consistent and reliable means of reporting data is to use standard scientific nomenclature. Therefore, in all Crow Canyon site reports, plants are referred to first by their scientific names, then by their common names. The latter are provided either parenthetically in text or in tables showing the correspondence between scientific and common names. Once the correspondence has been established in a given publication, both scientific and common names are often used interchangeably, depending on the context. The taxonomic nomenclature (both scientific and common names) used in Crow Canyon publications conforms to A Utah Flora (Welsh et al. 1987*1) whenever possible.

9
When a specimen is identified by a combination of two genus names—for example, "Amelanchier/Peraphyllum" or "Prunus/Rosa"—it means that the specimen could be a member of either of the named genera and that a more precise determination was not possible. The order in which the names appear does not indicate which genus identification is most likely correct; the names are simply listed alphabetically. Some specimens in a given sample are identified to the genus level; others in the same sample may be identified to both the genus and species levels. This occurs because some specimens are better preserved than others. For interpretive purposes, these items most often would be combined at the broadest level of identification (see discussion of taxon diversity, paragraph 28).

10
One special case involving the identification of plant remains to two possible genera requires explanation. Archaeobotanists use the common name "cheno-am" (lowercase) to refer to seeds that could be from plants in either the genus Chenopodium or the genus Amaranthus. Palynologists, on the other hand, use "Cheno-am" (uppercase) to refer to pollen grains that could be from either the family Chenopodiaceae or the genus Amaranthus. Except for the difference in capitalization, there is no obvious clue that the definition used by archaeobotanists is somewhat narrower than that used by palynologists.

11
The taxonomic level to which any given specimen is identified depends on a number of factors, including the "comfort level" of the individual analyst, the condition of the specimen, and whether or not other similar-appearing items are known from the region. (Only angiosperms, or flowering plants, are identified to the subclass level—that is, identified as Monocotyledons or Dicotyledons.) Following Bohrer and Adams (1977*1:41), analysts indicate one of three levels of confidence for each identification: "absolute," "-type," or "compares favorably." An "absolute" identification to the level of species indicates that all species of the relevant genus in the local environment have been examined, and the specimen in question appears identical to the named species. The label "-type" signifies that the specimen has morphological characteristics that closely resemble those of the named species, but that other plants in the area might also have similar-looking parts. "Compares favorably" (abbreviated "cf.") is used when a specimen is difficult to identify because of poor preservation or some other factor, but its characteristics suggest that it is more likely to be of the named species than of some other species; the use of this abbreviation indicates a "best guess" assessment. Although species is the taxonomic category used in the foregoing explanations, the three qualifiers can be used for identifications to any taxonomic level. In Crow Canyon reports, analytic confidence is indicated in tables by the inclusion of either "cf." or "-type" preceding or following the scientific name (for example, cf. Pinus or Pinus-type). Scientific names that do not include either of these qualifiers should be assumed to indicate "absolute" identifications (for example, Zea mays refers to a positive identification to this species). Including qualifiers every time a scientific name is mentioned in a discussion, however, would be cumbersome; for that reason, they are generally omitted from text. Readers should refer to the tables that accompany the text if they wish to know the level of confidence of a given taxonomic identification.

12
In cases in which a prehistoric specimen cannot be assigned to a specific taxonomic category, the item is recorded as "unknown." If similar or identical specimens are found with some frequency, additional efforts at identification are made. If those efforts fail, the "unknown" is described in terms of its key morphological and anatomical features. If recognized, nonbotanical materials such as bone, coal, termite fecal pellets, insect parts, gastropods, and "black spherical bodies" are recorded as present in samples.

Methods

Macrofossils

Wood Charcoal

13
Charred wood preserves well in archaeological contexts because it is more resistant than noncharred wood to decay and insect damage. Some specimens even retain an outer layer of bark, although most do not. Identification of wood charcoal in archaeological assemblages can provide valuable information about the types of wood used for fuel and in construction.

Subsampling

14
Because wood charcoal is usually plentiful in archaeological contexts, economy dictates that only a subsample of all the specimens submitted be examined. The first step in analyzing wood charcoal is to pour the entire contents of a given bag of plant material (which may contain both wood and nonwood vegetal specimens) onto newspaper or a lab tray so that each individual item is visible. The analyst then attempts to select an unbiased subsample of 20 pieces of wood charcoal by choosing specimens of different sizes and retrieving items from different sections of the newspaper or lab tray. If fewer than 20 pieces of charred wood are present, all are examined; if more than 20 pieces are present, additional specimens that appear morphologically distinctive to the naked eye are examined after the first set of 20 has been analyzed.

Examination

15
Each piece of wood charcoal is snapped in half so that a fresh transverse (cross) section is exposed. This section is then examined under a dissecting binocular microscope at magnifications of 10 to 45X. Often the pieces are placed in a sand-filled petri dish to stabilize them for ease of viewing.

Identification

16
Each piece of wood charcoal examined is identified to the finest of four possible taxonomic levels, listed here from most general to most specific: subclass, family, genus, and species. Wood charcoal identifications are based on anatomical traits (for example, rings, vessels, rays, and background patterning) viewed in cross section. Ancient wood charcoal is identified using Crow Canyon's modern wood charcoal comparative collection from the region, backed by plant voucher specimens in the University of Arizona herbarium in Tucson. Because of similarities among woods of some plant families or genera in the region, more wood types may be present in Crow Canyon archaeological assemblages than are actually indicated in the data summaries. For example, wood identified as Pinus may be either Pinus edulis or Pinus ponderosa. The identification criteria used in the analysis of wood charcoal collected from sites excavated by Crow Canyon are described in detail in Plant Identification Criteria, an on-line resource for researchers.

Charred Nonwood Specimens

17
Nonwood plant specimens include both agricultural and wild resources, and they usually consist of reproductive parts such as seeds or fruit. Excavators collect recognizable pieces of domesticates, such as maize (Zea mays) cobs and kernels, squash (Cucurbita) seeds and rinds, and bean (Phaseolus) seeds. They also collect pinyon (Pinus edulis) nutshells, yucca (Yucca baccata) fruits and seeds, and a wide variety of other wild plant parts, including such vegetative parts as leaves, stalks, and roots.

Examination

18
All charred nonwood specimens are removed from bags of macrofossils whose contents have been thinly spread out on newspaper or a lab tray so that all specimens are visible. Each item is examined under a dissecting binocular microscope (10 to 45X) and, if possible, identified. The nonwood materials are bagged and labeled separately from the wood charcoal found in the same sample.

Identification

19
The identification criteria for reproductive plant parts include size, shape, surface texture, and points of attachment. Seeds are identified using the author's modern comparative collection, backed by University of Arizona herbarium voucher specimens. Reference texts, such as Delorit (1970*1), Landers and Johnson (1976*1), and Martin and Barkley (1961*1), are also used. Vegetative parts such as leaves and stems are identified on the basis of internal anatomy and morphology. If an item cannot be identified by reference to the collections or texts, it is measured and described as an "unknown."

Modified Vegetal Items

20
Plant materials were sometimes modified (cut, split, woven, or otherwise shaped) in order to fashion them into various utilitarian or decorative items, such as baskets or other containers, mats, plaques, sandals, and tools. These items—whole and fragmentary, charred and noncharred—occasionally preserve in archaeological deposits and, when intact enough to be recognized in the field, are collected as macrofossil specimens. Because of their generally fragile condition, special care is used in handling them during excavation, processing, and analysis.

Examination

21
Modified vegetal artifacts are fully described both quantitatively (measured, counted) and qualitatively (text descriptions). Microscope examination is sometimes required to determine the anatomical details of the constituent elements. Very rarely, small portions of the items are broken off to expose fresh transverse (cross) sections.

Identification

22
The identification conventions cited above for wood charcoal and charred nonwood specimens (see paragraph 16 and paragraph 19) are also applied to modified vegetal items. The raw materials of construction are identified to the most specific taxonomic level possible. Although many modified vegetal items are constructed of a single plant or plant part, some are made up of more than one plant or plant part. In such cases, each of the constituent elements is identified and described, if possible. The terminology used to describe modified items as artifacts conforms to conventions published by specialists familiar with each type of artifact.

Microfossils: Flotation Samples

Processing

23
The steps used to process flotation samples are outlined in detail in the on-line laboratory manual, but the procedure basically involves placing an individual sample in a bucket, adding water, then gently stirring the mixture to free the organic materials. The inorganic material that settles to the bottom of the bucket constitutes the "heavy fraction," which is collected, allowed to dry, and stored. Charred and noncharred plant remains that float to the surface are poured into a fine (0.355-mm) mesh to be captured as the "light fraction." The light fraction is allowed to dry before being sifted through a series of geologic sieves. This process separates the light (organic) fraction into 4.75-mm, 2.80-mm, 1.40-mm, 0.71-mm, and 0.25-mm subsamples, which are then individually bagged and labeled. Although the size of the original mesh used to capture the entire light fraction (0.355 mm) is larger than the finest mesh used to create the subsamples (0.25 mm), smaller particles that adhere to larger particles when they are wet can detach as the residue dries and be caught in the 0.25-mm screen used during dry screening. Plant remains also continue to break into smaller pieces whenever a sample is handled.

Subsampling for Wood Charcoal

24
Analysts examine the flotation light fraction in two steps, using a dissecting binocular light microscope at magnifications ranging from 10 to 45X. The first step involves subsampling the wood charcoal. The analyst begins by choosing 20 pieces of wood (using the method described in paragraph 14) from the 4.75-mm portion, because the large size of the individual specimens allows for more-confident identification. If the 4.75-mm portion does not contain 20 pieces of wood, some of the larger specimens in the 2.80-mm portion are selected to achieve the goal of examining 20 pieces total. After the initial subsample of 20 pieces is analyzed, additional pieces that appear morphologically distinctive to the unaided eye are also examined and identified.

Subsampling for Charred Nonwood Specimens

25
The second step in analyzing plant remains from flotation samples involves looking for nonwood remains such as seeds, fruit, other reproductive structures, and nonwood vegetative parts. Samples less than 50 ml in total light-fraction volume are analyzed completely. For samples with light fractions that measure 50 ml or more, the following protocol applies: The 4.75- and 2.80-mm portions are completely examined under the microscope for seeds and for other reproductive and vegetative parts. An approach known as the "species area curve" (Mueller-Dombois and Ellenberg 1974*1:52–53) is then used to subsample the 1.40- and 0.71-mm portions. This approach maximizes the number of taxa recorded while minimizing the volume of sample sorted (Adams 1993*1:196). The goal is to identify the maximum number of taxa represented in a sample, rather than to record the total number of whole or fragmented items of each taxon. The 1.40- and 0.71-mm portions are sorted in increments of 0.90 ml. Each subsample of 0.90 ml is measured with a graduated cylinder. No new taxa must be identified in three successive 0.90-ml subsamples for the sample sieve size to be considered completely analyzed. If remains of new taxa are observed, then an additional three 0.90-ml subsamples are examined until no new taxa are identified. In a similar manner, the portion captured in the 0.25-mm screen is sorted in increments of 0.30 ml, again using the species-area-curve approach. Materials that pass through the 0.25-mm screen are not examined, because seeds of this size are usually also preserved in screens with larger mesh sizes and because fragmented items of this size are extremely difficult to identify.

Examination and Identification

26
The conventions cited above for examining and identifying macrofossils (paragraph 16 and paragraph 19) also apply to items recovered in flotation samples.

Indices

Taxon Ubiquity

27
Taxon ubiquity, that is, the number of samples in which the remains of plants of a given taxon occur within a total universe of samples analyzed, provides insight into the frequency of use of a plant resource in prehistory. Ubiquity is calculated for flotation samples only, and the measure is expressed as a percentage. To calculate ubiquity for a given taxon, the site or other sample set of interest must first be selected, then the number of flotation samples in which plant parts of the given taxon occur is counted. This number is divided by the total number of flotation samples analyzed for the site or sample set, and the resulting figure is converted to a percentage. For example, the presence of cheno-am seeds in six of 15 analyzed flotation samples would constitute a ubiquity of 40 percent. From this, it would be inferred that cheno-ams were a commonly used resource. Sterile and nonproductive samples—that is, flotation samples that yield no botanical remains and samples that yield only unidentifiable botanical remains—are included in the total number of samples analyzed.

Taxon Diversity

28
In any given sample set, and even within individual flotation samples, there may be items identified to similar, but not identical, taxonomic levels. When assessing taxonomic diversity within a given sample, analysts at Crow Canyon take a conservative approach and combine certain records for the purpose of counting the number of different taxa represented. For example, if a flotation sample contains both Artemisia tridentata–type wood charcoal and Artemisia-type wood charcoal, the two taxonomic levels are combined and considered to represent only a single taxon (Artemisia) for the purpose of determining the diversity of potential wood types in the sample. A single taxon represented by more than one plant part may be handled slightly differently, however, depending on the parts present and the types of prehistoric uses they are believed to represent. For example, Pinus-type bark scales, wood charcoal, and cone scales identified in a single flotation sample would be counted as a single taxon because all are believed to represent the use of pine for fuel. However, if Pinus-type nutshell fragments were identified in the same sample, they would be counted independently in discussions of food taxa because they are believed to represent the use of pine for food.

Taxon Density

29
Archaeobotanists often calculate the number of plant parts per unit of sediment volume examined—for example, the number of juniper seeds per liter of flotation sample. Taxon density is not calculated for Crow Canyon samples, because not all examples of each taxon/part combination within each sample are identified or counted, and only rarely are samples examined in their entirety. As stated earlier, the goal when analyzing flotation samples from Crow Canyon sites is to identify the maximum number of taxa represented in a sample, not to record the total number of whole or fragmented specimens of each taxon/part combination that is present.

Additional Resources

30
A number of additional resources available on Crow Canyon's Web site supplement the information presented in this document:

  • Ethnographic Uses of Plants: This database lists historic uses of all plant parts identified in plant assemblages from sites excavated by the Crow Canyon Archaeological Center. Full references to the primary ethnographic literature are provided.
  • Plant Identification Criteria: This document details all metric and nonmetric data relevant to the identification of all taxa and plant parts in assemblages from sites excavated by Crow Canyon. Descriptions of selected taxa are accompanied by photographs, most of them taken through the microscope.
  • The Crow Canyon Archaeological Center Research Database: This large database includes data for all analyzed archaeobotanical samples and specimens collected at sites investigated by Crow Canyon.

1Archaeobotanical samples and specimens are collected by Crow Canyon archaeologists and participants in the Center's research and education programs. The materials are processed and analyzed by both the author and numerous student interns working under the author's supervision.

2Five on-line chapters (Adams 1999*3; Adams and Brown 2000*1; Bowyer and Adams 2004*1; Murray and Jackman-Craig 2003*1; Rainey and Jezik 2002*1), as well as one chapter published in traditional print form (Adams 1993*1), also include descriptions of the methods used to process samples and analyze plant remains. The methods described in these earlier publications document the gradual refinement of the procedures used at Crow Canyon, culminating in the protocol described in the current document.

3Pollen grains, which are not visible to the naked eye, constitute a distinctive category of plant remains subject to specialized sampling, processing, and analysis techniques. Although pollen data are sometimes presented and discussed in Crow Canyon publications, those data are provided by independent palynological consultants, whose methods are described in their individual reports.


Karen R. Adams (Ph.D., University of Arizona, 1988) is an independent consultant in archaeobotany with more than 30 years of experience in the American Southwest and northern Mexico.


To cite this publication:

Adams, Karen R.
2004 Archaeobotanical Analysis: Principles and Methods [HTML Title]. Available: http://www.crowcanyon.org/plantmethods. Date of use: day month year.*

*Example: Date of use: 26 November 2004.


Copyright © 2004 by Crow Canyon Archaeological Center. All rights reserved.