Dendrochronology is the study of the dating of trees by their annual rings and, subsequently, other objects, both historical and environmental events and processes. Because of some tree species’ long life spans and growth responses, a reliable pattern of temperature and precipitation has been recorded in their woody tissue. Just beneath the bark of a tree is a tissue called the vascular cambium, a thin layer of cells that is responsible for the growth in diameter of the trunk. This tissue rapidly divides in two directions, with the outer cells forming the phloem, the transport system that carries food from the photosynthetic tissues to the other parts of the tree and also provides storage and support. The inner cells produce the xylem cells, which are thick-walled cells with perforated or missing end walls, resulting in the continuous vertical transport of water. The size of these cells is dependent on the seasons of the year and its concomitant availability of moisture and fluctuating temperatures. In the spring, in temperate climates, when days are cool and soil moisture is more plentiful, the cambium layer produces large-diameter xylem cells. As the summer temperatures increase, it produces smaller cells. This cycle repeats itself each year, resulting in distinguishable growth rings of large, dark spring wood and the smaller, lighter summer wood. By counting these rings, the age of the tree can be determined.
The method of extraction, after locating appropriate trees and sites, is to use a borer, a threaded hollow tube that is screwed into the trunk of a tree and an extractor inside the borer shaft that enables the dendrochronologist to pull the sample from the tree. Generally, a second bore is made on the opposite side of the tree, taken to the laboratory, and sanded until the cell structure of the wood is visible under a microscope.
Astronomer Andrew E. Douglass, of the University of Arizona, is credited with the modern-day study of tree rings. While studying sunspot cycles, solar energy, and climate, he speculated that these cycles should be evident in the growth of trees. Further study established the pattern of the varying tree ring widths with climate. Subsequently, he used a process called cross-dating to establish exact dates of specimens in an historical perspective. By comparing the pattern of rings of trees in which life spans partially overlap, these patterns can be extended back in time, whether in the same region or in distant locations. Using intact and overlapping sections of living, recently, or long-dead trees or timbers, researchers have been able to construct an unbroken record, stretching back nearly 9,000 years, using the bristle-cone pines in the American Southwest. In Europe, oak and pine chronologies reveal an 11,000-year-old record. With this information, scientists can accurately fix calendar dates to tree rings against a known chronological reference point.
Growth processes functioning within a geographical area are influenced by a variety of environmental factors, including water, temperature, slope, wind, sun, canopy removal, fire, and snow accumulation. Growth is at its best within a geographic area that is constrained only by the environmental factor that is most limiting. Trees with an ample supply of soil moisture show a minimal effect of climate on the ring width, but sites that have limiting factors favorable for growth show the largest degree of effect on the rings. The former rings are called complacent, and the latter sensitive. For example, trees are more sensitive to changes in their environment along the margins of their natural range and, therefore, display the widest variations in their annual ring sizes. The bristlecone pine has been extensively studied because of its long life span and sensitivity to drought conditions. The oldest known living specimens are almost 5,000 years old, surviving in marginal habitats in the arid mountainous regions of California, Nevada, and Utah in the United States, at an elevation of over 9,000 feet. These locations, where water supply is the main limiting factor, result in extremely slow growth patterns and, therefore, provide data about the climate over past ages.
Wood is organic and, as such, is biodegradable, but certain conditions, such as dry climate, waterlogging in bogs and swamps, and fossilization, preserve the dead wood for hundreds and even thousands of years, thus providing a means to extend tree ring chronologies beyond living organisms.
In addition to the record of climates, tree ring chronologies have assisted in the interpretation of data from many archaeological sites. Douglass’s early expedition to Pueblo Bonito, a prehistoric Native American settlement in the southwest United States, analyzed the wood used in its construction to determine its existence some 800 years before European arrival, exploration, and settlement on the North American continent. Exact dates of timbers, camp-fires, and artifacts from the ruins of many ancient sites have been established by tree ring study, together with evidence of the impact of natural events such as floods, fire, and droughts on Native human populations, history, and migration patterns. Cross-dating has also enabled the confirmation of times of construction of early structures of many buildings, barns, bridges, wells, and boats.
Prior to the study of tree rings, there was no way to independently confirm dates using radiocarbon methods. What was once an important assumption underlying radiocarbon dating, namely, that the amount of carbon 14 in the atmosphere had remained constant throughout history, had to be altered because tree ring evidence pointed to fluctuations in the amount of carbon 14 during different ages. While a green plant is alive, carbon 14 is incorporated into the organism by the photosynthetic reaction using carbon dioxide directly from the atmosphere. It stops when the organism dies because this atmospheric exchange ceases. The rate of decay is then calculated, but within a very large range. Instead of roughly dating an artifact within several centuries, scientists can use tree ring calibrated radiocarbon measurements that are within tens of years of the origin of organically derived archeological objects.
Fire scars are also evident in tree rings as a result of naturally occurring wild fires over the past centuries and have enabled scientists to reconstruct their frequency, extent, and effect on ecosystems and, subsequently, the impact on human populations.
Other recent areas of study using the principles of dendrochronology extend to solar variability, earthquakes, volcanism, erosion and deposition rates, glacial movement and moraine deposits, insect damage and geographic extent, and a variety of atmospheric pollutants, changes in the composition of the atmosphere, and other possible effects of hazardous wastes brought about by human interference with the natural processes.
References:
- Douglass, A. E. (1920). Evidence of climatic effects in the annual rings of trees. Ecology, 1(1), 24-32.
- Fritts, H. C. (1976). Tree rings and climate. New York: Academic Press.
- Webb, G. E. (1983). Tree rings and telescopes: The scientific career of A. E. Douglass. Tucson: University of Arizona Press.
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