Dendrochronology

Dendrochronology, a name derived from the Greek words for “tree” and “knowing the time,” is a set of techniques by which the annual growth layers of trees, called tree annual rings, can be assigned to the specific year of their formation. The history of changes in the tree's environment can be reconstructed using various properties of annual tree rings, for example, their width, cell size, wood density, trace element composition, and radioactive and stable isotope ratios. Andrew Douglass established the scientific basis of dendrochronology in the early years of the 20th century in North America. Douglass, the founder of the Laboratory of Tree-Ring Research at the University of Arizona in Tucson, invented the technique of cross-dating by means of skeleton plots, that is, the process of assigning year dates to annual tree rings by cross-comparison of rings from several trees growing in the same area. Cross-dating is a technique that aids the dendrochronologist in relating groups of specimens to each other by matching ring patterns and determining the exact date for each ring (Figure 1).

Cross-dating is a simple technique to undertake, although it requires an experienced person with a microscope, pencil, and graph paper for the task to be successful. The set of fundamental principles that have been established for cross-dating include the following:

• 1.
  Trees that will be used for cross-dating produce one annual ring for each year.
• 1.
  One environmental factor, such as precipitation or temperature, dominates in limiting the annual growth of the tree.
• 1.
  The intensity of the growth-limiting environmental factor varies from year to year, and the resulting annual rings reflect such variations in their growth.
• 1.
  The growth-limiting factor is effective over a large geographical area.

Tree-ring series have been used to reconstruct patterns in the variations in precipitation, temperature, soil moisture, frequency of extreme droughts, forest fires, pest outbreaks, and a diversity of other growth-limiting factors over past centuries and, occasionally, millennia. What can be reconstructed specifically depends mainly on the [Page 711]factor(s) that limits tree growth. Dendrochronology has made significant contributions to the science of geography, most specifically to physical geography. Physical geography centers on the spatial analysis of all the physical elements and processes that make up the environment, such as energy, water, climate and weather, landforms, soils, animals, plants, Earth itself, and the impacts of human activities on the environment.

Figure 1 Concept of cross-dating: extending chronologies by cross-dating living and dead wood samples

Source: Laboratory of Tree-Ring Research, University of Arizona.

Some of the earliest geographical writing on dendrochronology included investigations of tree rings as a means to understand arctic and alpine ecosystems and glacial environments. However, the first dendrochronological writings by a noted geographer, Ellsworth Huntington, were reports on investigations of giant sequoia tree rings, used by the author to understand climatic variations. Climatology is a time-honored theme in geographical studies because of the interest in the spatial variability of climates and the spatial and temporal variability of atmospheric circulations and related climate phenomena such as droughts, floods, and large storms. Seminal works in paleoclimatology such as Hays and Imbrie's investigations of the origins of Ice Ages and Hubert Lamb's compendia of global climate variations accelerated interest in past climate variations and their relationships to cultural and economic history and geography.

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