Dendrochronology


Geographical phenomena, regardless if they are physical or human, share one thing in common in addition to involving the surface of the earth--change. Soils evolve; sediment is transported; streams flow; and vegetation grows. Time is involved with each of these, but all too often we tend to envision the environment as static rather than dynamic. This is perhaps more so for vegetation than anything else. There are many ways of assessing vegetation change, but one of the oldest and most reliable is by dendrochronology--the study of tree rings [example].

Factors influencing growth. Most people think that trees add a ring for every year of growth. To a great extent this is true, but there is more [example]. Ring formation is as much a function of moisture as any annual cycle; years with greater-than-average rainfall result in thick rings whereas years with less-than-average rainfall result in thin ones [example]. Many scientists have been very successful in reconstructing past weather and climatic conditions and patterns by assessing tree ring widths. Coniferous trees have proven to be the best and easiest, but not the only, trees with which to work.

Trees also add one ring for each rainy season within a year. If the climate of a particular region is wet year-round, as in the tropics, rings tend to be very thick and almost indistinguishable. If the climate of an area has two distinct rainy seasons separated by periods of no rain, trees will add two rings per year. Now, here's a problem to consider. How might one interpret tree rings if an area with bimodal rainfall experiences an anomolous year in which there is only one rainy season? Clearly, dendrochronology isn't as easy and clearcut as it might seem at first glance.

Problems affecting growth. Complicating the interpretation of tree rings are other factors, three of the most common of which are burning, sloping terrain, and multiple trunks. Forest fires can burn off the bark and outer rings on one side of a tree and thereby affect the tree's growth, and hence ring formation, in following years [example]. "False rings" can make life difficult for dendrochronologists. Slopes can affect the centricity of tree ring formation. It is not at all unusual to find trees with thicker rings on one side of the tree than on the other [example]. In those cases were trees are growing on stable slopes, the rings tend to be thick on the downslope side. On unstable slopes, where landslides have disturbed vegetation, rings tend to be thicker on the upslope side. Trees with multiple trunks, junipers, for example, pose special problems. Growth patterns above points of bifurcation [example] are usually different from that below the fork although the ages of the two segments might well be the same.

Uses. Dendrochronology has its widest application in archaeological and forestry studies. Archaeologists study the ring patterns in timbers they find during excavation of prehistoric and historic sites. They do so principally to determine the ages of sites, but increasingly are concerned with understanding past environmental (climate) conditions.

How does one date an archaeological site with dendrochronology? By reconstructing the tree ring sequence if a preserved timber is found. This is done by examining the timber and numerous samples from several other sites and proveniences within reasonable proximity to each other [example]. The ring patterns from each sample are compared in anticipation of finding identical patterns in samples that overlap, but are from trees that sprouted and were cut at different times. Once the sequence is complete, the archaeological site under investigation can be dated in reference to the exact year in which the timber was cut [example].

In some places, such as parts of the American Southwest, a sufficient number of tree ring specimens have been collected, curated, and studied for so long that a very long dendrochronological sequence exists. In other places, for example northern Mexico, relatively recent and relatively old sequences have been established, but there is a gap spanning late prehistoric and early historic times (ca. A.D. 1500). This gap means that the actual dates of the prehistoric sequence cannot be determined at this time. Sequences of this nature are described as floating sequences [example]. Also, in some archaeological contexts, the outer "sap wood" has deteriorated, leaving only the inner "heart wood" for analyses [example].

Archaeological scientists deal with trees that have been cut down, and, therefore, are dead. Foresters and other scientists concerned with the age, health, and vigor of living trees, and their productivity in terms of timber resources, also use dendrochronology, but these people do not cut down trees in order to examine the growth rings. That said, much to the discipline's chagrin, it was a geographer who, although well-intentioned and working with official permission, cut down what was discovered to be the oldest living tree on earth--the Methuseleh Tree. Fortunately, an older tree has since been found. To study tree rings without cutting down trees, scientists use special coring device.

Increment borers have three parts--the borer bit which is hollow and threaded at the tip, an extractor which is a trough that slides inside the bit, and a handle which attaches to the bit in the shape of a T and also serves as a storage tube for the bit and the extractor [example]. To use an increment borer the bit is removed from and then attached to the handle. The extractor is removed and set aside. The bit lubricated with beeswax and screwed into the subject tree. Care is taken to insure that the bit is parallel to the ground and pointed directly at the center of the tree. It is inserted slightly past the tree's center. Once in place, the extractor is carefully inserted into the tube, between the inside of the bit and the top of the core, concave side down. The handle is then turned one-half turn counterclockwise to break the core loose. The core is removed in the extractor and either examined on the spot or placed in a plastic soda straw or specially made tray for transport back to the lab [example]. The bit is then extracted and the hole in the tree filled with puddy.

In the lab, cores are permanently mounted, labeled, analyzed, and stored [example]. An excellent website dealing with dendrochronology can be found by clicking here. A great video about the subject can be found by clicking here.


Suggested Additional Readings


Job 13


http://uts.cc.utexas.edu/~wd/courses/373F/notes/lec20den.html

 Created by William E. Doolittle. Last revised 26 June 2013, wed