Challenge of Dendrochronology

Dendrochronology is the technique of counting tree rings to determine a tree's age and measuring the width of these rings to determine characteristics of past climates. This might seem simple: each ring represents one year, and wider rings generally mean better growing conditions-plentiful rainfall, moderate temperatures, and so forth. But the seasonal growth of a particular tree is affected by factors other than the weather. Trees vary, one from another, just like people do. The genetic makeup of each individual tree is unique, so one particular tree may grow a bit more quickly that another. Highly local conditions can also change over time. It is easy enough to see that if part of the soil near a tree has been eroded, this will impact the tree's root system and limit its growth, at least until the situation stabilizes. Then again, an infestation of insects may affect a tree in one valley more than the same type of tree ten miles away. Or one tree may suddenly start to get a lot more sunlight when an old, big tree in the neighborhood finally falls. These kinds of factors produce significant variations among individual specimens, and that fact means that researchers need to average together samples from many specimens of a single tree species in one region over the same time period. Some dendrochronologists think that measuring an average of twenty-five to thirty tree-ring records in a locale is an essential first step in getting around the problem of individual variability. While it may be easy enough to find thirty samples in some locations for particular periods, it obviously becomes less and less likely the more ancient the wood samples are.

/

Another issue is more general. Trees that are fortunate enough to live on good soil and near local sources of groundwater often grow at steady rates. Such growth translates into attractive trees that are tall and well formed; they also have rings that are wide and quite uniform in thickness, but their uniform growth rings make them entirely useless when it comes to inferring anything about past weather patterns. That is why, instead of looking at superb botanical specimens, dendrochronologists focus their work on wood from trees that are living a tough life due to poor soil, steep slopes, the absence of local groundwater, or some other challenge. It is these "tortured" trees that are the most likely to grow very little during years of scarce rains or do poorly after a harsh winter and a late spring. What this means, of course, is that few trees in the woods are likely to be good samples for the scientist. Indeed, it may be quite a small fraction that yields useful ring patterns. Again, this increases the challenge of finding enough good samples to say with much certainty what past conditions were like.

Another factor of dendrochronology relates to wood itself. In the spring, a tree grows rapidly, creating new cells on the outside of its trunk and branches, just under the bark. These cells, called "earlywood" or "springwood," are large and have thin cell walls; both these factors contribute to making the wood relatively lightweight for its volume. In the summer, growth slows. Denser "latewood" is formed, creating the band that is relatively dark when you look at the end of a piece of lumber. But occasionally the sequence of a perfect pair of springwood and latewood does not hold up. If conditions-weather or disease-severely test a tree one year, it will not grow over all its surfaces. That may mean that a particular sample of wood taken by the dendrochronologist will have a missing ring in it, which will result in the scientist's inferences being off base by a year

A few trees also may trip up scientists by revealing a "false ring" made of latewood that's in the middle of springwood. These features, sometimes known as double rings, usually can be distinguished from true rings because the unusual dark ring is likely to change gradually rather than more abruptly into the springwood that lies on either side of the false latewood. It is not clear what creates such double rings, although people have speculated that unusual conditions during the middle of the growing season or even highly local issues might be the cause.