Envirothon Data Dendrochronology can be a very powerful tool in answering ecological questions about the recent past. It also has applications in archaeology, geomorphology, forestry, climatology, and even law. Schweingruber's Tree Rings: Basics and Applications of Dendrochronology for a much more comprehensive coverage of the technique and its many applications. A tree ring is composed of two more or less distinct bands of cells figure 1.
The early wood, the light-coloured band, is laid down in the spring and early summer, when water availability is highest. The xylem cells produced by the cambium are then rather large in diameter, and have thin walls.
Late wood is produced later on in the summer and in the early fall. Late wood cells are somewhat smaller than the early wood cells, and have a much thicker cell wall and much smaller lumen, accounting for the darker color of the late wood. At the end of the growing season, wood production shuts down until the following spring, when large, thin-walled cells are again produced by the cambium, making a very sharp contrast with the previous year's dark, tight late wood at least in conifers!
Water availability and warmth of the growing season are the two main factors affecting tree ring width. A wet, warm season will lead to the formation of wide, light-coloured bands in most boreal evergreen trees, with ring width generally increasing with the length of the favourable growing season. Dry or cold summers will result in narrower rings. Although the tree's response to growing conditions is species-specific and also depends on other factors such as nutrient and light availability, the previous generalizations usually apply to most of our boreal conifer species.
Deciduous species are more difficult to work with as the late wood is not much different form the early wood, and observation of the cells themselves is sometimes the only way to distinguish the annual rings.
A fire scared spruce tree Many events will happen in the life of a tree which may be recorded in its wood either at the cellular level or as more obvious scars or deformations in the tree itself. Frost, insect epidemics, and droughts are a few examples of events that will be recorded in the rings at the precise year or series of years at which they occurred. Fire and other cambium-destroying accidents will stop the production of wood where the cambium was damaged, leaving a scar that might eventually close with time; this scar can also be dated.
Cutting down a tree will stop lateral growth altogether and tell us the year the tree was felled. Tree death by flooding, burying, or uprooting can also be dated. Tree rings will indicate the tree's age, growth rate and give clues about the climatic conditions of an area. The core samples that have been prepared will provide you with information about the forest growth rates and the rates at which the forest accumulate and store carbon from atmospheric carbon dioxide.
How to core sample trees Sampling, preparation, and dating of wood material The first step in any research project or study relying on dendrochronology for answers is to decide exactly what the question is we want answered. This is important as it will influence the species we will sample, the sites where we will sample them, the number of samples we need, and the kind of samples we take. For example, a study wanting to investigate past outbreaks of the spruce budworm will involve sampling trees sensitive to defoliation by the insect fir and white spruce as well as trees that are not sensitive to it pine to distinguish the insect signal from any background climate signal.
A project looking for links between tree-ring width and recorded rainfall to make predictions of past precipitation regimes will involve sampling trees that are very sensitive to variation in rainfall: Sampling of the trees can be done in a few ways. It is possible to obtain cores from trees using an increment borer.
This type of sampling does not kill the tree, but can be difficult if the trees are rotten. Taking a tree disk or 'cookie' is destructive but tree disks are the best samples as incomplete rings can be detected, narrow rings can sometimes be seen better somewhere else around the stem, and patches of rot can be avoided when dating the disk. Scars such as fire scars are almost impossible to date without a disk. Dating live trees is very straightforward: Counting back from the outside ring towards the inside of the disk will give an approximate age for the tree.
Trees are very rarely sampled at the root collar; therefore a few rings may be missing from the beginning of a tree's life. Sampling as close to the ground as possible helps avoid underestimating the age of a tree. Counting from the inside out towards a scar will then allow dating of the scar. Dead trees can be cross-dated with live trees to allow for age determination. Use one small dot on the core holder by each decade, two dots by every fifty years and three dots by each century.
This process helps in keeping track of the number of rings counted. Pencil marks are recommended over pinprick as they can be erased should a dating mistake be detected.
Narrow rings are the most useful of all marker rings. They appear faithfully in almost all of the trees of an area since they are usually caused by climate felt at a regional scale. In Yukon narrow rings quite often correspond with fire years dry years. A series of narrow rings may indicate a few years of unfavourable growing seasons.
It may also indicate an insect outbreak causing intense defoliation and therefore little growth. If that is the case, the series of narrow rings will only be present in the species that are affected by the defoliator, and other species will not show that insect signature.
The most common way to build such a marker year chronology is by drawing what we call a skeleton plot. In a skeleton plot, marker years are recorded by a vertical line on a time axis figure 4: Narrow rings are the most reliable marker years to record in a skeleton plot, although frost rings, false rings and light rings are usually noted as well.
In general, 15 to 20 samples will be enough to build a reliable skeleton plot. Marker rings are recorded on a separate time axis for each sample, then a composite skeleton plot for the site is compiled.
Skeleton plots are very useful in identifying patterns of tree growth, and are quite often good enough to allow cross-dating of most events and even date dead trees and wood samples. Unfortunately, a wood sample that happens to fall completely in between two reliable sets of marker rings cannot be cross-dated in this manner. Skeleton plots also do not allow for climate reconstruction studies, as the picture they give is too coarse.
This is when the need for master chronologies and computerised cross-dating arises. Ring widths have to be measured for every sample and every year, using a micrometer table or one of the software programs that allow on-screen measurements of digital samples.
Dead trees and fossil wood that can be cross-dated to the live-tree chronology can contribute to the building of very long master chronologies figure 6. Those can be useful in reconstructing long-term climatic trends, or to date wooden structures and objects that were built or made a long time ago, if they are reasonably well preserved. Software such as COFECHA allow the comparison of ring width patterns in undated samples with dated master chronologies to find the best statistical match and therefore assign calendar dates to the undated samples.
Some master chronologies developed in Europe and in the south western United States can reach back as far as a few thousand years.