Now that we have a basic understanding of what tree rings are and how they form, it’s
time to go over some common methods that dendrochronologists use in the field and laboratory, as well as
some methods of analysis and chronology building.
Field work in any discipline can always lead to surprising and unpredicted situations.
Therefore, it is imperative to plan ahead as much as possible.
We will need to understand some of the most standard dendrochronological practices, while
also acknolwedging that certain situations may call for modifications of these practices.
The first step in a dendrochronological study, after either receiving an assignment or determining
the best methodology to answer your research question, is site selection.
Sometimes a site will already be defined by the research question, while other times it
is up to the dendrochronologist.
It is important to select a site that will accurately represent the area and topic of
study.
Much of science involves the process of observing patterns and determining the cause of that
pattern.
Dendrochronology is no different.
Some studies may call for random sampling, and others for targeted sampling.
In random sampling, plots are randomly selected in order to represent a forest type using
either Global Information Systems, or GIS, excel, or a compass bearing and random number
generator.
In targeted sampling, the researcher looks for specific conditions and chooses areas
that are best suited to answering the research question.
Regardless of the method, plots are generally chosen before entering the field to reduce
bias on the part of the researcher.
Next, a researcher must determine how they will be sampling the trees in their sites.
They may create a circular plot.
These are created by drawing a rope a specific radius from a central point and sampling trees
within that radius.
Square plots are created similarly to circular plots but require a bit more set-up, while
transects are essentially long lines that form rectangular plots with a predetermined
start point.
The researcher samples every tree within one meter while moving along the rope.
After a site and sampling method has been selected but before anyone goes into the field,
the researcher will need the proper gear.
Along with sufficient water and something for taking field notes, like a small waterproof
notebook, a dendrochronologist usually carries the following: an increment borer and a duplicate
or two just in case something goes wrong, since in the field something always does,
a map tube for storing samples, reusable paper or plastic straws with punctured air-holes
for storing individual cores, ensuring they’re wide enough in diameter to fit a core sample
from the increment borer, diameter tape also known as DBH tape, masking tape, permanent
markers, a hand lens, beeswax, a golf-tee or chopstick for removing pieces of wood that
get stuck, WD-40 along with cloth pads for cleaning out increment borers, gloves, rope,
a good backpack, a compass, a knife, a sharpening kit, a first aid kit, a map, camera, and GPS
unit or smartphone.
Depending on the study, a dendrochronologist may choose to leave some of these materials
at a nearby basecamp, or bring additional materials like chainsaws, pry bars, and helmets
depending on what they’re studying.
After a dendrochronologist arrives at their site, sets up their plot, and determines which
trees are to be cored, these trees are catalogued and their diameters are measured at “breast
height” which is considered to be 1.3 meters off the ground in most countries.
Then they are cored, usually at breast height as well, but sometimes also at their base
if tree establishment dates are needed, since the earliest years of growth will be missed
at breast height.
Before going into exactly how to core a tree, let’s address what is probably the most
commonly asked question about this field.
Does coring hurt the tree?
The short answer is, yes, coring does open the tree up to potential pathogens, but some
trees, like conifers, will secrete a substance called pitch into the hole, sometimes within
hours to seal it up.
Angiosperms will compartmentalize the wound by creating a barrier around the damaged areas,
sealing them off from the rest of their body.
Remember that most of the wood the dendrochronologist is studying is actually dead heartwood anyway.
Multiple studies have confirmed that coring, done properly and without any fungicides or
sealants, does not cause an increase in tree mortality or a worsening of general health.
To successfully core a tree, a dendrochronologist turns an increment borer to drill into the
tree.
As the borer is turned, it cuts away the wood around it and compresses the wood away from
it.
This way, the core stays in its original orientation and fills the borer, so that when it is extracted,
you have a full sample of rings from the bark to the pith.
Often dendrochronologists will take two cores from an individual tree to aid in crossdating.
The extracted core is placed into a straw, labeled, and carefully placed in the map case
for later analysis.
An increment borer must be properly cared for in the field, as well as sharpened and
cleaned when not in use.
Some studies may last weeks or more depending on how many cores the study calls for.
After the cores are brought in from the field, the next step is to prepare them for further
analysis.
First, the cores must be fully dried, after which they are properly oriented and glued
down to a labeled core mount.
Then they are carefully sanded with progressively finer grades of sandpaper, from 80-grit to
400-grit, and even down to 1200-grit or 1 micron paper, to reveal their rings.
After the cores are prepared the rings are counted backwards from the present year, which
is the outermost core, and every tenth year is marked.
Crossdating may reveal missing years or other ring anomalies later, so the initial counting
is often revised and cores are only gently marked.
Following this initial estimate, the cores need to be crossdated using either a skeleton
plot, list method, memorization method, marker method, or computer assisted program like
WinDENDRO, which requires high quality core scans.
A major goal of all these methods is to search for marker rings.
These are rings that are particularly narrow or have other identifiable characteristics,
and that are consistent between different trees.
An example would be a pattern of several average width rings followed by an extremely narrow
ring and then back to regular widths.
If all trees in an area have a similar pattern, and especially the same marker ring, then
the researcher can use that ring to calibrate the other trees.
These marker rings correlate to a specific limiting factor that impacted all trees in
the area, usually, but not always, a drought or catastrophic event.
These extraordinary rings are marked, and locations where the marker rings match up
better than others are noted.
This will allow a researcher to then go back and check their initial counts using the marker
rings as reference points.
Visual dating allows the dendrochronologist to use all aspects of the wood, such as color,
latewood thickness, and marker rings, to determine the dating of the sample of wood.
This may seem arbitrary or specific to each person, but evidence shows that the same dating
of a sample can be agreed upon by multiple trained dendrochronologists using their own,
even potentially idiosyncratic approach.
There are many different systems that can be used to determine accurate measurements
of tree rings.
Most of them have a moving stage that can be moved with the rotation of a lead screw
or by an optical linear encoder, while the researcher looks at the core through a microscope,
such as Velmex combined with measure J2X to store data.
Others use high quality scans and digital markers like WinDENDRO or LignoVision.
Regardless of the system used, the goal is usually to measure the width of each annual
ring in a core sample.
This is done by measuring perpendicular to its boundary.
The crossdating of these measurements can then be verified using free software called
COFECHA.
COFECHA checks for correlations between sample measurements and highlights both strong correlations
and anomalies.
By statistically generating a master chronology with the cores that the operator enters into
the program, COFECHA can then check the dating of each core sample against the synthesized
master chronology.
Depending on its read out, a dendrochronologist may need to revise their measurements or check
back on their original cores for any anomalies.
Most importantly, best attempts at accurate crossdating are conducted by the scientist
prior to the use of COFECHA.
The entire process of crossdating takes more time than any other step in the lab, but this
investment is absolutely essential before any further analysis.
The goal of crossdating is to create a reliable master chronology of tree growth in an area
that can be used to answer other questions.
In addition, once a master chronology is created, it can be used to check any unknown samples.
The master chronology is a continuous time series containing all marker rings that agree
between trees for the length of the chronology.
It is the best tool to date any remaining samples.
This master chronology is then often checked and verified by a different dendrochronologist,
or other established master chronologies in the region before being used by another program,
like ARSTAN, to build a stand-level chronology.
This program applies various statistical approaches to the measurements from each core, often
dividing ring width by a modeled curve, and averages together the resultant index for
each core to create a tree-level index.
After this step, ARSTAN then averages together the tree indices to develop a stand-level
chronology.
This is known as “detrending” and it is the final process utilized to convert the
raw master chronology measurements produced by COFECHA into standardized measurements
that can be used for further statistical analyses towards answering the research question.
Note that this was a very brief overview of just a few of the statistical analyses that
can be used to cross-date cores.
There are other programs that could be discussed, like YUX, Outbreak, or the suite of functions
offered by the R package dplR, or the Dendrochronology Program Library in R, which essentially means
code written by someone else that is freely available for other researchers to use for
data analysis.
Additionally, we could have discussed how these data sets can be further analyzed and
checked against climate and historical data.
While most of these programs simply perform the basics, there are many more specialized
programs and statistics available to the different sub-disciplines of dendrochronology.
But that was still probably more than enough information for anyone to take in upon their
first exposure to the methodology and practices of this field.
To wrap things up, let’s move forward and talk about some of the specific sub-disciplines
of dendrochronology next.