Humans have been fascinated by trees for as long as we’ve been around.
Many early scientists like Leonardo da Vinci and even Pliny the Elder saw the
potential in examining tree ring structure centuries ago. However, as a formal science,
dendrochronology is only about one hundred years old. Over the last century it has become firmly
based on a series of principles and concepts. The topics we will present in this tutorial
deal with processes in the natural world, tree growth sampling, modeling,
dating, and building chronologies. Before we get started, there are some
important terms that need to be defined, or nothing that follows will make much sense.
The first term is part of the name of the field itself, chronology. A tree ring chronology
is a reliable representation of tree growth. It’s basically a record of tree growth from at
least ten sampled trees in an area, though some contain records from more than one hundred trees.
Later we will examine exactly how chronologies are built. For now, just remember that it’s a catalog
of growth for multiple trees in a given area. Another important term is calibration.
In dendrochronology, calibration is the process of comparing an independent record
of some environmental variable to a reliable tree ring chronology. This is used to determine tree
growth in response to that variable. Being sessile organisms, meaning they are fixed in one place,
trees cannot run from the swirling changes in their environment. And in that sense,
they are supremely sensitive to changes in their environment. The importance of the
factors limiting or driving tree growth depends on what species they are, where they live, and
the many changes that come with living over long, extended periods of time, as so many trees do.
So that’s it for vocabulary, now let’s explore some broader concepts. In this tutorial we will
cover five principles. Those are the principles of uniformitarianism, crossdating, limiting factors,
aggregate tree growth, and replication. We will also cover two concepts, those being
autocorrelation, and standardization. The Principle of Uniformitarianism is the notion
that the same natural laws apply everywhere in the universe and have always applied without variance
throughout the lifetime of the universe. This is a basic assumption of geology,
and is applicable through the statement that the Earth has always changed in a uniform manner.
The processes occurring today are the same processes that have occurred in the past,
meaning that geological events occur at the same rate now as they always have, so we can
say that the present is the key to the past. Dendrochronologists use this principle
when constructing past climates using tree ring chronologies. Essentially,
the climate changes over time, and this change may alter how a tree responds to its surroundings.
Using this principle, dendrochronologists can compare meteorological data and historical
records of environmental events to tree ring data in order to construct a model.
This model can provide an enhanced understanding of how trees respond to different variables.
It’s important to note that in actuality, present processes do change, especially when human
activity is involved. This is evident in the fact that anthropogenic emissions of carbon dioxide,
meaning those originating in human activity, have led to an atmospheric carbon dioxide concentration
that is currently outside the natural range recorded over the last 100,000 years.
We know this quite firmly from other forms of proxy data, like ice cores. This elevated level
of carbon dioxide influences tree ring growth and must be considered when calibrating a chronology.
It is also important to note that trees, being biological organisms,
adjust internal processes in response to changes in the surrounding environment.
Next up, the Principle of Crossdating is vital to dendrochronology. This is the primary way to
determine the exact year of each ring. Without crossdating, a simple ring count is virtually
meaningless. A dendrochronologist needs to assign precise annual dates of tree rings before they can
perform any other analysis. Crossdating allows us to match up the wide and narrow patterns found in
tree rings. Think of a tree core like a bar code with distinct patterns. You can match the patterns
from one tree with other trees to check cores for missing years and build stronger chronologies.
There are many methods used to date tree rings. The process can be done
entirely using a pen and paper, or it can be done with the assistance of computer programs.
One of the earliest paper methods of crossdating, which is rarely used today, is skeleton plotting,
where a straight line is drawn on graph paper and each vertical line represents a year of growth.
The length of each vertical line represents the importance of the ring to the chronology.
Once a dendrochronologist has enough skeleton plots for the trees in the collection,
they can start to compare them in order to build a chronology.
There are also other forms of cross dating, such as the list method, which involves writing down
narrow ring years and matching them up to other trees, the memorization method, which involves
identifying narrow years and memorizing them to then search for them in each sample, and the
marker method, which involves physically marking key years on the actual cores. These methods can
also be used in conjunction with one another according to the preferences of the researcher.
Next, the Principle of Limiting Factors states that whichever environmental feature
most significantly restrains growth, that factor will control the rate of growth for that organism.
Trees are limited by many different factors such as precipitation,
sunlight, temperature, insect damage, soil nutrients, and wildfires. Dendrochronologists
use this principle as a baseline when trying to determine what could
be influencing growth trends in chronologies. The Principle of the Aggregate Tree Growth Model
is actually something that was mentioned in the first video in this series. This principle states
that trees record all factors that influence their growth. This is essential, since trees record more
than just information on a single limiting factor. Dendrochronologists must take this into account
when studying a chronology, since annual ring width is dependent upon a wide range of variables.
This is why statistical analyses are necessary to understand growth trends. We’ll go over this in
more detail when we examine sampling and analyses. Then, the Principle of Replication really just
means that more samples are better. This refers to the fact that in order to develop a reliable
chronology, researchers require many samples going far back in time. Taking multiple samples
at a single site allows for better ring width pattern matching and strengthens chronologies,
given that patterns in the data can be replicated from one tree to another.
This principle can allow researchers to study more than just the lifespan of an individual tree.
So that wraps up the five principles. Now let’s check out two important concepts. The Concept of
Autocorrelation is something that refers to data correlating with the self, or internal biological
processes, and this technically applies to any living organism that creates macromolecules which
can potentially be utilized much later in time. This is specifically relevant to dendrochronology
because during an event like a drought, the internal metabolism of the tree is affected, which
shows up in the rings. So the width of tree rings are influenced not only by the climate of the year
they formed, but also the past climate, since the tree stored energy in the form of macromolecules
in previous years. We also must take into account the potentially unique physiological
processes of a species, the location in which a tree lives, the time period in which it lives,
or a combination of all of the above. It is well-established that the growing conditions
of an individual year have a lagging effect on the next year and the years that follow.
All of this is to say that the current year’s growth is affected by the previous year’s growth,
and everything is more complicated than just considering broad trends in climate.
Lastly, the Concept of Standardization is a procedure used to increase the validity and
reliability of research. Here, this is referring to the standardization of tree ring data.
While dendrochronologists primarily measure rings, remember that tree growth is influenced by many
variables, and perhaps most strongly by the biology of the species and the
immediate environment in which it lives. This is especially true for the history of dense forests.
The concept of standardization allows researchers to remove
variance using sophisticated statistical analyses such as splines or negative exponential curves.
And with that we have introduced the major principles and concepts of dendrochronology. With
these understood, we are ready to move forward and discuss the structure and growth of wood.