Earlier in this series we learned a lot of terminology regarding plant tissues and organs,
as well as plant classification, including the characteristics of gymnosperms and angiosperms.
These are the kinds of plants that dendrochronologists study,
so we are now ready to go over the basic structure of wood and the concept of ring
anomalies. There are some new terms that will come up as well, which we can define as we go.
Since dendrochronology is the study of the growth of tree rings,
we are only concerned here with woody plants, specifically gymnosperms and angiosperms.
Recall that gymnosperms are plants which produce naked seeds, like the cone-bearing pines,
while angiosperms are flowering plants. Within the angiosperms, woody monocots, like palm trees,
do not produce annual rings and are not studied in dendrochronology. However, eudicots and
magnoliids, as well as many gymnosperms that can be found in seasonal climates,
produce one ring of growth every year. It is this ring that is so essential to dendrochronologists.
It’s important to note that tree rings form in forests that have an annual cycle consisting of
a growth period followed by a dormant period. Such patterns are common in temperate zones, and are
entirely or partially absent in tropical zones. So let’s now examine a typical set of tree rings
and try to understand what we are looking at. In temperate zones, the annual ring of some,
but not all, species can be further separated into earlywood and latewood. Earlywood
is usually lighter in color and produced in the spring and early summer, when the tree is
growing more rapidly, while latewood is usually darker in color and formed in the late summer
and early fall, when growth has slowed down. In some species the contrast is readily apparent,
while in others it’s much harder to see. This annual ring is due to the secondary
meristem produced in most gymnosperms and eudicots which enables a trees to grow in circumference.
Cellular division in the cambium produces the xylem towards the inside of the tree, becoming
the wood structure that supports the tree, and phloem is produced towards the outside of the
tree and becomes the inner bark. Be sure to check out the previous tutorial on the types of plant
tissues if you are rusty on this terminology. The cells that make up the woody part of the tree
continue to thicken their cell walls within a few days or weeks before they die.
Following their death, they function as the support system of the tree, acting as conducting
or strengthening tissue. If this “deadwood” is inside the middle of the trunk we call it
heartwood. And it is this heartwood, the darker, colored, dense inner part of the tree trunk
which provides the best timber. In older trees it’s much larger than the lighter colored living
part of the inner wood, known as the sapwood. To visualize tree growth, imagine a series
of stacked cones, each new cone is just slightly larger than the one before it,
and each cone represents a new layer of wood that is put on the tree every year.
The tree grows upward and outward causing the cones to stack. When a dendrochronologist cores a
tree, a sample is removed from bark to pith, which is the bundle of cells at the center of the tree.
This is like drilling a hole into all the cones, so that each layer, or each ring, is collected.
If you were to cut the cones straight across you’d see the width of each year, and this is
what you see on a tree stump. This cross sectional view is the most common way to study tree rings,
though most often dendrochronologists study cores, or slices known as “cookies,”
instead of whole stumps. Cross-sectional views of a eudicot and
a gymnosperm look very different from one another, because gymnosperms do not have pores. Angiosperms
on the other hand, can be ring porous, with a row of vessels at the beginning of each ring,
semi-ring porous with some vessels at the beginning of the ring, or diffuse porous
with vessels distributed throughout the ring. Pore arrangement in angiosperms varies by taxa
and can be used in identification. For example, the rings of the genus Acer have solitary pores,
Populus have pore multiples, Quercus have nested pore clusters, and Ulmus have wavy bands of
pores. Diffuse genera in particular are more challenging to study because the trees produce
a large number of pores throughout a ring making it difficult to identify a ring boundary.
Before we study ring anomalies, we need to cover reaction wood. If you’ve ever seen a tree growing
on a slope or otherwise inhospitable area, you’ve noticed that no matter what the tilt in the base,
the top of the tree often reaches straight up towards the sun.
This is due to reaction wood, which trees produce to maintain a vertical orientation.
Angiosperms and gymnosperms produce reaction wood in two different ways. Gymnosperms produce
compression wood on the downhill side of the tree. This results in wider than average rings on the
downhill side and narrower than average rings on the uphill side. This pushes the pith, the center
of the tree, up vertically. Angiosperms produce tension wood on the downhill side of the tree,
causing wider than average rings on the uphill side and narrower than average rings
on the downhill side. The reinforced, thickened wood pulls the pith of the tree up vertically.
Now we’re ready to study ring anomalies. Let’s start by considering absent rings. Remember
from the tutorial on mechanisms of plant growth that growth hormones can trigger cell division,
and cell elongation. Trees produce more growth hormones in good growing conditions
and less in stressful conditions. In extremely stressful years,
trees may not produce enough growth hormone to initiate growth in some parts of the stem.
This is especially common near the base of the tree, where dendrochronologists typically take
measurements. What this means is that sometimes a tree core might be missing rings entirely,
due to lack of growth in that time period. In addition, sometimes trees produce micro rings
which are only two cells wide, and are incredibly difficult to find on a cross section even with
extensive sanding. False rings occur when tree growth shuts down during some part of the year due
to limiting factors, but then starts growing again at a different part of the year. Diffuse ring
boundaries occur when trees don’t go dormant for part of the year. And pinching rings are produced
due to damage or limited nutrients such that a ring does not grow all the way around a tree.
These ring growth anomalies are further complicated by the fact that many tree species
do not show circuit uniformity. Circuit uniformity is when a tree has the same amount of growth
around its circumference, such that a core sample taken from any position around the stem will
result in an identical core. Since many species do not have circuit uniformity, dendrochronologists
must create chronologies from multiple samples. In addition, at high latitudes, frost rings form
when the air temperature drops below freezing during the growing season. In any area damaged by
wildfire, fire scars can form due to cellular death at high temperatures, sometimes leaving
charcoal behind. Aphids are insects that suck sap from trees, and they can leave behind pith flecks
that look like bubbly-textured wood. So as we can see, examining tree rings is
not always as simple as the ideal scenario we are most likely familiar with. There are many factors
and variables a dendrochronologist must consider. So to get a better sense of the strategies a
researcher must employ, let’s move forward and discuss the methodology of dendrochronology.