This manual describes an elementary procedure
for the preparation of a tree trunk cross-section and subsequent counting of annual growth
rings. The information will then be used to sketch a graph of the dynamics of tree growth
in years and further analyze the tree growth in connection with changes in environmental
factors.
This field study has instructional video
featuring real students conducting the ecological field techniques in nature. Each video
illustrates the primary instructional outcomes and the major steps in accomplishing the
task including reporting the results.
Introduction
The given educational task is devoted to the determination of the age of a tree
based on the number of annual rings and the study of peculiarities of tree growth
during various years based on the width of these rings.
The annual rings seen in a cross-section cutting of a tree trunk of a tree
grown in a moderate climatic zone occur as a result of the varying rates of growth during
a vegetative season. The cells that are formed in the spring and summer have a lighter
tone. The cells that are formed at the end of the vegetative season form wood that is
composed of smaller cells whose cell walls are thicker than the ones formed in the spring
and summer. The color of these smaller cells is darker than those formed in the beginning
of the summer. Thus, an annual ring has light and dark components, and as a result, we can
see borders of annual rings on the cut cross-section of the tree.
This occurs only in those zones of the earth where there is a noticeable change of
seasons. In regions without a change of seasons, for example on the equator, annual rings
are also formed, but they are practically invisible - wood has a even coloring.
By looking at the number of annual rings on the cut cross-section of a trunk it is
possible quite accurately to define the age of the tree. The width of one annual
ring, i.e. annual growth ring, varies from one year to the next. The width depends on tree
state (condition) during the given vegetative season, which, in turn, depends on annual
climatic peculiarities (features), health of the tree and many other factors.
The average width of rings of different tree species varies and is basically associated
with the species, the place it is grown, and individual features of the given tree.
However, there are some typical features for all tree species or for the majority of trees
of the same species. For example, rings are wider on the well-illuminated side of the tree
than on the side of the tree that is in the shadow, therefore the stumps that remain from
lone standing (grown) trees are used for the determination of Cardinal points (where North
and the South are). Within a species beginning to grow under the forest canopy (in our
woods they are, for example, spruce and oak) it is possible to observe objective changes
of ring width during the first years of the trees life: while the young tree is growing in
the shade, rings are narrow, when it begins getting more sunlight, rings become wider.
Natural loss (fallout) and the cutting of neighboring trees during forest management
(maintenance) can also influence the width of the rings. When a "window"
(opening) is formed near a growing tree, it begins to actively grow, growing both in
height and in thickness.
The goal of this lesson is the preparation of a tree cross-section and the
construction of a graph of its growth. Interesting results can be determined if students
prepare cross-sections of several trees, growing in one general area but under different
environmental conditions (for example different habitats) and then comparing the graphs.
We recommend doing this laboratory-based activity as part of winter ecological field
studies during cold weather, while other outdoor field studies may be difficult and
uncomfortable to execute.
For this lesson it is necessary to have: two-handled saw and metal ruler. For
preparation of a permanent cross-section for multiple years, it is necessary to have a
metal strip and nails.
Preparation of the cut cross-section
The study of growth dynamics of a tree can be conducted at any time of year and without
dependence on weather conditions. For conducting the study, a saw, a ruler, a pen, in some
cases a magnifying glass and some painting substance such as bluing or potassium
permanganate are required (if no prepared cross-sections are available).
A wind-fallen tree or a standing dead tree should be found in the forest, whose year of
dying off can be defined. If the needles still remain on a tree, then the current year can
be taken for the year that it died. If there are no needles, but the smallest branches are
present - the tree died the previous year. If there are no needles and there are no
small-sized branches present, but the cortex is well preserved - it died about two years
ago. It is advisable not to use older trees as it is impossible to define the year of the
tree's death precisely, so all further attempts will produce no results due to absence
of the "starting point" of chronological scale.
An ideal tree for the preparation of cut cross-section is a recently wind fallen
tree. Its death is not caused by natural reasons, i.e. by illnesses or pests, but by
the influence of...
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This manual describes an elementary procedure
for the preparation of a tree trunk cross-section and subsequent counting of annual growth
rings. The information will then be used to sketch a graph of the dynamics of tree growth
in years and further analyze the tree growth in connection with changes in environmental
factors.
Our Paperback and eBooks on Amazon, Google Play and App Store