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Tree Rings - What they Are and Why they Vary
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Tree Rings - What they Are and Why they Vary
Notes by J. Zimmerman, Ph.D.
How can you estimate how old a tree was? And can you estimate what kind of
climate variations occurred during the life of a tree? The answer lies in those
light and dark rings that you see on a tree stump or a sawn log.
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A tree ring is simply a layer of wood produced
during one tree's growing season.
Tree rings have many uses.
A cross-section of a tree often shows a distinct pattern of concentric tree
rings. You can see such rings on a stump or on a fallen tree truck that has been
sawn through to clear a trail.
It is very interesting to count the tree rings, so that you can estimate the
age of the tree. For example, I counted rings of a newly fallen redwood recently
(see the highlights for November 2001 of Redwoods and
Waterfalls: A Year of Walking in the Forest of Big Basin: The Berry Creek Falls
Loop).
Each tree ring marks a line between the dark late wood that grew at
the end of the previous year and the relatively pale early wood that
grew at the start of this year. One annual ring is composed of a ring of early
wood and a ring of late wood.
The growth occurs in the cambium (the thin, continuous sheath of
cells between bark and wood). In spring, the cambium begins dividing. This
creates new tissue and increases the diameter of the tree at two places:
- Outside the cambium. The outer cells become part of the phloem.
The phloem carries food produced in the leaves to the branches, trunk, and
roots. Some of the phloem dies each year and becomes part of the outer bark.
[Phloem is pronounced "FLO-em." The word is derived from
phloos, a Greek word for bark.]
- Inside the cambium. The inner cells become part of the xylem.
These cells contribute most of a tree's growth in diameter. The xylem carries
water and nutrients from the roots to the leaves. These cells show the most
annual variation:
- When a tree grows quickly, the xylem cells are large with thin walls.
This early wood or springwood is the lighter-colored part of a tree ring.
- In late summer, growth slows; the walls of the xylem cells are thicker.
This late wood or summerwood is the darker-colored part of a tree ring.
[Xylem, which is a useful word for Scrabble games, is
pronounced "ZI-lem." The word is derived from xylon, a Greek word for
wood.]
Other features of tree rings include:
- Trees growing in California add one annual ring per year. By comparison,
trees in tropical regions may have more than one growth ring per year, or may
appear to grow continuously and have no rings.
- Tree rings are easily seen in conifers (e.g., pine, spruce) and hardwoods
(e.g., oak, ash).
- Rings may be closely spaced or widely separated.
What Causes Variations in Tree Rings?
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When conditions encourage growth,
a tree adds extra tissue and produces a thick ring. In a discouraging year,
growth is slowed and the tree produces a thin ring. Much of the variation in
tree rings is due to variations year-to-year in:
- Higher springtime temperature. If spring starts early, the growing season
is likely to be longer than usual, causing a tree to have a wider ring.
- Lower springtime temperature. A late spring is likely to shorten the
growing season, causing a tree to have a narrower tree ring.
- Abundant rainfall increases growth, producing a wider ring.
- Drought decreases growth, producing a narrower ring.
- Species of tree do differ in their response to weather changes. One might
respond strongly to changes in overall rainfall, another might be more
sensitive to the amount of rain during the late summer, and another to a
temperature change that alters the length of the growing season.
- Crowding from neighboring trees. This causes a series of narrow rings.
Crowding is suspected when the series of narrow rings is more than three,
because droughts are usually only one to three years.
- If the rings are narrow on one side of a tree with wide rings on the
other, the tree was crowded on the side of the tree where the rings are
narrow.
- A series of many narrow rings followed immediately by wide rings probably
means that an encroaching neighbor died, releasing the crowded tree into a
growth spurt.
- Fire scars suggest past forest fires. The number of annual rings between
fire scars shows the period between fires.
- Scars due to insect plagues indicate insect infestations.
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Forest and Tree Home,
Glossary pages:
A, B,
C, D,
E, F,
G, H,
I, J,
K, L,
M, N,
O, P,
Q, R,
S, T,
U, V,
W, X,
Y, Z.
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