Friday, November 22, 2013

Dendrochronology - Studying Tree Rings with Third Grade Students

One activity that I do with students is called Michigan Trees and Plants.  During this activity, we discuss the parts of plants and the roles of each part, students learn to classify leaves based on their appearance and also how to determine the age of trees.

Trees can be aged by counting their annual growth rings.  This process of is called dendrochronology from the Greek words dendron (tree limb), khronos (time), and logia (study). In temperate climates, most trees will produce an annual growth ring each year.  Annual growth usually consists of a wide light-colored ring (spring growth) and a narrow dark-colored ring (summer growth).

An individual tree does not grow at the same rate each year, but instead grow at rates that depend on the temperature and availability of water.  A year with little precipitation may be marked by a very narrow growth ring, while a year with abundant precipitation will be marked by a wide growth ring.  These pattern can be used to determine climate patterns for a region even when precipitation records do not exist.

Here are photos of six different species showing different patterns of growth.  Each photograph was taken a 2X magnification with a digital microscope.  Each photograph shows an area of the same size on the sample of tree trunk.

The first sample is of an American Beech.  American Beech is a slow-growing hardwood species.  The rings on this sample are running from left to right.  Counting from the top of the photograph over one dozen closely-packed rings are visible in this small sample.  Each ring is approximately the same size.

American Beech (Fagus grandifolia)
The second sample is another hardwood species, Green Ash.  Green Ash is typically found in moist soils and grows much faster than American Beech.  This sample has  parts of seven rings visible in the photograph.  Each ring is of a different width depending on the that year's precipitation.

Green Ash (Fraxinus pennsylvanica)
The third sample is from a Red Pine.  Like the American Beech, the Red Pine grows on upland sites, but grows at a faster rate - parts of four growth rings can be seen in the photo.

Red Pine (Pinus resinosa)
Silver Maple is another fast growing hardwood that is found in wet sites.  This Silver Maple shows parts of three wide annual growth rings in this photo.

Silver Maple (Acer saccharinum)
The last two photos show growth rings on samples of petrified wood a Sycamore and a Willow.

Sycamore (Platanus spp.) - petrified

Willow (Salix spp.) - petrified

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