Many people are familiar with reading the rings on the stumps of cut trees, but that would be impossible if you wanted to read the rings of a live tree. To view a cross-section of a tree with plainly visible rings, click here, and then click "Back" to return to this page.  (Go here for a Photo Gallery of Tree Rings).  So, you might ask, how does one read the rings of a live tree?

You use a tree corer to take a slender sample of wood from a live tree.

The tree coring method

A tree corer consists of a metal tube with two teeth at its opening and screwdriver-like threads on its outer side.   (The teeth help it “bite” into the tree and the threads help it “screw” into the tree.)  The tube is fixed to a handle that you can grip with both hands on either side of the tube.  This handle allows you to apply the strong pressure necessary to screw the corer clockwise into the stem of the tree.

The corer should enter the tree approximately at breast height since that is the height at which you take the dbh (diameter breast height) measurement for growth.  For total tree age, you want to core on the bole (main trunk) as low as possible.  In order to get a quality core that is easy to read, you must take care to keep the corer perpendicular to the tree’s bole as it enters the tree.  We aimed for the center of the tree with our corer, but since we were not trying to read the overall age of the tree, we did not have to hit the actual center.

Once you feel that the corer is into the tree far enough to yield a sufficient core, you “break off” the core by turning the corer a half-turn counter-clockwise.  You then remove the core from the tree by inserting a core extractor into the outer opening of the corer tube.  As you carefully pull the core from the corer, you must make certain that the bark and wood of the core stay together.

We immediately placed our tree cores into plastic straws that we sealed and labeled in the field.  Our labels stated the species, dbh, and site location (mesic or xeric) of each tree.  We also assigned a number to each tree.
 
Pre-coring considerations

Although taking a core does leave a small hole in the tree, the wound does not seriously impair the tree’s future health.  We determined our method by first asking a few questions:

1. What are we trying to find out?

We decided to look at the different rates of growth among sugar maples, hickories, and white oaks located on two different kinds of sites -- mesic and xeric – on Totoket Mountain.  Mesic and xeric refer to the availability of water in a specific site.  A mesic site is characterized by soil with enough water holding capacity (deep or fine texture), such that the trees have enough water -- there is no water stress during the growing season.  A xeric site is characterized by excessively drained soils that do not retain enough water.  Trees are thus stressed in dry periods.

We also wanted to see if we could match the trees’ growth rates with certain years in which pest outbreaks occurred.  We were specifically looking for the effects of a gypsy moth infestation in 1980-81 on oak trees in particular and a pear thrip outbreak in 1988 which effected sugar maples.

2. How many individual trees of each species should we include in our sample?

Our sample sizes included at least five trees of each species per site.  For example, on our mesic site we took five white oak cores, five sugar maple cores, and five hickory cores.  (Note: a second trip was needed to retrieve additional core samples due to processing and preparation errors.  These were denoted with a letter "a".)
 

Back at the Lab

Preparing the Cores for measurement

Once you have taken your tree cores, you must dry, mount, and sand them to prepare them for examination under a microscope.  To dry the cores, you remove them from the straws and place them between two small grooved wooden slats.  We let them air dry on a window sill for about a week before we glued each core into a groove on a slat.  (Photo)  After the glue dried, we sanded the cores to create a finer finish.  This finishing made it easier to distinguish individual tree rings under the microscope.

Although the potential for making errors exists at anytime during the tree coring process,  this preparation gave us the most difficulties.  While we were mounting and drying the cores, the bark on some samples fell away from the wood.  If you do not have the bark-wood interface on the core, people can not be sure if the wood has been broken off the outer core which would change the age of the tree.  This poses real uncertainty when you are attempting to count and measure tree rings backwards – if you start counting from the wrong end, you might be measuring the growth for the incorrect years.

How to measure tree rings?

To read our tree rings, we used a stereo microscope with a ruler imbedded in one eyepiece.  The ruler enabled us to measure   the width of each individual ring.  By measuring the width of each ring, we will be able to gauge the rates growth for individual years.

Counting tree rings is a laborious process, so we decided to limit our counting back to the year of 1980.  The time period of 1980 to 1997 allowed us to look at the trees’ growth patterns during both the gypsy moth and pear thrip years.

We tackled the ring measuring in teams of two people.  One person looked through the microscope and read off the ring widths to a second person who recorded the widths alongside the corresponding year.  We started measuring at year 1997 and measured backward to year 1980.

Challenges to accurate measurement

  
        Hickory, Ring Porous                 Sugar Maple, Diffuse Porous             White Oak, Ring Porous
 
Annual growth is measured from spring to spring.  The earlywood, which represents relatively fast growth in the spring, is characterized by bands of large cells. The latewood appears later in the summer and is characterized by smaller, more dense cells.  Generally, the earlywood is lighter in color than the latewood.

Ring porous species (oaks and hickories) possess distinctive bands of earlywood.  We encountered difficulty in reading these cores when the springwood cells did not form easily recognizable bands.  It was important to always measure the bands perpendicular to the growth rings.

Diffuse porous species present a much greater challenge to measurement.  In measuring sugar maple, a diffuse porous species, we had to begin by sanding the cores by hand to a smooth finish.  Although this alleviated some of the difficulties of reading the cores, more often than not it was extremely difficult to distinguish the earlywood and the latewood.

Data Analysis
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