Tree Judging

prepared for Washington Farm Forestry Association field day exercise

By Thembi Borras, Forester (thembi@mcn.org)

Judging trees is required of field foresters, it informs which trees are retained and how the forest will behave and look after treatment. Tree judging is a skill forest landowners may wish to develop. There are numerous contexts, including improving forest health, in which tree judging is valuable. The criterion on which a tree may be judged within the context of improving forest health may include crown quality, diameter, height, ring count and basal area.

Indicators of crown quality include live crown ratio, crown class and crown vigor. Live crown ratio is the ratio of crown length to total tree height. For example, if the crown length, measured from the leader tip to the base of the crown, is 40 ft and the total tree height is 80 ft, then the live crown ratio is 50%. There are four commonly recognized crown classes dominant (trees with well-developed crowns that extend above the general level of the crown cover and receive full light from above and partly from the side), codominant (trees with medium-sized crowns that form the general level of the crown cover and receive full light from above but comparatively little from the sides), intermediate (trees with small crowns that extend into the crown cover formed by codominant and dominant trees, but receive little direct light from above and none from the sides) and suppressed (trees with very small crowns that are entirely below the general level of the crown cover and receive no direct light either from above or from the sides). Crown vigor indicators include branch mortality, twig dieback, thin foliage, foliage discoloration and missing crown area. Trees with a live crown ratio of less than 30%, which are suppressed or intermediate and have poor crown vigor are candidates for removal when thinning. Remember to evaluate the crown quality; the observer must get far enough away from the base of the tree to see the crown.

Diameter is usually measured with a diameter tape or a Biltmore stick, but in the absence of these tools a flexible measuring tape can be used to measure the diameter. The diameter of a tree is measured at breast height, which is defined as 4.5 feet above the ground on the uphill side of the tree. The circumference of the tree is what is measured by wrapping the tape around it, making sure there are no kinks in the tape and the tape does not sag. To convert the circumference to the diameter, divide by 3.14. For example if the circumference is 82", dividing by 3.14 will yield a result of 26"; therefore the diameter at breast height (dbh) of the tree is 26". Height is usually measured with a clinometer, but basic tools including a measuring tape and two sticks of equal length can be used to measure the height of a tree. Find the center point on one of the sticks; hold the other stick perpendicular to it, effectively making the letter "T". Step away from the tree until the top and bottom of it are in view. Put the base of the "T" between your eyes, and then walk away or toward the tree until the top and bottom of the stick line up with the top and bottom of the tree. Make a mark on the ground, the distance between your mark and the tree is the total height of the tree, if you are on flat ground. If you are on sloped ground, the slope distance will need to be converted to horizontal distance. Tree height in relation to tree age is a measure of site quality. Site quality expresses the average productivity of an area for growing trees. The higher the site the more healthy trees it can support.

Tree rings represent the annual growth of a tree and can be measured with an increment borer. The tree rings are easily counted, measured and visually inspected on the core extracted from the increment borer. The higher the number of rings per given length of core, the slower the growth. Trees that are growing slowly as compared to their similarly aged neighbors may be candidates for removal when thinning.

Basal area is a measure of stand density and is often measured with a prism, but tree diameter can be converted to basal area. Basal area is the cross-sectional area in square feet taken up by an individual tree trunk at dbh; basal area per acre is the sum of these individual values for all the trees growing in 1 acre. Basal area of a given tree is calculated by squaring the dbh and multiplying the result by .005454. For example, a tree with a dbh of 13.54” represents 1 sq ft. Trees may be thinned to reduce basal area, so limited resources are allocated on fewer trees, to improve forest health.

A portion of this information was gleaned from Log Scaling and Timber Cruising by Bell and Dilworth, 1988 and Tree Judging: A Quantifiable, Hands-On Tool To Teach Forest Genetics And Applied Silviculture by Christopher C. Schnepf.