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.
Showing posts with label tree height. Show all posts
Showing posts with label tree height. Show all posts
May 28, 2009
March 19, 2006
Determining Tree and Log Volume
By Thembi Borras
Given the tree diameter at breast height (dbh) and the tree height to a merchantable top, there are tools available to help you estimate the volume in a standing tree. Volume tables are one such tool. Volume tables are, in part, specific to species and log rule. Log rules are ways to estimate the number of boards a milled tree will yield. The log rule used regionally is Scribner Decimal C. Volume is most commonly reported in board feet for trees that will be made into lumber. A board foot is 12" wide by 12" long by 1" thick. These are familiar units to anyone who buys lumber.
The volume of a tree may be measured several times before it is made into lumber. When I mark a tree to be cut, I record the dbh and height in logs to a merchantable top. In the local species specific volume table developed by Peter Joos that I use, logs are defined as 16' long and the merchantable top diameter is defined as 6". For example, a 26" dbh redwood with (5) 16' logs to a 6" top is reported in the table to have 620 board feet. As a general rule, height to a 6" top is usually about 30' less than the total height, but can vary greatly depending on the form of the tree. This tally of tree volume helps the landowner, logger and forester plan.
The tree, once it has been cut and bucked into logs, may be measured again by the faller. Henceforth each log is considered individually. The volume of each log is determined by measuring the small end diameter inside the bark and the length of the log. Given these inputs a different volume table is used to estimate the board foot volume. For example, a log 24" in diameter and 20' long with trim has a gross volume of 500 board feet. After the log is skidded, loaded and trucked to the mill, people called scalers will make the same measurements, but will reduce the gross volume of the log by the defect that they identify in it. The resulting figure is called the net volume. Commonly, the landowner is paid by the mill based on the net volume scaled at the mill.
Given the tree diameter at breast height (dbh) and the tree height to a merchantable top, there are tools available to help you estimate the volume in a standing tree. Volume tables are one such tool. Volume tables are, in part, specific to species and log rule. Log rules are ways to estimate the number of boards a milled tree will yield. The log rule used regionally is Scribner Decimal C. Volume is most commonly reported in board feet for trees that will be made into lumber. A board foot is 12" wide by 12" long by 1" thick. These are familiar units to anyone who buys lumber.
The volume of a tree may be measured several times before it is made into lumber. When I mark a tree to be cut, I record the dbh and height in logs to a merchantable top. In the local species specific volume table developed by Peter Joos that I use, logs are defined as 16' long and the merchantable top diameter is defined as 6". For example, a 26" dbh redwood with (5) 16' logs to a 6" top is reported in the table to have 620 board feet. As a general rule, height to a 6" top is usually about 30' less than the total height, but can vary greatly depending on the form of the tree. This tally of tree volume helps the landowner, logger and forester plan.
The tree, once it has been cut and bucked into logs, may be measured again by the faller. Henceforth each log is considered individually. The volume of each log is determined by measuring the small end diameter inside the bark and the length of the log. Given these inputs a different volume table is used to estimate the board foot volume. For example, a log 24" in diameter and 20' long with trim has a gross volume of 500 board feet. After the log is skidded, loaded and trucked to the mill, people called scalers will make the same measurements, but will reduce the gross volume of the log by the defect that they identify in it. The resulting figure is called the net volume. Commonly, the landowner is paid by the mill based on the net volume scaled at the mill.
March 12, 2006
Measuring Tree Diameter and Height
By Thembi Borras
Trees are in part valued on their volume. To determine the volume of a tree the diameter and the height of the tree must be measured.
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 you will measure 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 at breast height to the diameter at breast height (dbh), divide by 3.14. For example if the circumference is 82", dividing by 3.14 will yield a result of 26", therefore the dbh of the tree is 26".
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 you can see the top and bottom. Put the base of the "T" between your eyes, 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.
These are exercises for interested persons that do not have the tools of the forestry trade available to them. Foresters use diameter tapes, biltmore sticks and relaskops to measure diameter and height, or their eye to discern diameter and height more efficiently.
Trees are in part valued on their volume. To determine the volume of a tree the diameter and the height of the tree must be measured.
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 you will measure 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 at breast height to the diameter at breast height (dbh), divide by 3.14. For example if the circumference is 82", dividing by 3.14 will yield a result of 26", therefore the dbh of the tree is 26".
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 you can see the top and bottom. Put the base of the "T" between your eyes, 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.
These are exercises for interested persons that do not have the tools of the forestry trade available to them. Foresters use diameter tapes, biltmore sticks and relaskops to measure diameter and height, or their eye to discern diameter and height more efficiently.
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