Tree Pruning

June 8, 2009

By Thembi Borras

Pruning involves removing the lower branches from live trees.

Why prune?
Pruning is an important practice to reduce fire hazard, but must be done properly so tree health is not compromised. Pruning may also improve safety, aesthetics, timber quality and tree vigor. Trees along roads are excellent candidates for pruning to create a fuel break or to facilitate quicker road surface drying.

How to prune? Following are guidelines for pruning:

• Make pruning cuts where two limbs intersect or where limbs intersect the main tree trunk.


• Make the final pruning cut just outside the branch collar. Avoid "flush cuts" which remove the branch collar or "stub cuts" which leave branch stubs protruding beyond the branch collar. The branch collar is the swollen area at the base of a branch where it connects to the trunk and contains special tissue that prevents decay from moving downward from the branch into the trunk.


• For large limbs, make an undercut so that when the branch separates it doesn't strip off bark from the bole of the tree. The following figure (copied from USFS publication NA-FR-01-95 entitled, How to Prune Trees by Peter J. Bedker et al and illustrations by Julie Martinez of Afton, MN) displays a cutting sequence to minimize pruning damage:


• If there is reason to remove a large number of live limbs, it may be prudent to prune the tree in stages increasing the distance from the ground to the lowest limb slowly, pruning a portion of limbs each year.

• Consider staggering the distance between the ground and the height of the lowest limbs to avoid the “lollipop look”.


• Hand saws can be used if the branch can be reached. Otherwise, use a telescoping pole saw or shear. Visit http://ext.nrs.wsu.edu/handtools/tools/pruning/index.htm to view power and hand tools for pruning.


• Be careful not to scar the bole as the scar may invite the introduction of stem rot fungi into the tree.


• There is no need to apply any kind of “wound dressing” to the prune scars.


• Use recommended safety equipment, such as eye protection, hard hat, gloves and sturdy footwear. Use common sense and be safe.
  

When to prune?
Pruning can be done anytime during the year. However, consider pruning during the dormant season when the sap is flowing less. Try to avoid pruning during the spring and early summer when the tree is actively growing and is easily damaged. Another reason to avoid pruning during the growing season is that beetles are most active and are attracted to the sap from the pruning wound.

The area out to 200 feet from a home is referred to as defensible space. Trees in the defensible space and along emergency vehicle access routes should be pruned first.

Which trees should be pruned?
Pruning takes time and energy so to optimize these precious resources pick the best trees to prune and thin out the worst. There is an excellent article containing characteristics of “leave” trees and characteristics of “cut” trees in the winter 2008 edition of SW Oregon Woodland News entitled Thin for Quality, Not Spacing and can be accessed at http://extension.oregonstate.edu/sorec/sites/default/files/documents/WoodlandNewsJan08newsletter2.pdf.

The following is excerpted from it:
GUIDELINES: So what makes a high quality tree?
Following are some guidelines for “leave” and “cut” trees that relate to tree quality. These are all characteristics you can “eyeball” out in the woods.

Characteristics of “leave” trees
• Good live crown ratio (30% or greater)
• Good height growth for species and age
• Symmetrical crown with “pointy top”
• Abundant foliage with good color
• Good form (straight, without sweeps, crooks, forks, etc.)
• Species is well suited to the site over the long term

Characteristics of “cut trees”
• Poor crown ratio (<30%)
• Poor height growth and crown form (flat or rounded top, lopsided)
• Foliage is sparse or off-color
• Poor form
• Species not well suited to the site

Additional considerations include exposure to sun or wind. Sunscald is a tree bole injury that can result from pruning. This injury is more likely to occur on trees on the southwest side of a low density stand. To avoid sunscald a buffer of unpruned trees can be left on the south and west side of the stand. Similarly, to avoid wind damage unpruned trees and trees that would otherwise be “cut” trees can be left on the side of the stand from which prevailing winds blow to serve as a buffer.

How much to prune? Live crown ratio is an important concept to understand in determining how much of the crown to prune. Live crown ratio is the percentage of the total height of the tree that is occupied by the live green crown. For example, if the crown length, measured from the leader tip to the base of the live crown, is 55 ft and the total tree height is 100 ft, then the live crown ratio is 55%. Post pruning, the live crown ratio should be no less than 40% and preferably 50%. Therefore in this example, you can safely prune the lower 5 feet of live crown and no more than 15 feet. Dead limbs below the live crown do not factor into this calculation and should be pruned.

Another important consideration, especially within the defensible space is reducing ladder fuels which is vegetation that connects ground vegetation to tree crowns facilitating a ground fire becoming a crown fire, which moves much more rapidly. Reducing ladder fuels may be accomplished by, in part, removing “cut trees” and pruning “leave” trees. The minimum distance between the shrub layer and the canopy should be, at least, 4 times the height of the shrub layer. For example, if the shrub layer is 3 feet tall then the lowest limbs of the overstory trees should be 15 feet above the ground.

A portion of this article was gleaned from Conifer Pruning Basics for Family Forest Landowners available at http://cru.cahe.wsu.edu/CEPublications/eb1984/EB1984.pdf, the chapter on pruning in the Oregon-Washington Master Gardener Handbook entitled Sustainable Gardening and the article entitled Thin for Quality, Not Spacing in the winter 2008 edition of the SW Oregon WOODLAND NEWS.

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.