PNW 184

Thinning An Important Timber Management Tool

W.H. Emmingham and N.E. Elwood

hinning is the practice of removing some in an immature Contents stand to increase growth of the remaining trees and the total Basic growth...... 2 Tyield or value of usable . You thin stands in which the How thinning affects trees are approximately the same age. individual tree growth...... 3 There are two methods of thinning: commercial and precommercial. In commercial thinnings, the immediate value of the removed trees How thinning affects pays the thinning costs. If the value of the trees does not return enough stand growth...... 4 to pay thinning costs, the practice is a precommercial thinning. How to thin...... 7 A good thinning program meets one or more of these six objectives. Some final notes...... 8 1. Use or sell trees that otherwise would die and decay. 2. Redistribute the total fiber growth of the stand to fewer trees of higher quality, thereby increasing the value of usable fiber. 3. Increase the profitability of growing trees by reducing the invest- ment in standing volume. William H. Emmingham, Extension 4. Provide money to pay off investments, such as , pre- specialist emeritus; and Norman E. Elwood, Extension commercial thinning, and other stand improvement activities. management specialist; Oregon State 5. Enhance nontimber uses of the such as grazing, wildlife, and University. This is a revision of earlier recreation. work by Larry Streeby, former Exten- sion forestry management specialist, 6. Provide more frequent periodic incomes. Oregon State University. The authors acknowledge the assistance of Exten- In order to understand thinning, it is necessary to have some knowl- sion at University of Idaho edge of how seedlings grow into mature trees and how groups or and Washington State University. stands of trees develop.

A Pacific Northwest Extension Publication Oregon State University • Washington State University • University of Idaho 1 that is light in color. Later in the summer, Basic tree growth when growth slows, cells have thicker Trees, like all living things, grow by walls, which appear to be darker. These al- forming cells. New cells form in the cambi- ternate light and dark layers make up annual um (Figure 1), which is just inside the bark rings, which you see on a stump or log end. along the entire length and circumference of A paired light and dark layer form an the tree stem, branches, and . Adjacent annual ring—the amount of wood a tree to this growth area is the transportation produces in one growing season. Annual network for the tree. rings enable you to trace the growth history Water and nutrients move upward in of the tree. the xylem (sapwood), and food manufac- tured in the leaves moves downward in the phloem, a region between the cambium and the bark. If a tree is “girdled” (by cutting Basic growth processes a notch all the way around the stem), the Knowledge of three basic growth phloem is severed and the tree will die un- processes—photosynthesis, respiration, and less the notch is bridged by new growth. translocation–assimilation—is important Except in the extreme tips of growing to understand thinning. You might also call , new cells formed in the cambium these processes energy storage, energy re- remain in place. The cambium layer moves lease, and food transport–cell formation. outward as the tree grows in diameter, ever surrounding the tree and forming new cells outside the old ones. Energy storage (photosynthesis) Thus, a nail driven in 5 feet above the This process converts carbon dioxide and ground in a tree with a 10-inch diameter water into carbohydrates, the basic food of will always remain 5 feet above the ground the tree. Sunlight supplies the energy neces- and will always be in the center 10 inches sary for photosynthesis, which takes place of the bole (trunk), no matter how large the in the leaves. The tree stores this energy tree grows. New wood eventually will bury in a form of reusable food. When the nail by growing around and over it. water and nutrients are plentiful, light and Cells formed in the spring, when growth temperature determine and limit the rate of is rapid, have thin walls and produce wood photosynthesis.

A B Figure 1.—Types of tree cells and their functions. The outer bark (A) insulates the tree from extreme heat and cold, helps keep out rain, and protects the plant against insect enemies. The phloem (B) conducts food from leaves to the rest of the tree. Eventually, it becomes part of the tree’s bark. Immediately inside the phloem is the cambium layer (C). The cambium produces new bark and new wood annually in response to hormones, called auxins, that stimulate the growth of cells. Sapwood, or xylem, (D) is the pipeline for water and nutrients moving from roots to the leaves. When its inner cells lose their vitality, they turn into heartwood (E). Heartwood is the central, supporting column of the mature tree. Although it C D is dead, it will not decay or lose strength as long as the tree’s outer layers remain intact. (Illustration courtesy of The St. Regis Paper E Company, New York, NY.)

2 Energy release (respiration) produced, making more carbohydrate avail- This process occurs in all living cells able for new cell formation and growth. of the tree. It releases energy by breaking The increased volume growth of individ- down the carbohydrate manufactured in ual trees normally occurs as diameter rather photosynthesis. Although this breakdown than height growth. With a wide range of and energy release are necessary, break- tree density per acre, height growth is rela- down may be excessive, i.e., use more tively constant for a given species and site. carbohydrate than necessary. Temperature The primary effect of thinning, there- is very important in regulating the rate of fore, is to increase diameter growth of the respiration. remaining trees. Effective thinnings will stimulate this growth within a few years. Food transport–cell formulation (translocation–assimilation) Effect on species and age Carbohydrates manufactured in the The growth response to thinning var- leaves must move out to all parts of the ies with the age of the tree (more so for cambium so new cells can form and the some species than others). Foresters have tree can grow. The carbohydrates must be long observed that old and large trees do assimilated into new cells. Transporting not respond to thinning as readily nor as food and converting it to new cells requires dramatically as small, young trees. This is energy. This energy is liberated from the particularly true for species that are intoler- food during respiration. ant of shade. Intolerant species require nearly full Summary sunlight to thrive and grow. Tolerant tree Photosynthesis, the energy-storing pro- species can survive and grow in low light cess, is controlled by temperature and the levels, such as those that develop beneath a amount of sunlight, water, and nutrients full canopy of intolerant species. Shade- tol- available to the tree. Temperature controls erant species are more likely to respond to respiration, which uses food to maintain the thinning at older ages. life processes of the tree and to release the The tolerance of common Pacific North- energy necessary for growth. The amount west species varies (Table 1). of growth depends on how much energy or food is left over after the tree uses what it needs for respiration. Thinning shock There are exceptions to the generaliza- tion that thinning increases the growth rate of remaining trees. In dense stands of young How thinning affects trees, the trees may continue to grow at the individual tree growth same rate—or they might display up to 40 percent slower growth. This undesirable reaction to thinning is called Effects on the tree’s thinning shock. environment Removing some of the trees that compete for limited water and soil nutrients makes Table 1.—Tree species’ tolerance to shade. more water and nutrients available for the remaining trees. Thinning also opens the Tolerant stand’s canopy, making more light grand fir, mountain hemlock, Pacific silver fir, spruce, available for the remaining trees. western hemlock, western red cedar Intermediate Douglas-fir, sugar pine, western white pine Effects on tree growth Intolerant The increased water, nutrients, and light alder, cottonwood, lodgepole pine, noble fir, ponderosa that result from thinning increase photosyn- pine, western larch thesis in the remaining trees. More food is 3 How thinning affects stand growth C C C Thinning applies to D I D D I D W stands of trees rather than

Crown length Crown to individual trees. The fact that thinning ordinar- M ily increases the growth of Total height Total individual remaining trees M S does not mean total stand S growth is increased—there M are fewer trees in a thinned stand. The effects of thin- ning on stand growth are Figure 2.—Crown type classifications of trees in even‑age stands. D = Dominant, best understood after a C = Codominant, I = Intermediate, W = Wolf, S = Suppressed, M = Mortality. The discussion of stand develop- “crown ratio” is the proportion of total tree height that is occupied by live crown. In ment under natural, un- this illustration, the dominants have a 50 percent crown ratio; the wolf tree has an thinned conditions. 80 percent crown ratio.

Shock occurs in trees with small crowns where the vertical crown length is less than Crown classification one‑third the total tree height (see Figure 2 A tree crown classification system is use- for an illustration of crown ratio). ful in discussing stand development. Fig- Repeated thinning stimulates vigorous ure 2 illustrates a commonly used system, growth and produces crowns greater than which has the following six classes. 40 percent of the total tree height. Shock is 1. Dominant. Trees with the crown extend- unlikely in such stands. ing above the general level of the crown canopy receive full sunlight from above and some from the sides. The sides of the What causes thinning shock? crowns are well developed but (possibly) There is no universally accepted reason. somewhat crowded. One explanation is that the limited crown 2. Codominant. Trees with crowns forming size prevents an increase in photosynthesis the general level of the crown cover re- large enough to offset the increased respira- ceive full light from above but little from tion that may result from direct sunlight on the sides. The tree crowns are medium the stems of remaining trees. size and more crowded on the sides than Another explanation is sunscald—the are dominants’ crowns. death of the bark and cambium of young These trees usually are trees on the newly exposed south side of the 3. Intermediate. shorter than those in the two preced- tree. A possible reason for this type of shock ing classes. They have small, crowded is that trees in dense stands tend to have sides. The crowns extend into the canopy thinner bark, which may contribute to scald. formed by dominant and codominant A third explanation is tree leaves that trees; they receive a little direct light develop in shade cannot control their water from above but none from the sides. loss when they are exposed suddenly to full sun. They sometimes wilt and die and leave 4. Suppressed (overtopped). The crowns the tree with less leaf area, which reduces on these trees are below the level of the photosynthesis. crown canopy. They receive no direct light from above or from the sides. 5. Wolf. These trees develop and grow in the open. They have full crowns on all sides, with branches well below the canopy

4 level. The crowns are uncrowded on two Trees that develop in the open often become unde- or more sides and receive full light from sirable “wolf” trees with large branches. above and well down on two or more sides. 6. Mortality. These are dead trees within the Effect of thinning stand. Suppressed trees usually die, and on stand development trees of any crown class may die from Thinning can alter the way a stand develops by disease or insect attack. influencing the proportion of trees that grow into each crown class. The extent of the alteration depends on the particular way you thin. If you remove trees early Stand development in a stand’s development, you reduce the competition under natural conditions the remaining trees face. As illustrated in Figure 3, a very young, With reduced competition, fewer remaining trees even‑age stand is composed entirely of will develop into intermediate and suppressed trees. dominant trees because the seedlings do not They will grow faster and will be larger than trees in shade one another. As the trees grow, height an unthinned stand on a similar site. becomes critical to their survival. Tall After competition begins and the stand develops all trees shade shorter trees. At some point the crown classes, removing only intermediate and sup- increased shading reduces photosynthesis, pressed trees may not significantly reduce the com- which further reduces growth. petition faced by the larger dominant and codominant The tallest trees remain in the dominant trees. Suppressed trees, in particular, do not compete crown class, while shorter ones drop down significantly with larger trees. to the codominant, intermediate, and sup- A successful low thinning removes all suppressed, pressed classes. Once a height advantage most intermediates, many co-dominants, and even is lost, normally it is not regained. As this some dominant trees. The remaining stand consists of process continues year after year, the stand uniformly spaced dominant and codominant trees. develops into one with varied crown class- es. The process continues as long as the even‑age stand exists. All trees compete for light, water, and nutrients. Some trees in the domi- nant class remain there; others fall behind gradually until they are in the codominant class. Some trees in the codominant class remain there, but others fall behind and D D D D D become members of the interme- S C diate class. Trees in the intermedi- ate class often become overtopped M and eventually die. With increased stand age, crown class differentiation be- comes more pronounced, and the D I D D D C D number of trees in the stand is S reduced constantly. In an unman- age Increasing aged stand, the remains of dead trees rot in place. D C D D D C D Stagnation of tree stands occurs I when normal stand development does not take place. It is usually found on poor sites and results in very slow stand growth and many D D D D D D D D suppressed trees.

Figure 3.— Trees differentiate into crown classes as stands become older.

5 In a high thinning, removing relatively Large logs usually have a higher value few dominant and codominant trees will per unit of volume than small logs, for two release a large number of remaining trees. basic reasons. The cost is lower High thinning is not likely to stimulate a because a few big logs cost less to handle good release response if competition has than many small logs; and the products produced a stand where few trees have that can be made from large logs (veneer, crowns extending 30 to 40 percent of the large timbers) are worth more than those tree’s height. made from small logs. In a natural stand, part of the gross growth is lost when trees die from com- Effects of thinning petition or other factors, go unsalvaged, and eventually decay. Thinning “captures” on total stand yield this mortality—it increases the amount On a forested site, the total amounts of of usable (and salable) fiber that a stand water, nutrients, and light are fixed, which produces. limits the total volume of wood fiber a stand Thinning does not produce increased will produce within a given time. This im- growth, but because thinning concentrates plies that total fiber growth is also fixed and growth onto a few large stems, individual you cannot increase it by thinning. How- tree and stand value increase. ever, thinning can increase the usable fiber The way to increase stand production a stand produces. is to supplement elements that stimulate Thinning channels the limited light, wa- growth, such as increasing nitrogen by ter, and nutrients into fewer trees, enabling fertilizing or water by irrigating. Genetic each of them to grow faster. Unless thinning improvement programs, which can cre- is so severe that too few trees are left to ate trees that grow more efficiently, also fully occupy the site, the total fiber growth can increase production. You obtain best that the site is capable of producing is con- results with fertilization, irrigation, and centrated on fewer stems. This results in a genetic improvement when they are com- few large logs rather than many small logs. bined with a careful thinning program.

B A Response to thinning

Thinning should have been done here (age 7) rather than waiting Thinnings done until here (age 11) at appropriate ages.

Figure 4.—Some effects of thinning frequency. Thin often so that the width of growth rings remains relatively constant. Cross sections A and B are of 21‑year‑old trees. Thinning for tree A was postponed too long, causing the diameter growth restriction shown. Upon thinning, diameter growth increased but then slowed again as the tree crowns closed. Tree B was properly thinned, allowing its diameter growth to remain fairly uniform.

6 than heavy thinnings. In the Northwest, How to thin thinning intervals commonly range from In deciding how to thin, you must answer 5 to 15 years. several questions: When should I thin? How many trees should I remove? Which trees Time of year should I remove? What logging methods should I use? Late summer and fall are the best times to thin. Trees grow in spring and early sum- mer, and it is easy to scrape the bark from residual trees during and yard-ing When to thin operations. Also, during this season, cut To get the biggest increase in usable fi- trees can attract insects that can kill the ber, begin thinning early, when trees are 10 remaining trees. to 20 years old. Actually, the density of the For example, if you thin ponderosa pine stand and size of trees—rather than stand or lodgepole pine in spring, large numbers age—dictate when thinning is effective. of bark beetles may be attracted to the cut Thinning before competition begins accom- trees. Thin pine stands after August 1 to plishes little, but thinning when the crowns minimize the danger associated with bark of adjacent trees begin to touch will reduce beetles. competition. Winter is also an acceptable time for In stands with 4- to 12-inch diameters thinning, with one note of caution. Soils are (diameter at breast height, or dbh), thin more prone to compaction and erosion in before the crown ratios drop below 40 the wet season, so regulate use of ground- per-cent. If crowns have receded beyond based skidders carefully. In areas where this point, thin lightly to prevent thinning snow accumulates or the ground freezes, shock. Use early thinnings (which are likely winter thinning is perfectly acceptable. to be precommercial thinnings) to shorten the time until you can harvest merchant- Thinning intensity able logs and to keep stands vigorous and If the intensity of thinning is too light, healthy. the growth is spread over too many trees to Make subsequent thinnings as competi- achieve maximum benefits. If the intensity tion recurs but before diameter growth is is too heavy, considerable amounts of nutri- severely reduced. As stand competition ents, water, and light are outside the reach increases, annual rings of individual trees of the remaining trees, and the total produc- become thinner. Rapid increases in the tive capacity of the site is underused. Also, width of the trees’ growth rings after thin- wind throw after thinning may be a problem ning often are dramatic examples of the for some species or on highly exposed sites. benefits of thinning. Ideally, however, you For these reasons, the intensity of thinning should thin before diameter growth drops is an important management choice. off severely (see Figure 4). In Douglas-fir stands, a common guide The interval between thinnings depends for spacing trees to be left is the “D-plus” on the intensity of thinning and the produc- (D+) rule. It specifies that the average tivity of the site. Intensity of thinning refers spacing (in feet) of remaining trees should to the number of trees (and competition) be equal to the average stand diameter (in you remove with respect to full, natural, inches), plus some constant. or “normal” stocking. A heavy thinning re- For example, if the average diameter of moves many trees and reduces competition trees left in the stand will be 12 inches and for a longer period than does a light thin- you follow a D+5 rule, the average spacing ning, which removes fewer trees. of the remaining trees will be 17 feet, leav- When you do precommercial thinning ing 150 trees per acre. (usually a heavy thinning), space the trees Using the D+5 spacing guide does not in- you leave so they will reach merchantable dicate an arbitrary 17‑foot by 17‑foot spac- size before the stand closes and competition ing between all trees. Some variation, both becomes severe. Crown closure and com- greater and less than 17 x 17, is necessary to petition resume soon after light thinnings; accommodate differences in tree vigor and therefore, conduct them more frequently tree quality and for other considerations.

7 It is commonly advocated that you make competition. Aim for maximum fiber pro- D+4 or D+5 thinnings in early years (until duction from the site on good quality trees. trees are about 8 inches dbh) and follow If you are a novice at thinning, visit with somewhat heavier thinnings (perhaps stands that were thinned in different ways to D+6) in later years. Consider this a rough get a better feel for tree‑selection criteria. guide only. Experience in thinning over the years Species selection in the Northwest has resulted in a gradual Thinning offers a means of controlling increase in the intensity of thinning. Other the species composition of a stand by leav- techniques for determining the spacing of ing the more valuable or better growing trees to leave, such as density management species to mature. This is often done during diagrams, are now available and can pro- precommercial thinning when you remove duce equally satisfactory results. large shrubs along with undesirable hard- The main point is to use some thinning wood or conifer species. guide so you can determine the spacing of residual trees. Consult with a professional Logging methods about the guides used in your area. Selection of logging methods and equip- ment is important. Logging systems differ Tree selection in their capacity to handle logs of differ- The type of thinning you select depends ent sizes, adaptability to steep slopes, road on your objectives, on the age, size, condi- access requirements, site disturbance, soil tion, and species composition of the stand, compaction potential, potential damage to and on its management history. A cardinal remaining trees, and cost. rule when thinning is to improve the stand’s You can reduce damage to residual trees condition for future growth. by not logging during spring and early Low thinning is most common if residual summer when the bark easily is broken trees are in the dominant and codominant loose from the trunk. Planning and mark- crown classes. Residual trees should be ing skidtrails can reduce the amount of soil straight and vigorous, have relatively small compaction. limbs and considerable clear bole, and be spaced as uniformly as possible. A young Douglas‑fir stand that has been precommercially thinned presents an Some final notes entirely different set of selection problems Thinning is an important stand manage- than does a middle‑age Douglas‑fir stand ment tool available to woodland owners. that has never been thinned. In the middle‑ State service foresters and industrial and age, unmanaged stand, concentrate your consulting foresters can give you valuable initial thinning on diseased trees or rough, assistance in planning and conducting thin- limby trees in the dominant and codominant ning operations. Educational programs and crown classes and on poor quality trees in other literature on thinning are available the lower crown classes. Pay less attention from most county Extension offices. It is to the spacing of the remaining trees than important that you understand thinning and you would in a thinned stand. The thinning define management goals before contacting primarily upgrades stand quality. professional help. In a younger, previously thinned stand, focus on crown classification, spacing, and

© 1983 Oregon State University.

Published and distributed in furtherance of the Acts of Congress of May 8 and June 30, 1914, by the Oregon State University Extension Service, Washington State University Extension, University of Idaho Extension, and the U.S. Department of cooperating. The three participating Extension services offer educational programs, activities, and materials—without regard to race, color, religion, sex, sexual orientation, national origin, age, marital status, disability, and disabled veteran or Vietnam-era veteran status—as required by Title VI of the Civil Rights Act of 1964, Title IX of the Education Amendments of 1972, and Section 504 of the Rehabilitation Act of 1973. The Oregon State University Extension Service, Washington State University Extension, and University of Idaho Extension are Equal Opportunity Employers. Published August 1983. Reprinted December 2010.

8