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Tree Growth, Management, and Their Implications for Quality

John Punches

ood has long served as one of the process by which grow allows our most versatile and heavily to anticipate the effects of Wused raw materials. From their activities on the products that it we derive solid wood for structural ultimately will be produced from their and ornamental applications, composite trees. Similarly, understanding products in the forms of panels and growth processes helps wood products beams, numerous products, manufacturers comprehend how various chemicals, fuel—the list could go on wood characteristics develop and what and on. Foresters have worked for many constraints foresters face while guiding years to increase wood yield from , the growth process. while manufacturers have refined their This publication provides basic techniques and become increasingly information on tree growth, efficient in turning wood into products. characteristics that define wood quality, To a large extent, however, each group and the implications of common has worked toward its objectives silvicultural (tree tending) activities on without understanding the needs and/or wood quality. It is at best a summary— constraints of the other. tree growth is an immensely complex Wood is the result of a biological process. process, and not all aspects of wood It grows under a wide range of genetic formation are fully understood. Most and environmental influences and has of the relationships discussed in this a similarly wide range of properties publication apply to trees in general; John Punches, and characteristics. Understanding however, some items apply more Extension , specifically to a species group. Douglas County, These limitations are noted Oregon State in the text. University.

PNW 576 • September 2004 • A Pacific Northwest Extension publication Oregon State University • University of Idaho • Washington State University The Biological Process Growth hormones are produced near apical meristems and are transported through of Tree Growth the tree’s vascular system. As they spread, Trees, like most plants, grow taller over they activate the cambium, and secondary time. What distinguishes trees from other thickening (diameter growth) occurs. Since plants is their ability to extend growth over hormones must spread from the crown (the long periods and to add successive layers of region of living branches and the stem to growth in both height and in diameter. The which they are attached) down the stem, process is magnifi cently orchestrated, and secondary thickening does not begin all while science cannot claim to understand it at once. The cambium is activated fi rst in completely, the basics certainly are within the crown and then progressively down grasp. the stem, responding to a concentration gradient of hormones. A tree increases in height through the activity of apical meristems, growth regions Once activated, the cambium produces at the tips of its both wood and bark cells. Inner bark cells, known as phloem, are produced in Apical branches and stem meristems (Figure 1). a narrow layer just outside the cambium Diameter (Figure 2). They are the mechanism by growth, also which carbohydrates (sugars produced in called secondary the tree’s crown during photosynthesis) thickening, occurs are transported throughout the tree in any via a lateral direction needed and are made available Year 1 Year 2 Year 3 meristem called the to feed the cambium’s activity. Growth Figure 1.—Height cambium, a layer hormones also are transported through the growth. of living cells between a tree’s bark and its inner bark. woody core (Figure 2). As new layers of bark form, older layers are In the Pacifi c Northwest, trees typically progressively crushed and forced outward. lie dormant during the cold, wet, winter Within these layers are specialized cork months, then renew growth in the spring. cambiums in some species, forming corklike Growth begins with activation of the apical cells that further thicken the bark to form a meristems. Buds burst, and new foliage protective outer shell around the tree’s vital appears. All height growth and branch inner tissues. The fi ssured bark of Douglas- elongation results from activity at, rather fi r and ponderosa results from this than between, apical meristems, so a branch process. located a foot off the ground will remain Wood cells, known as xylem, are produced at that location even though the tree may to the inside of the cambium layer and grow considerably taller. typically form a new ring of wood each season (Figure 3). Wood cells formed early in the growth season are known as earlywood or springwood. Once mature, Heartwood (xylem) they are relatively long and hollow and have thin cell walls. They are primarily conduits Outer for moving and from the Sapwood bark root system to the crown. In conifers, (xylem) earlywood production coincides with the tree’s period of height growth. Inner bark (phloem) When apical meristems terminate their Vascular activity (typically in late spring), height cambium growth stops and hormone production Figure 2.—Stem cut-away showing tissue layers.

2 Latewood In temperate regions, tree age can be (Summerwood) estimated by counting growth rings, but Earlywood use this technique cautiously. Trees may (Springwood) take several years to reach the height at End of which the rings are being observed, so the previous resulting ring count would underestimate season’s total age. (For example, if a tree took growth 5 years to reach 8 feet in height, it always will have five more rings at its base than at the point 8 feet above ground.) Trees also can add false rings when conditions Figure 3.—Earlywood encourage them to stop and then resume and latewood cells growth during a single season. A dry in a conifer. summer followed by early, warm makes this phenomenon fairly common decreases. As hormone levels in the in the Pacific Northwest. Finally, when cambium fall, the tree begins to produce faced with particularly stressful conditions, thick-walled cells called latewood or some trees may produce no visible ring summerwood. Latewood production begins whatsoever, making them older than a at the points farthest from the crown (i.e., simple count of rings would suggest. the places where hormone levels are lowest) and gradually works its way up the stem. Xylem cells, both the earlywood and Latewood cells serve the tree primarily latewood varieties, can be further as mechanical support; their thick walls categorized as mature wood, juvenile wood, make them considerably stronger than or reaction wood. their earlywood counterparts. Latewood production is facilitated by the fact that In a cross section of a tree’s stem, wood the now-mature foliage in the crown is formed first (at that point in the stem) producing a surplus of carbohydrates that will be found at the core. Core wood is can be distributed to regions of active formed in close proximity to the tree’s growth. Since it has considerably more living crown and often is called crown- wood material within its cells, latewood formed wood. It tends to have a high is often noticeably darker in color than its proportion of earlywood, which makes neighboring earlywood. sense given the factors discussed above. However, core wood cells differ from those The process just described results in the yearly addition of a new sheath of wood within a tree’s stem, branches, and roots, stretching from just behind the root tips Growth rings to just below the buds at the branch and leader tips. The sheath consists of earlywood and latewood and lies just beneath the cambium layer. Each new layer Sapwood of wood is added on top of the previous Heartwood year’s layer. These growth rings or annual rings are readily observed on a cross section of a tree stem (Figure 4). Healthy trees with abundant water and little competition generally produce wide rings; those facing intense competition, drought, or disease produce narrower rings. Figure 4.—Tree stem cross section.

3 in wood formed farther from the core at that serve as conduits for transporting the same height, and this has some distinct water from roots to the crown. Roughly implications for wood quality. Core wood, 5 to 10 percent of sapwood consists of or juvenile wood, cells are prone to high specialized living cells that help transport longitudinal shrinkage during drying, and and store products, produce defensive they tend to be weaker than the mature chemicals, and transport waste byproducts. wood cells found beyond the core (which Sapwood is generally light in color, but formed farther away from the crown and as it ages its few remaining living cells later in the tree’s ). In general, juvenile die, and a region of dead wood, known as wood is less dense than mature wood. heartwood, accumulates at the center of However, in many conifer species, wood the stem (Figures 2 and 4, pages 2 and 3, closest to the pith (tree center) will have respectively). Heartwood is often darker relatively high density; density will fall in color than the surrounding sapwood. rapidly in the next few rings and then The color change might be attributed to will increase over subsequent rings as the chemical changes in cells as they die and to transitions from juvenile the accumulation of chemical byproducts to mature. The more uniform, generally produced during the tree’s growth process. stronger, and more stable mature wood is These chemicals, known as extractives, are considered to have superior characteristics deposited in the older xylem cells at the for solid wood applications, and trees that tree’s center. The heartwood of trees with have higher proportions of mature wood high extractive content often is resistant generally are considered to be of higher to decay. Redwood, western juniper, and quality. western redcedar are good examples of species with high extractive content. Trees develop yet another form of wood cell, called reaction wood, to straighten leaning stems or support branches. Coni- fers form a type of reaction wood called Crown Effects compression wood, while Since the crown is the tree’s source of wood- Tension wood broadleaved trees form building carbohydrates, larger crowns can tension wood. As implied by be expected to support higher levels of the names, compression wood wood production. Numerous forest man- forms on the side of a stem agement activities seek to increase crown or branch under mechanical vigor and thereby increase tree growth rate. compression, while tension It should be apparent by now, however, Compression wood wood is found where the stem that a tree’s crown has a tremendous impact or branch is under tension not only on growth rate but on the type Figure 5.—Types of reaction wood. (Figure 5). Reaction wood, while serving a of wood produced within the tree’s stem. necessary function within the tree, reduces Crown-formed wood tends to be mostly wood quality in most applications. It tends earlywood of the juvenile variety. As such, to be weaker and more prone to shrinkage it tends to be low in density and high in than mature wood, often has a different shrinkage, with obvious impacts on wood color, may machine poorly, and can be quality. Mature wood generally appears difficult to finish. after the living crown has receded past that point on the stem, and higher density New wood produced by the vascular cam- latewood is most prevalent in the portion of bium is known as sapwood. It comprises the stem farthest from the crown. both earlywood and latewood and might contain mature wood, juvenile wood, or Taper is largely a crown effect. Since the reaction wood if growing conditions favor cambium activates first within the crown, their formation. Fully developed sapwood growth rings tend to be widest here, and consists primarily of dead, hollow cells maximum diameter growth often occurs

4 near the base of the crown. Thus, trees with Density is rarely uniform large crowns tend to have highly conical across any given growth stems, while stems of trees whose crowns ring; this phenomenon have receded gradually become more is known as density cylindrical (Figure 6). High-taper trees have variation. It is caused by smaller scaling diameters (because diameter the inherent differences is measured at the log’s small end) and thus between earlywood and are of lower value to the grower. When its neighboring, higher sawn or peeled, they will yield less product density, latewood. Wood and will be more prone to cross grain. with considerable density variation can give rise Knot size is directly impacted by crown to uneven machining size. Persistent, live crowns give branches characteristics or might more time to grow, and knot size increases. Figure 6.—Crown wear differently under use. For example, effect on taper. Receding crowns limit branch size, thus when wood with a high degree of density limiting knot size. Clear wood can develop variation is used as flooring, ridges of high- only after branches have been removed density latewood may appear as the lower (either by natural or manual ) and density earlywood is crushed or worn away. the branch stubs have been covered with new layers of wood. Ring width and ring count (rings per inch) are used in various grading rules as indicators for a number of wood quality Defining Wood Quality factors. Wide rings, which equate to a low Wood quality is defined by many ring count, have long been employed as a characteristics and is properly assessed predictor of low density. This is somewhat only as related to a specific application. unfortunate, because density actually is not Wood intended for structural applications related to ring width but to the proportions may be judged by its strength, stiffness, of earlywood and latewood within the and dimensional stability, while wood for rings. At the time the rules were developed, architectural millwork may require specific however, most timber had grown slowly grain patterns or color. In the and under relatively high competition, and the paper industry, wood quality may be based observation was largely valid. Wide rings on fiber length and relative proportions of also make density variation problems more cellulose and lignin (two of wood’s basic, likely. A tree’s widest rings are formed chemical building blocks). within its crown, and the wide rings near a tree’s pith almost always will be composed Several wood characteristics serve of juvenile wood. as predictors of wood quality across applications. The most common are Branches affect wood density, density variation, ring width or ring quality by showing up as count, and knot size, type, and placement. knots in sawn and peeled wood products. Knot Density is mass per unit of volume. In size and placement are wood, it simply means that more wood important wood-quality fiber is packed into a given volume. Wood factors and are reflected density often directly correlates with its in log and grading utility as a raw material. Dense wood tends rules. Knots come in two to be stronger, stiffer, better able to hold varieties (Figure 7). Where Live branch, Dead branch, fasteners, and more wear resistant and a living branch intersects which would which would impact absorbent. In any given volume, it result in a result in a the stem, a tight knot or tight knot loose knot will have more wood material and less void intergrown knot forms. space than a less dense sample and will yield Figure 7.—Branch type more wood fiber for paper products. dictates knot type.

5 The wood grain the controlling factor in wood quality is is distorted at this genetics. It is less successful, and may even point, but the be counterproductive, when site conditions knot is attached have been the controlling factor. physically to modifies site conditions and can increase surrounding wood quality if production of clear wood wood. Dead with acceptable characteristics is enhanced. branches form It can decrease wood quality if the crown loose knots or persists and knot size increases or ring black knots. They width becomes too great. have no physical connection to the Enhance nutrition Figure 8.—Stem and surrounding wood Fertilization is used to increase growth crown forms vary and interrupt the grain. While any knot rate and, to some extent, tree health. Its among trees. reduces wood strength to some extent, loose primary function is to increase the volume knots do so to a far greater extent. Lumber of living crown on the tree. Its effects on and log grades often designate maximum wood quality are difficult to predict. Some acceptable knot sizes and numbers, and studies show increased quality, while others lumber grades place additional emphasis on show declines. Fertilizing should not be their placement (since a knot at the edge of dismissed, however, particularly when a board will have a greater negative impact combined with other activities. on its strength than if at the board’s center). Control crown extent Crown extent (length) can exert greater Influencing Wood Quality influence on wood quality than all other What can forest managers do to influence factors combined. A large crown produces wood quality? Here are some common more carbohydrates, thus supporting faster answers to this question—and reasons why and more extensive growth. However, they may or may not work. crown-formed wood tends to be mostly earlywood and may have a high proportion Use genetically improved of juvenile wood. Production of latewood planting stock and (to some extent) of mature wood is Most improved planting stock is selected favored by increasing distance from the based on growth rate; crown, stem, and crown. Crown persistence directly relates branch form; and disease resistance. While to knot size, and clear wood will form only not specifically developed for enhancing after the crown has receded past that point wood quality, many of these characteristics on the stem. do lend themselves to production of high- Controlling crown extent should be a pri- quality wood. Tree genetics is only one of mary goal when managing for wood quality. several factors affecting tree growth, so it cannot by itself ensure high-quality wood. Concentrate on the butt log A tree’s bottom (butt) log often contains Favor superior trees more volume than other logs in the tree during thinning and has the greatest potential for quality. It On any given site, some trees will produce has the greatest proportion of mature wood higher quality wood than others relative to juvenile wood, is most likely to (Figure 8). Concentrating the site’s growth be free of branches, and can be pruned most potential in these trees (by thinning) might easily. Pick a butt log length appropriate for increase production of quality wood. This anticipated market conditions and manage strategy is particularly applicable when to maximize its value.

6 Impacts of Common less taper, a higher proportion of latewood, and higher density. Again, juvenile wood Silvicultural Activities content is difficult to predict, but it is likely With this background in tree growth to be lower than in a low-density planting. processes and wood quality factors, now we Thinning reduces competition among should be able to see the implications of a trees and delays crown recession number of common silvicultural practices. (Figure 11). If properly applied, however, it can increase both diameter and height growth. Since it stimulates crown expansion, it generally encourages trees to retain more taper. The larger, more persistent crown will grow larger knots and a greater volume of juvenile wood. Its effects on wood density below the crown or in regions old enough to produce mature Figure 9.—The effect of planting at a low density. wood depend on how far the crown was allowed to recede and the tree’s age before thinning. If the trees Low planting density delays competition between trees and forestalls stand closure have little stem length (Figure 9). Trees are encouraged to produce below the crown, full crowns, and, lacking competition for wood quality will be light, they tend to sacrifice height growth. highly affected by the At the end of a rotation, they generally will crown, and density have larger knotty cores, larger knots, more will suffer. Trees with taper, higher proportions of earlywood, greater stem length and lower density than they would if below the crown may have very acceptable grown under higher levels of competition. Figure 11.— Juvenile wood impacts are more difficult to latewood components and would add layers The effect of anticipate, but it is likely these trees would of mature wood over their knotty, juvenile thinning. include a significant proportion of juvenile cores, both factors that serve to increase wood. wood quality. directly manipulates crown extent High planting density hastens competition Pruning between trees and speeds stand closure and so has profound implications for wood (Figure 10). Trees concentrate on height quality (Figure 12). Forcing the crown growth, and crowns recede relatively rapidly up the stem limits knot growth, decreases due to shading of lower limbs. Under these taper, encourages latewood production, conditions, diameter growth is slowed, but minimizes the size wood quality tends to increase. At the end of the knotty core of a rotation, these trees can be expected to and allows clear have smaller knotty cores, smaller knots, wood to develop. In some species, it might reduce juvenile wood production, but evidence Figure 12.—The effect of this remains elusive. Aggressive of pruning. pruning can reduce growth rate (by reducing the amount of food- producing leaf area) and can stimulate Figure 10.—The effect of planting at a high density..

7 development of epicormic branches impacts wood quality primarily through (branches originating from dormant buds) its effect on crown extent. Stand density if done in open stands, but appropriate management (both at planting and, later, pruning can minimize these effects. in stand development), pruning, and fertilization each has distinct and fairly Fertilization stimulates crown vigor, predictable impacts on crown condition. thereby increasing growth rate Understanding tree growth processes and (Figure 13). It also encourages the crown implications allows foresters to production and retention of sapwood, seriously consider wood quality as one of resulting in greater their management objectives. It also helps sapwood depth. Its manufacturers understand the limitation of impact on wood their wood raw materials. quality is highly dependent on crown position. If the crown How to Order This Publication covers much of the To order copies of this publication, send stem, wood quality the complete title and publication number is likely to decrease. (PNW 576), along with your payment, to: If the crown has Figure 13.—The effect Publication Orders of fertilizing. receded, the branch-free stem may produce Extension & Station Communications perfectly acceptable wood at an enhanced Oregon State University rate. Fertilization can even impact relative 422 Kerr Administration earlywood and latewood proportions. Corvallis, OR 97331-2119 If fertilization increases the period of fax 541-737-0817 height growth, earlywood production will e-mail: [email protected] be prolonged and latewood production delayed. If, however, the height growth Copies are $2.50 each; orders of 100 copies period is unaffected but late-season growth or more of a single title receive a 25-percent is extended, latewood production could discount. Shipping and handling charges be enhanced. There is some evidence that are per order, as follows: adding certain micronutrients can extend $10.00 or less ...... $3.00 the period of latewood production, thereby $10.01–$20.00 ...... $4.00 increasing latewood proportion and wood $20.01–$40.00 ...... $5.00 density, but the jury is still out on the $40.01–$60.00 ...... $6.00 specifics of this interaction. $60.01–$80.00 ...... $7.00 $80.01–$100.00 ..... $8.00 over $100.00 ...... 10% of total order Some Parting Thoughts Outside continental U.S., request price quote In the forest, tree growth and wood formation are dictated by a complex Payment can be by check, money order, interaction of site, climate, genetics, and or purchase order in U.S. funds, made competition. Forest managers seldom payable to Oregon State University, or by can control these factors completely, Visa or MasterCard (please provide account but they can influence forest conditions number and expiration date). using basic silvicultural tools.

© 2004 Oregon State University. Produced and distributed in furtherance of the Acts of Congress of May 8 and June 30, 1914. Extension work is a cooperative program of Oregon State University, the U.S. Department of Agriculture, and Oregon counties. Oregon State University Extension Service offers educational programs, activities, and materials—without discrimination based on race, color, religion, sex, sexual orientation, national origin, age, marital status, disability, and disabled veteran or Vietnam-era veteran status. Oregon State University Extension Service is an Equal Opportunity Employer. Published September 2004 $2.50