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Forest Landowner's Guide to Field Grading Hardwood Trees

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DIVISION OF AGRICULTURE RESEARC H & EXTENSION Agriculture and Natural Resources University of Arkansas System FSA5015 Forest Landowner’s Guide to Field Grading Hardwood Trees Kyle Cunningham Many forest landowners own economic value. Therefore, this fact hardwood stands and are not aware of sheet will refer to tree grade and AssociateExtension Instructor Professor - of the great differences in quality and butt-log grade interchangeably. Forestry value between individual trees within these stands. Tree (or log) grade, among other factors, is essential in determining the quality and economic value of hardwood trees. The objective of this fact sheet will be to explain the tree grading process in a simple, straightforward manner. Why Is Tree Grade Important? Because grading is the method for estimating the wood quality a particular tree may produce, it provides information for establishing Figure 1. Log locations within the main a tree’s monetary value. If a land- stem of a tree. owner is interested in timber produc- tion, maintaining higher grade trees is important. The reason for this prin - Because grade is the determining ciple is simple: high-grade trees factor in establishing the type of saw- contain much more economic value timber product (or products) a tree than low-grade trees. If a forest may produce, grades are typically landowner is interested in economic applied to sawlog-size trees return, tree grade is as important as (diameter > 12 inches). tree volume in determining the rate of return for a hardwood stand. Field Grades Three field grades are used to What Is a Tree Grade? rank the quality of tree logs: F1, F2 Tree grade is a measure used to and F3. An F1 tree grade represents express a tree’s wood quality. Grade is stems capable of producing large determined using the amount of clear amounts of high-quality wood wood (defect-free stem length) within products. This means that F1 grade the butt-log portion of a tree trees will produce a large proportion (Figure 1). The butt-log of a tree is the of wood that can be used for furniture, Arkansas Is lowest 16-foot section above the stump flooring, dimension lumber and other height (approximately 1.5 feet). high-value wood products. Our Campus Though the upper-logs may be graded at a sawmill, the butt-log is generally An F2 tree grade represents stems the portion used to establish a stand- capable of producing some high-value wood products but not enough to be Visit our web site at: ing tree’s grade because this portion http ://www.uaex .edu contains the majority of a tree’s ranked as a grade F1. University of Arkansas, United States Department of Agriculture, and County Governments Cooperating An F3 tree grade represents stems containing a of a tree log. It is used to determine how much wood small proportion of high­value wood products. F3 can be extracted from a log. Because it is difficult to grade stems will often be used for pallet or contain­ obtain this measurement from standing trees, an er material. assumption is applied that the SD is equal to 80 percent of a tree’s DBH (Table 1). Therefore, a tree If a sawtimber­size hardwood tree does not with a 20­inch DBH would have a 16­inch SD contain enough quality to make grade F3, it is classi­ (20 × 0.80 = 16). The 80 percent figure refers to a fied as “no grade” and will either be used as pulp­ tree’s “form class,” which expresses the ratio between wood, tie­logs, other products or be determined DBH and SD. Most sawtimber­size hardwood trees “non­merchantable” (or cull). have a form class near 80 percent. Components of Tree Grade Table 1. Estimated Scaling Diameter for Form Tree Size Class 80 Trees One of the most important aspects of tree DBH SD grading is the size or diameter at breast height (inches) (inches) (DBH = diameter of tree at 4.5 feet above ground) of 12 10 a tree. First, there are differing minimum DBH sizes 14 11 required for a tree log to be considered an F1, F2 or 16 13 F3 grade. For example, a tree must have a DBH of at 18 14 least 16 inches to be a candidate for a grade F1, at least 13 inches for grade F2 or at least 10 inches for 20 16 grade F3. 22 18 24 19 Also based on DBH, there are varying lengths of clear wood (defect­free sections) required to make a 26 21 specific tree grade. For example, a tree with a DBH of 28 22 20 inches must have two clear sections of wood that 30 24 are each a minimum of 5 feet in length to be a grade F1. However, a 16­inch DBH tree would need two clear sections that are a minimum of 7 feet in length. Grading Faces Essentially, smaller diameter trees require longer lengths of clear wood to make a grade. Establishing grading faces is another key component in determining tree grade. Typically four equal sections are established by dividing the butt­log Scaling Diameter vertically (Figure 2). The first face is established by Scaling diameter (SD) is another important placing as many defects as possible into one grading measure used in tree grading. The SD can be defined face. This grading face becomes the “worst face” as the diameter (inside bark) at the small (or top) end because it contains the highest proportion of tree . Figure 2. Establishing 4 grading faces on the butt­log. Source: Rast and others, USDA Forest Service, 1973 defects and the least amount of clear wood. The is considered a defect. On trees with a scaling remaining three grading faces are established by diameter 14 inches or larger, count every other vertically dividing the remainder of the butt­log into epicormic branch (> 3/8 inch) as a defect. three equal sections. It is important to note that the second worst grading face of the butt­log will be used to determine a tree’s grade. Tree Defects Being able to identify tree defects is essential in accurately establishing a tree grade. These defects can include knots, epicormic branching, decay, frac­ tures and/or crook and sweep. Tree defects directly affect tree grade in that they reduce the length of defect­free stem sections in the butt­log. – Knots are probably the most common defect found on hardwood logs. Knots are simply theKnots remnants of old limbs (Figure 3). As a forest stand develops, some of the lower limbs of trees are shaded and eventually die and fall off. The point on the trunk where the limb prunes itself establishes a knot. All knots are counted as defects, because they affect the wood all the way to the core (or pith) of the stem. Figure 4. Example of epicormic branching. – Stress fractures may include frost cracks, seams or splits. The major determina­Stress Fractures tion that must be made with a stress fracture is whether or not it is on the surface only or extends into the interior of the trunk. For stress fractures that extend into the interior of the trunk, measure the length affected to be counted as a defect. – In standard log grading, crook, sweep and decay areAssumptions for Decay, Crook and Sweep handled by one of two methods: (1) use a series of formulas derived by the USDA Forest Service based on the length and severity of the defect or (2) simply reduce the length of the log so that it does not include the portion with the defect (remembering a log must be at least 12 feet in length). To keep things less compli cated, this fact sheet will only demonstrate the formula for sweep (which is straightforward). Crook Figure 3. Example of a knot. and decay will be handled by reducing the log length – Hardwood trees contain to exclude the defect. dormant buds all along their trunks. Given the rightEpicormic Branches conditions (light, stress and others), these buds are – Decay or rot triggered to sprout a new limb on the trunk of a tree may be caused by severalDecay called an “epicormic branch” (Figure 4). Timber factors, but it is always buyers will usually not grade a tree very high if it viewed as a defect. Decay contains epicormic branching. However, epicormic may be initiated by fire branching is often not as severe as it appears. In damage (Figure 5), insects, most cases, the majority of epicormic branches occur disease, mechanical dam­ above the butt­log of a tree (remember the butt­log age and/or others. In contains most of the economic value). Also, epicormic standing trees, it may be branches must be 3/8 inch in diameter or larger to be difficult to determine the considered a defect. On trees with a scaling diameter area of the trunk affected under 14 inches, every epicormic branch (> 3/8 inch) by decay because only the Figure 5. Example of decay. exterior damage can be viewed. When establishing the length of the trunk affected by decay, a measure Clear Cuttings of the length of the exterior decay and an estimate of Clear cuttings are sections of clear wood or the length of interior decay should be determined. defect­free stem in a grading face (Figure 8). These This portion of the log should be excluded, if possible, sections are used to establish the grade of a particu­ and the resulting shorter log is then graded. lar face. Clear cuttings involve two primary charac­ teristics: (1) the number of cuttings present and – Sweep and crook refer to (2) the length of each cutting.
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