| Quantitative sciences such as mathematics and statistics are used in the measurement of trees, forests, and the products derived from them. Regardless of the land management objectives—timber, wildlife, recreation, watershed, or a combination of these resources—the tree overstory must be quantified for informed decision making. Timber management requires knowledge of the location, species, ages, and sizes of trees. Forest cover is an important component of wildlife habitat. The recreation potential of wildland is a function of many variables, including the size and number of trees present. Water yields are related to the composition and density of the tree canopy.
Measurement of Trees
Standing trees are commonly measured for diameter, height, and age. Diameter and height measurements are used to estimate the volume (or weight) and value of individual trees; ages are used in assessing site quality and predicting growth.
Diameters. The tree measurement made most frequently by foresters is diameter at breast height (dbh), defined in the United States as the stem diameter outside the bark at a point 4.5 ft (1.37 m) above the average ground line on the uphill side of the tree. In countries that employ the metric system, dbh is measured at 1.3 m or 1.4 m above ground. The instruments (e.g., diameter tapes, calipers) used to measure tree diameters are called dendrometers.
Heights. Height-measuring devices, called hypsometers, usually embody basic trigonometry principles. The observer stands at a fixed horizontal distance from the tree. Tangents of angles to the top and base of the tree are multiplied by the predetermined horizontal distance to give the height of each measured section of the tree. For smaller trees, direct linear measurement of tree height may be made using height poles, which consist of sections of lightweight material that can be extended to form a measuring stick of length equal to the height of the tree being measured.
Foresters generally measure either total tree height or merchantable height, although in some instances both are measured. Total tree height is the linear distance from ground level to the upper tip of the tree crown. Merchantable tree height refers to the usable portion of the tree stem and is defined for single-stemmed trees as the length from an assumed stump height to an arbitrary upper-stem diameter. If the main trunk does not extend as a single stem to the tree tip, the upper limit of merchantability is generally dictated by the branching characteristics.
Age. Many tree species found in the northern temperate zone grow in diameter by adding each year a single, distinctive layer of wood called an annual ring. Each ring consists of a band of early or spring wood and a band of denser, darker late or summer wood. These layers appear in a cross section of the stem as a series of concentric rings that can be counted to give the age of the tree. Ages of standing trees are often determined by extracting a radial core of wood with an instrument called an increment borer. The reliability of annual ring counts depends on the species and the growth conditions.
Measurement of Forests
Timber inventory. The usual purpose of a timber inventory is to determine, as precisely as available time and money will permit, the volume (or value) of standing trees in a given area. To attain this objective requires a reliable determination of the forest area and measurement of all or an unbiased sample of trees within this area. Under limited circumstances—such as when scattered, high-value trees occur on small areas—a complete or 100 percent tree enumeration may be feasible. However, except for those circumstances in which a complete tree tally is justified, the conduct of a timber inventory is a sampling process. The choice of a particular inventory system is governed by relative costs, size and density of timber, area to be covered, precision desired, number of people available for fieldwork, and length of time allowed for the estimate. Among the considerations involved in developing an efficient sampling scheme are sample size, plot size and shape, and the sampling design. Other things being equal, the intensity of sampling tends to increase as the size of the tract decreases and as the value of the timber increases.
Many forest inventories are carried out using fixed-area sample plots located throughout the area of interest. These plots may be any shape—square, circular, triangular—but circular plots are most often used. The sample plots can be located by any random or systematic sampling design, but the most common procedure is to space the plots according to a predetermined grid pattern. As an alternative to fixed-area plots, sample points—which are somewhat analogous to plot centers—can be located within a forested tract, and an instrument can be used to establish a fixed angle of view to sight trees at dbh. Tree boles close enough to the observation point to completely fill the fixed sighting angle are tallied; stems too small or too far away are ignored. This method of selecting sample trees with probability proportional to size, commonly referred to as “point sampling,” is an efficient field method for determining which trees to include and measure.
Forest monitoring. Land managers need trend data in addition to inventories aimed at determining current conditions. Monitoring consists of collecting information over time, generally on a sample basis by measuring change in key indicator variables, in order to determine long-term trends. The sampling design for monitoring generally involves repeated measurements on the same sample plots or individuals.
Geographic information systems. Forest inventory information is commonly stored, updated, and retrieved through geographic information systems (GISs). A GIS is a computerized database for storing, manipulating, and displaying map (spatial) data and tabular (attribute) information. With a GIS, forest inventory information can be stored in a computer and directly linked to associated forest maps, making it easier and faster to analyze and graphically display the results of forest inventories.
GISs can make forest inventory information more powerful by integrating it with other data commonly needed to make management decisions. Managers may need to know the location of roads, streams, or soil types when developing management plans. Combining forest inventory data with other land-resource information allows managers to make more‑informed decisions.
Measurement of Forest Products
Products cut from tree boles are measured in different units, depending on local customs and end use. The most common units are cubic volume, stacked wood measure, board-feet, and weight. Tree sections 8 ft (2.4 m) or more in length are called logs; shorter pieces are referred to as sticks or bolts.
Cubic volume. Cubic volumes of logs or bolts are computed from the length and the cross-sectional area, which may be taken at the midpoint of the log or averaged from the areas of the ends or from the weighted average of the ends and the midsection. Tree cross sections are assumed to be circular for purposes of computing cross-sectional areas. Areas and volumes may be computed in U.S. Customary units (square feet or cubic feet) or in metric units (square meters or cubic meters).
Stacked wood. Pulpwood and firewood are commonly sold by stacked wood measure. Stacked wood is generally measured in terms of cords. A standard cord of wood measures 4 ft x 4 ft x 8 ft (1.2 m x 1.2 m x 2.4 m); its volume is 128 ft3 (3.6 m3) of wood, bark, and air space. Cordwood customers are primarily interested in the solid-wood volume rather than the total space occupied. The solid content of a stack of wood varies with species, method of piling, diameter, length and straightness of sticks, and length of branch stubs.
Board-foot log rules. Logs that will be converted into lumber are generally scaled for their estimated board-foot content [a board-foot is equivalent to a plank 1 in. (2.5 cm) thick and 1 ft (30 cm) square]. Scaling involves determining the log diameter inside the bark at the small end and the log scaling length (in multiples of 1 or 2 ft; or 0.3 or 0.6 m). The log scaling dimensions are then used to enter a log rule, which is a table or formula showing the estimated volume, usually in board-feet, for logs of various diameters and lengths.
Weight scaling. Much of the forest products industry now buys wood by weight scaling rather than volume measure. Price/weight equivalents are usually established by making thousands of paired weighings and scalings of loads of freshly cut wood. A weight-per-volume conversion is derived, and prices are quoted on a per-volume (e.g., cord or thousand board feet) basis. When sawlogs are bought by weight, adjustments are made to account for the fact that lumber yields per unit weight are less for smaller or defective logs.
It is now common practice at large conversion facilities to scan logs prior to processing. Laser scanning devices are used to generate detailed profiles of logs. By combining multiple sensors, a full three-dimensional image can be produced. The detailed profiles developed are fed into computer programs that supply information on optimal utilization of the logs.
References and Further Reading
Avery, Thomas Eugene; Burkhart, Harold E. Forest Measurements. New York: McGraw-Hill, 2002.
Burkhart, Harold E. “Forest Measurement.” In McGraw-Hill Encyclopedia of Science and Technology. New York: McGraw-Hill, 2002. pp. 430-435.
Fonseca, Matthew A. The Measurement of Roundwood: Methodologies and Conversion Ratios. Wallingford: CABI, 2005.
Husch, Bertram; Beers, Thomas W.; Kershaw, John A., Jr. Forest Mensuration. Hoboken: Wiley, 2003.
West, P. W. Tree and Forest Measurement. Berlin: Springer, 2004.
Posted: August 2006
Updated: 23 August 2007
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