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Suggested Citation:"UNITED STATES TIMBER INVENTORY." National Research Council. 1976. Biological Productivity of Renewable Resources Used as Industrial Materials. Washington, DC: The National Academies Press. doi: 10.17226/18425.
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Suggested Citation:"UNITED STATES TIMBER INVENTORY." National Research Council. 1976. Biological Productivity of Renewable Resources Used as Industrial Materials. Washington, DC: The National Academies Press. doi: 10.17226/18425.
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Suggested Citation:"UNITED STATES TIMBER INVENTORY." National Research Council. 1976. Biological Productivity of Renewable Resources Used as Industrial Materials. Washington, DC: The National Academies Press. doi: 10.17226/18425.
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Suggested Citation:"UNITED STATES TIMBER INVENTORY." National Research Council. 1976. Biological Productivity of Renewable Resources Used as Industrial Materials. Washington, DC: The National Academies Press. doi: 10.17226/18425.
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Suggested Citation:"UNITED STATES TIMBER INVENTORY." National Research Council. 1976. Biological Productivity of Renewable Resources Used as Industrial Materials. Washington, DC: The National Academies Press. doi: 10.17226/18425.
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Suggested Citation:"UNITED STATES TIMBER INVENTORY." National Research Council. 1976. Biological Productivity of Renewable Resources Used as Industrial Materials. Washington, DC: The National Academies Press. doi: 10.17226/18425.
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Suggested Citation:"UNITED STATES TIMBER INVENTORY." National Research Council. 1976. Biological Productivity of Renewable Resources Used as Industrial Materials. Washington, DC: The National Academies Press. doi: 10.17226/18425.
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Suggested Citation:"UNITED STATES TIMBER INVENTORY." National Research Council. 1976. Biological Productivity of Renewable Resources Used as Industrial Materials. Washington, DC: The National Academies Press. doi: 10.17226/18425.
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CHAPTER 3 UNITED STATES TIMBER INVENTORY INVENTORY MEASURES Units of forest measurement were developed at a time when there was only one product: lumber. The board-foot is still used today. It is as though we were to measure what is in a granary in terms of loaves of bread. The farmer would then have to think of his crop as consisting of so many loaves of bread rather than bushels of grain. So would the buyer, regardless of how he intended to use the produce. A feedlot owner buying wheat for stock food would have to use one conversion, and a baker another. The baker would be no better off, because he might be making 14-ounce loaves of whole wheat bread when he had bought wheat in terms of one- pound loaves of white bread. Board Foot Measure One board-foot is theoretically a piece of lumber one- foot square and one-inch thick. In practice, however, this is never the case. The variations arise from many causes, but the result is that the board-foot is a highly inaccurate measure even of sawn lumber. Lumber is often cut from a green log. It shrinks as its moisture content is reduced below the fiber saturation point. Some lumber is used rough from the saw, and some is surfaced. Lumber widths and thicknesses are specified in terms of their nominal dimensions though their actual dimensions may be quite different. These varying dimensions are catalogued in the "American Lumber Standards," an industrially accepted specification originally promulgated some 50 years ago and changed only moderately since. Under these lumber standards a nominal 2" by 8" dry, surfaced board must have a minimum dimension of 1-1/2" by 7-1/2". Thus a board of minimum dimensions in this nominal size would have a cross section slightly more than 82 percent of the nominal cross section area. The average dimension in thickness and width of lumber from any particular mill is likely to be a function of the manufacturing precision of that mill. Those that are precise are set nearer the minimum than those that are less precise. The American Lumber Standards for variation in sawing (miscutting) of softwood lumber are also archaic. Variation in sawing is defined as deviation from the line of cut. The - 19 -

standards prescribe that slight variation is not over 1/16 inch scant in 1 inch lumber, 1/8 inch in 2 inch, 3/16 inch in 3 inch to 7 inch, and 1/4 inch in 8 inch or more. These are precisely the same standards for miscutting that were contained in the original American Lumber Standards promulgated in 1925. The situation has now remained static for 50 years. There have been improvements in equipment and manufacturing procedures, but they have not been reflected in the American Lumber Standards. These standards protect the worst practices in the industry and fail to reflect the best available technology. The situation with respect to precision in the manufacture of hardwood lumber is even worse. Current standards for miscutting of hardwood lumber are more relaxed in 1975 than they were in 1945. These lax standards might be tolerated in a situation where the basic resource is available in excess of need, but they are not acceptable when applied to a commodity whose raw material base is in short supply. Crude specifications and standards for measurement foster large-scale material waste and make it difficult to estimate available supplies with any precision. Log Rules Even greater inaccuracies in estimating the volume of wood result from the crude but persistent methods by which the number of board feet in logs is estimated. Logs are bought and sold for the most part on the basis of log rules, devices that evaluate the useful content of a log in terms of an estimated recovery of mi11-run ungraded lumber. A number of log rules of this type are used by the American lumber industry. Given the lack of precision in measuring lumber and timbers and the lax manufacturing procedures reflected in lumber standards, the task of attempting to estimate recovery of these materials from round logs of various sizes is a difficult one. Two general types of log rules have been widely adopted by industry and government, and several of them are in current use. One type of log rule is that developed by graphically diagramming circular cross sections of various diameters to project the reduction of a log to boards and timbers. The Scribner log rule is such a diagram rule. It was devised over 100 years ago and is still widely used in the United States. It is the only diagram rule still in extensive use. A second type of log rule is the formula log rule, which attempts to express lumber recovery from a log through the use of an algebraic formula. Two such log rules are still - 20 -

commonly used in the United States. The Doyle rule, the oldest of the formula rules still commonly used, is crude and hiqhly inaccurate. The International rule, which uses an algebraic formula that is more complex and sophisticated, is the most widely used of the formula loq rules. While it is of more recent vintaqe than the Scribner and Doyle rules, it is still a very old one. Unlike the Scribner and Doyle rules, the International rule recognizes taper in a log. The nature of all log rules reguires that they be based upon at least implicit assumptions regarding the lumber manufacturing process. These assumptions include the mix of board sizes to be produced, the width of the saw kerf and the slabbing and edging practices to be employed. Regardless of the general validity of the geometrical and algebraic procedures used in devising a log rule, it is obvious that assumptions based upon manufacturing practices of a century ago bear little relationship to modern tech- nology. Manufacturers have, of course, accommodated to the inaccuracies and imprecision of log rules, and such criteria as "overrun" and "board foot-cubic foot ratio" represent efforts to estimate true volume from the inaccurate estimates in board-foot volume provided by the log rule. Log rules are commonly used in the timber trade as a basis for log and tree marketing even in situations where the use is for veneer rather than lumber. Here the log rules, based as they are upon estimated lumber recovery, make no technological sense at all. With the advent of large-scale electronic computers, the possibility has been explored of using simulation models to predict product recovery under the manufacturing conditions actually projected for use. A large number of such computerized models have been constructed and are in use. Evaluation of logs in terms of actual solid volume or weight can be accomplished with much greater precision than can be achieved in attempting to predict actual product recovery through the use of log rules. Evaluation of log content in terms of cubic volume or weight has the advantage of permitting more meaningful comparisons among alternative uses for the same log and perhaps more importantly for evaluating multiple uses of the same log. There has been some progress in substituting the cubic-foot measure or weight for the board-foot measure, although the use of old log rules is still a common practice fostered by tradition, archaic laws, and government practices at national and state levels. - 21 -

Volume Tables The measurement of tree volumes presents many of the same problems that are associated with the assessment of log volumes. Tree volumes are customarily estimated through the use of volume tables which estimate the merchantable volume of a tree in terms of its diameter 4.5 feet above the ground and its height. Most volume tables are developed by making assumptions concerning the choice of a geometric solid configuration that approximates the shape of the tree bole, the probable stump height and minimum usable top diameter, and, sometimes, the mixture of logs that are potentially recoverable. In some cases, the volume estimate by volume tables is the cubic volume of the whole stem, but most volume tables have concentrated upon estimation of the volume of the so-called merchantable bole. This is defined as either the number of 16-foot logs or the portion of the stem between the top of the stump and the point of minimum merchantable diameter. When volume tables are based upon estimates of product recovery, they must include assumptions concerning the nature of the product conversion process. Commonly, the log rules previously discussed are used as the basis for the conversion assumptions. When this is done, the merchantable quantities estimated are obviously no better than the conversion assumptions built into the log rules. The geometric solid configurations most commonly used are designed to give good estimates of total cubic volume in the middle portion of the bole—the so-called merchantable portion of the stem. They are less precise when used to estimate those portions of the bole that are traditionally non-merchantable; i.e., the stump, top and large branches. As use of the full tree becomes more widely practiced, failure to estimate accurately the volumes in the "non- merchantable" fraction of the tree is an important menaurational deficiency. Similarly, volume tables based upon single product use will be of limited value in estimating product recovery potential when the trees are used for products other than those anticipated, or when they are used for multiple product recovery. As larger components of the tree are used for fiber products and chemical feed stocks, it becomes increasingly important to be able to assess raw material in trees in terms of weight rather than volume, the conventional basis for forest resource inventory. - 22 -

INVENTORY OF PRESENT STANDS An inventory is essential for adequate appraisal of the current resource: the standing volume; current annual increment; and, for stands still in the developmental stages, mean annual increment, or the average growth per year from the year of establishment to the present time. Standing volume is measured in a number of ways. The basic unit of measure may be board feet, cubic volume, or weight. The volume of the total tree bole or only a part of the tree bole may be estimated. Only trees above some arbitrary size limit may be counted, and even then, large trees with varying amounts of defect or deformity may be omitted. The user who makes chips from any size or species of tree, the sawmiller who wants a specific size or species, and the user with fully integrated manufacturing facilities must appraise the value of the resource from the same inventory. Adeguate sampling of extensive forest areas through the management of sample plots in the field is time consuming and expensive. Even the best designed forest surveys are of questionable accuracy because of the problems already described with the basic units of measuring, the difficulty of making accurate measurements of basic parameters in the field, and the high cost of adequately sampling highly variable forests. The result is that the forest manager is typically confronted with an absence of needed inventory data. Even when forest inventories provide good estimates of the cubic-foot volume of tree boles, such data do not include the stump, top of the bole, root system, large branches, and foliage. Botanists studying total forest biomass have been restricted by cost and basic-science* directed personal decisions to small and arbitrarily chosen samples. The nature of these samples does not permit generalization of the results to large forest areas. Botanists and foresters are both concerned with forest biomass, but the two groups work independently by and large with little or no joint efforts to use and interprete data. ESTIMATING GROWTH The measurement of growth is subject to the same problems inherent in the measurement of current forest volume or weight. The larger amount of effort required to estimate growth, however, renders adequate sampling of the forest more expensive and therefore more unlikely. Furthermore, growth, involving as it does a projection over - 23 -

time, is inherently less easy to assess than current volume or mass. Current growth or annual increment is measured either by successive measurements of sample plots or by reconstruction of the stand at an earlier time through the extraction of increment cores from the tree to count and measure growth rings. The first technique is better, but the number of plots remeasured in practice is seldom enough to permit adequate sampling, and the precision of the repeated measurements is seldom good enough to provide accurate measurements of the differences that constitute growth. The use of increment cores in the stand-table projection method is adeguate to provide data on the gross growth of individual forest stands, but may not provide accurate estimates of net growth because of the difficulty of measuring tree mortality over a specific period of time. In addition, tree mortality tends to be episodic rather than continual, thus leading to even greater errors in prediction. Stand table projection is much less satisfactory when applied to an entire forest or region than when used on a stand-by-stand basis. In the absence of actual growth studies, increment must be measured by growth or yield tables that generalize expected growth in such terms as site, age, and degree of stocking. Conventional yield tables based on age are applicable to even-aged stands only. In irregular, mixed, and uneven-aged stands, adequate growth tables based on variables other than age are generally not available. A considerable expertise exists about how to measure current volume or mass and its increment over time. Inadequate funds have been allotted, however, to applying this knowledge to our present forest resources. In view of the critical nature of growth data in the forecasting of future supply and the time periods necessary to obtain such data, we cannot overemphasize the importance of an immediate remedy to this situation. POTENTIAL PRODUCTIVITY In taking a long look into the future, we need to know what a new forest under man's management will produce using current and projected technology. Forest development in volume follows the familiar S- shaped growth curve, with the X-axis in years or decades. The grand period of growth is followed by a leveling off. If the forest type is one of the early successional stages, the volume will decline substantially as it progresses to the climax forest and will eventually level off at a lower - 24 -

level of volume with no growth. Even if current annual growth could be accurately determined, its significance can only be judged if the observer knows where the forest is on the growth curve. If the development curve is described in terms of biomass, it is quite different from one described in board foot volume of merchantable trees only. Furthermore we must still predict the growth of managed forests from observations of natural untended stands. Consequently, we are underestimating biological potential because managed stands can produce more than unmanaged. Data on the response of forests to intensive management practices are generally restricted to periods of only a few decades. Few studies cover longer segments of the life of the forest. We need to develop data collection techniques and analytical procedures to predict long-term responses of forests to silvicultural treatment based upon short-term monitoring of the forest. We need yield tables for managed stands showing the production expected using various silviculture and management options. The development of definitive predictions requires watching and measuring a managed stand throughout an entire rotation. The obvious necessity, as pointed out in this report, for exploiting today the potential productivity of the forest cannot wait for the development of such yield tables. Interim prediction techniques are needed to guide investments in improved technology that must be made today to insure healthy, productive forests for the future. A CONTINUOUS FOREST INVENTORY The United States forest resource is on the one hand extensive and valuable, and on the other remarkably poorly defined as to extent, content, and current and potential growth. This is not to say that large efforts by competent professional foresters have not been productive. The primary limitations are three: (1) most inventories contain a mixture of biological and physical data and local management data, the latter often playing a dominant role in setting procedure and pace; (2) inventories have most often been designed to produce data for the moment, with no deliberate intention of keeping current; and (3) the onset of the quickening pace of operations due both to demands of intensive management and the pressure of conflicting interest groups is rendering much of forest inventory practice obsolete. - 25 -

Procedures for forest inventory, even of total occupied land, are so tedious and time consuming that it can take 10 years to complete the cycle of a national inventory. Rapid methods such as use of satellite imagery must be developed and adopted so that planning can proceed based upon data that are current. (At present many industrial and national forests have no inventories more recent than 1955.) Even where more appropriate inventory techniques are available, forest surveys have tended to become limited or delayed because of lack of funds, disagreement between interested parties as to objectives, and restraints imposed by management. The Forest Survey, an on-going project of the United States Forest Service, has provided invaluable data, but it is so underfinanced and understaffed that the statistical base of the 1974 Outlook Study is often necessarily scanty and out of date. The principles and programs it stands for, however, are important and need immediate further development, expansion, and application. If forest policy planning is to be based on adeguate data, a continuous nationwide inventory of forests should be reinstituted and maintained. Instead of using the board- foot or the merchantable cubic-foot as the unit of measure, we should express forest inventories in the dry-weights and volumes of their component parts. The objective should be to provide biological and physical data that can be interpreted over a range of terms from total biomass at one extreme to current merchantable harvest at the other. The reinstituted continuous forest inventory should make full use of remote sensing and modern sampling techniques. It should be based upon sufficient actual measurements in the field that current inventory and growth data can be provided on a unit-area basis for the important (both biologically and economically) forest types, forest site classes, forest age and size classes, major ownership classes, geographical regions, and other stratifications needed to form the basis of public management and policy decisions. The time is past when we can afford to plan without facts. The potential of forests as a renewable resource is more than great enough to justify the effort and cost of creating and maintaining a nationwide continuous forest inventory. - 26 -

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