Woodland Inventory

In developing management plans, planning harvests, and making other management decisions, an inventory of the timber resources is essential. Timber volumes can be expressed in cords, cubic feet, or other units of measure. However, volume measurements are usually expressed in board feet. Such volume measurements attempt to estimate the number of square feet of 1″ thick boards (board feet) which can be sawed out of a given log or tree. Three different rules or methods of doing this have been developed. These are the Doyle, the International, and the Scribner rules. The Scribner rule is seldom used, the Doyle rule tends to under scale the smaller trees and logs, and because the International ¼” rule is more accurate in predicting lumber output it is in more widespread use in Pennsylvania.

A common measurement for determining tree size is diameter at breast height, (dbh). This is defined as the average diameter of the tree in inches measured at a point 4-1/2 feet above the ground on the uphill side of the tree. Dbh can be determined by dividing the circumference (or distance around the tree) by 3.14. However, dbh is usually measured by use of specially designed measure sticks, tapes, or calipers. Landowners can choose a few trees in their own woodlot and periodically measure their diameter to help gauge the growth of their timber stand.

Height is usually expressed and measured in terms of 16-foot logs. A trees “merchantable height” is its height measured to a 10″ top or the first major defect (usually a fork). Using these two measurements and a volume table, it is possible to calculate the volume of standing timber. Although usually done by a professional forester, active landowners can calculate the volume of their own trees in a woodlot.

It is most essential for the forest landowner to be able to determine species of the individual tree. There are many excellent tree identification books available. Time spent learning to identify trees is certainly not wasted because the growth characteristics, site requirements, and log or lumber values vary widely among different tree species.

Another important characteristic of the timber stand is the degree of stocking, or how fully trees utilize the existing growing space. This usually is expressed as number of trees per acre or basal area. Basal area is the number of square feet per acre occupied by tree stems at breast height. A stand should be managed to achieve a fully stocked stand. This is a stand in which trees are neither too crowded nor too widely spaced for maximum growth per acre. Over the life of the stand, the number of trees per acre will decrease drastically as the slower growing trees are crowded out of the stand by more dominant trees. A stand which has just been reproduced may have several thousand stems per acre while the stand when ready for harvest, will probably contain less than 100 stems per acre.

Quality of the individual tree may range from a highly valuable prime veneer type tree to a hollow tree with no timber value whatsoever. Tree quality is primarily determined by the amount of crook in the stem, number and location of knots, presence of decay, and marketable length of the stem.

Benefits obtained from timber in the forest are determined to a large extent by the rate of growth which is in turn determined by the stocking of the stand and the quality of the site. Although diameter increases fastest in young trees, there is no sawtimber volume until trees reach 10 to 12 inches dbh and very little sawtimber value until well past this point.

Studies have shown that the most profitable size at which to harvest timber is neither at the minimum marketable size nor at biological maturity, but at a point in between. After the annual volume or value increase reaches a maximum. This point is referred to as economic maturity. Accurate determination of this point and preparation of a cutting budget requires growth measurements. These are usually done either by measurement of annual growth rings or by comparisons of diameters and volumes over a period of time.

The primary objective of timber management is to produce the most volume of high quality timber of desirable species possible in the shortest amount of time. This is usually accomplished by efforts toward regulating the stocking and composition of the stand. Volume and stocking tables such as those on the Timber Inventory Worksheet are helpful in planning these efforts.

Besides inventory of his land and timber resources, the wise woodland owner will want to consider and inventory any minerals such as coal, oil, gas, etc., present on his land. Improvements to the land such as buildings, established roads, developed recreation areas, lakes, ponds, etc., should also be listed and valued in the inventory. Although difficult to evaluate in monetary terms, intangible values may, in some cases, be of over-riding importance. These include aesthetic, recreational, and historical values.

Woodland Inventory – Land

The Woodland Inventory is the first step in developing a management plan, what is there to work with. Land is evaluated for its woodland productive potential based on a number of site factors and with the economics of woodland management as they are today, it is good management to identify the better sites and begin management efforts on them. There are several factors which influence the productivity of a site. They are:

  • Soil

    This is probably the single most important factor. Different soils have different levels of natural fertility, depth, moisture holding capacity, slopes, etc. Moisture holding capacity and fertility have obvious influences on tree growth. Soil depth is a limiting factor only when it’s less than 4′ deep. Tree roots rarely go deeper than 4′ and most feeder roots are in the top 6″. Slopes are limiting factors of equipment usage and water infiltration rates. As part of a Conservation Plan, these factors are taken into account and soils are evaluated as to the productivity potential for woodland.

  • Aspect

    This is probably the second most important factor. Aspect is the direction a hillside faces. Generally speaking, north and east facing slopes are more productive than south and west facing slopes. The reason for this is that south and west facing slopes get direct sunlight and are hotter and dryer than north and east facing slopes. This causes trees on south and west facing slopes to experience stress which open them to insects and diseases because of reduced vigor. Northeast facing coves are the best sites in this area for growing trees.

These two variables can go a long way in making a determination about site productivity. Other site factors to consider are:

  • Slope

    This can limit equipment usage, water infiltration, and road construction.

  • Slope Position

    Usually toe slopes are more productive than ridge tops mainly due to availability of water and soil depth.

  • Drainage

    In forest management, it is best to suit a tree species to a site rather than fit a site to a tree species. Different types have different site requirements. Some need wet soils, some need well drained soils.

    In inventorying his land, the landowner will want to take into account the site classification and determine species best suited to those sites present.