Site Quality & Stand Density
Site quality tells us how much timber a forest can potentially produce. Stand density is a measure of how many trees are growing per unit area. Together, site and density tell us how much timber we can produce, as well as what kind of wood quality we can expect at harvest time.
The productivity of your forest land is defined in terms of the maximum amount of volume that the land can produce over a given amount of time. Site quality is measured as an index related to this timber productivity.
Of all the indirect measures investigated, the rate of height growth has been the most practical, consistent, and useful indicator of site quality with respect to timber production. The standard practice has been to define site index in terms of the total height of the dominants - the largest, full-crowned trees in a stand. These trees capture the the most light, moisture, and nutrients of all trees in a stand.
Numerically, site index is the total height of the dominant trees in a stand at specified ages, usually 25, 50, or 100 years. In the South, pines are commercially mature between 25 and 40 years, depending on the product output, and a 25-year base age is often used.
Expressed as a number, a site index of 40 on a 25-year base means that the dominant trees will average 40 feet in total height at 25 years of age.
It is very important to understand that the productivity of timberlands varies greatly by site. On one site you may observe very good growth, while on another site, the same species at the same age may grow very poorly. Site quality can be changed by fertilization, vegetation control, irrigation, or drainage. Only highly intensive treatment can make a productive site out of a poor one.
Density is a measurable attribute of a stand. Stand density can describe how much a site is being used and the intensity of competition between trees for the site's resources (i.e., water, light, nutrients, space). At higher densities, the growth rates of individual trees slow down because there are more trees competing for the site's limited resources.
Stocking refers to the adequacy of a given stand density to meet some specified management objective. Hence, stands are often referred to as understocked, fully stocked, or overstocked. Stocking is a relative concept - a stand that is overstocked for one management objective may be understocked for another
Measures of Density
In homogeneous, even-aged stands of known age, site quality, and history, the number of trees per acre is a useful stand density measure.
Another commonly used measure of density is the cord or cords per acre. A cord is 128 cubic feet of stacked roundwood (whole or split, with or without bark) containing wood and airspace, with all the pieces of similar length and lined up on approximately the same direction. Example: a pile of firewood 4' x 4' x 8'.
Since many objectives relate to volume, it is often used as a measure of density. Volume is interpreted in relation to some standard, such as the volume represented in a yield table, and is given as a percentage of stocking for a specified objective.
Basal area is a measure of stand density developed by foresters. It is the total cross-sectional area of the trees in a stand, at breast height (4.5 feet above the ground), measured in square feet per acre. Basal area (BA) of a given tree is calculated using the formula below:
- BA = .005454 * d2
- Where: d = diameter of tree at breast height (dbh)
Spacing rules are often used to guide thinning in order to leave a desired density in developing stands. While the space occupied by the roots and crowns of individual trees is difficult to determine, average space-size relationships can be estimated.
In general, trees occupy and need space in proportion to their size. Density, as measured in basal area can be expressed quite readily in terms of tree diameter and number of trees per acre.
For example, let's say that you want to leave 90 square feet of basal area per acre in your pine stand. What number of trees and average between-tree spacing is necessary to get a density of 90 square feet of basal area per acre?
- If the average basal area of the trees is 0.196 square feet, then 90/0.196, or 459 average-sized trees should be left per acre.
- The average land area per tree is the number of square feet per acre, 43,560, divided by 459, or 95 square feet. Assuming the trees occupy the corners of equal-sized squares, as in a pine plantation, the average between-tree spacing is the square root of 95 or 9.7 feet.
Since all trees in your plantation are likely not the same size, it would make life a lot easier to express spacing as a ratio of diameter. For any specific basal area per acre, the ratio between diameter in inches and spacing in feet is a constant. The formula used in computing the "diameter times" spacing figure for any given basal area or BA is:
- C = 15.4 / square root of desired BA
C = spacing figure or constant
BA = basal area
In our example, we want to leave 90 square feet of basal area per acre, so the spacing figure would be calculated:
- C = 15.4 / square root of 90 = 1.62
Therefore, the spacing between trees is:
- Spacing (feet) = dbh * C
Using this relationship, the spacing required for a tree 10 inches dbh, at 90 square feet basal area per acre, would be 10 * 1.62, or 16.2 feet.
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