Stocking, also known as stand density, is a measure of tree crowding. A stand may be under-stocked (too few trees), fully stocked (just the right amount of trees), or over-stocked (too many trees) for good tree growth. Stocking is a relative concept?\a stand that is over-stocked for one management objective may be under-stocked for another. Knowing your stand?fs stocking level can help guide its management by helping you determine whether to let trees grow longer or to thin the stand now and provide more growing space around the residual crop trees. Foresters use several measures of stocking, including basal area, stocking charts, and crown cover.
The basal area of a tree is the cross-sectional area of its main stem in square feet measured 4.5 feet above ground. Basal area per acre is the sum of cross-sectional areas (in square feet) of all trees on an acre (Figure 2-7). The optimum basal area for a stand depends on its species composition and tree size or age. Look for basal area recommendations in Chapter 6: Managing Important Forest Types.
The wedge prism and angle gauge described in Stand Measurements (see pg. 15) can be used to measure basal area. When using a 10-factor wedge prism or angle gauge, count all the trees in the variable radius plot and multiply by 10 to determine basal area. For example, if there are 14 trees in a plot, multiply 14 by 10 to determine that the basal area is 140 square feet per acre. Take several basal area measurements and average them to determine the average basal area for a stand.
Figure 2-7. The basal area of a tree is its cross-sectional area in square feet measured 4.5 feet above ground.
A stocking chart uses information about a stand?fs basal area, trees per acre and their average diameter to recommend optimum stocking to sustain rapid tree growth. For example, Figure 2-8 (see pg. 18) is a stocking chart for red pine stands managed for maximum fiber production. Trees grow best when stocking is between the A and B curves. If a red pine stand had a basal area of 190 square feet and 350 trees per acre, those corresponding lines intersect at the A level. The closest average stand diameter is 10 inches. Follow the heavy black line for the 10-inch average stand diameter down to the B level and read the basal area and number of trees per acre at that point. We see that trees of this size would grow faster if the stand were thinned back to approximately 85 square feet of basal area and 160 trees per acre. But a forester would point out that cutting 45 percent of the stand to reach that level is too much to cut at one time. The rule of thumb is to cut no more than 33 percent of the stand at any one time. It will take two thinnings over time to reduce this stand to a stocking level where red pines grow best. Stocking charts for selected tree species and forest types are shown in Appendix C. A forester can help you understand stocking charts and identify a target spacing for your stand.
Crown cover refers to the percentage of sky area blocked by tree crowns when viewed from ground level. Some foresters use a mirror with grid lines on it to view the reflected canopy and measure crown cover, but most simply look upward at the sky from each plot center and estimate crown cover. Crown cover is used as a measure of stand density mainly when setting up a shelterwood harvest. This is a partial harvest aimed at allowing more sunlight to reach the forest floor where it can sustain the growth of seedling trees (see Chapter 4: Regenerating Woodland Stands). For example, a shelterwood harvest may aim for a 75 percent residual crown cover after the harvest. More specific guidelines are in Chapter 6: Managing Important Forest Types for specific forest types that rely on shelterwood harvests for regeneration.
Figure 2-8. Stocking chart for red pine.