Determining an Appropriate Level of Residual Stocking

Residual stocking is the stand density resulting from cutting to select reserve trees in the shelterwood method or seed trees in the seed-tree method. The residual stocking for a cutting varies by method (shelterwood vs. seed-tree method) and can vary appreciably from stand to stand, even for a single method. One must take into account the minimum number of trees needed for a reliable seed supply and the minimum needed for sufficient canopy cover. At the same time, shade from the overstory canopy must not preclude germination or inhibit seedling development. Additional factors to be considered include (Nyland 1996):

• The minimum volume to eventually sustain a commercial removal cutting
• The volume of wood that will accumulate on the seed trees between the seed and removal cuttings
• How rapidly crowns of the residual trees close and light near the ground drops below acceptable levels
• The ease and cost of logging during the seed and removal cuttings
• The extent that logging will damage the regeneration during removal cutting

Advance Regenerating Systems

In southern Appalachian oak forests, some of the larger trees are left primarily to inhibit competition from shade-intolerant species. In these systems, the success of shelterwoods depends in a large part on the presence of abundant advance oak regeneration. In stands having these advance seedlings, foresters can reduce the overstory to about 60 to 70 percent of the stocking found in unmanaged stands, removing trees in lower crown positions and keeping the overstory largely intact. On site indexes of 70, 80, and 90 feet, recommended residual basal areas are 60, 65, and 70 percent of initial basal area, respectively. Once the oak seedlings reach about 3 feet (0.9 m) tall, foresters should reduce the stocking to about 50 percent of that in unmanaged stands (Loftis 1990, Loftis 1990).

Non-advance Regenerating Systems

In community types where seed trees are left to serve as a seed source for a new cohort, as in true seed-tree methods, foresters must consider all the factors that affect natural seeding from these residual trees when determining residual stocking. These factors include (Nyland 1996):

• Durability of seed trees to exposure
• Quantities of seed produced per tree based upon its health, vigor, age, and species
• Frequency of anticipated good seed crops, and amounts produced during subsequent years
• Direction and velocity of prevailing winds
• Heights of seed trees
• Duration of seed viability after dispersal
• Seedbed characteristics
• Expected germination rates, and subsequent survival of new germinants
• Density of seedlings for effective site utilization

The following formula can be used to determine the average numbers of seed trees to leave in the shelterwood or seed-tree methods (Nyland 1996):

seed trees/unit area = (c / F) * D / (G * N * Y)

where:

c = land unit conversion (= 43,560 ft²/acre, = 10,000 for m²/hectare)
F = area of ground space (ft² or m²) covered by seeds falling from a single tree
D = the density of seedlings needed per unit of area (ft² or m²) to occupy the site
G = percent seed viability
N = number of seeds cast per tree
Y = seedling survival factor

See: Stocking