When to thin

While there is no “magic” number of years to wait until applying a thinning, several indicators suggest when thinning should occur. As trees grow and mature, their crowns will begin to compete for available sunlight. Eventually this crown competition will result in a forest with a closed canopy, where sunlight does not reach the forest floor. This indicates that the tree crowns no longer have space to grow and expand. A thinning should be applied. A more quantifiable estimate of when to thin based on crown structure is when live crown ratios are less than 33 percent. The live crown ratio is the height of the crown divided by the height of the tree. Live crown ratios decrease because sunlight is not getting to the lower branches, decreasing the photosynthetic area of the tree, and
resulting in mortality of lower branches.

Keep in mind

Basal area per acre is another good estimate of when to thin. Basal area is a measure of stand density. Basal area is the sum of the surface area in square feet taken by an individual tree trunk at 4.5 feet for all the individuals growing on an acre. As basal area exceeds 120 square feet per acre, individual tree growth declines and the stand becomes unhealthy. For most pine stands, a thinning that results in 70 to 90 square feet of residual basal area per acre represents an ideal stocking rate. A 14-inch tree is approximately 1 square foot of basal area.