Site quality assessment

Regenerating red pine

Managing established red pine stands

• Thinning and stand density
• Prescribed under burning
• Pruning
• Growth and yield
• Rotation age
• Harvesting

Managing for ecological complexity

Thinning and stand density

	Mechanical thinning treatment. (E. Sagor)

Foresters manage stands to meet a variety of landowner objectives. In cases where timber production is the primary objective, management often focuses on maximizing financial return. This usually involves encouraging growth on some collection of desirable trees, while capturing mortality from the less vigorous ones. Density control is the primary means by which this is accomplished.

There are a number of thinning methods which can be used over the life of a stand. A few of the methods often used in even-aged red pine management include mechanical thinning and thinning from below. Mechanical thinning is the removal of trees in rows or strips at fixed spacing intervals throughout the stand. This method is commonly applied as the first thinning in red pine stands that are densely crowded and have a relatively uniform crown class. Typically every second or third row is removed until the stand matures and most of the remaining trees are large, high quality growing stock. At this point, row thinning becomes less suitable and thinning from below becomes the more appropriate thinning method. Thinning from below is the removal of intermediate and codominant trees to favor the large, high quality trees in the upper crown classes. Depending on the users objectives the trees removed during low thinnings are typically high risk trees, trees that are crowding potential crop trees, undesirable species and trees of poor quality and low vigor. As a general rule, high quality leave trees which will be used as a seed source should be disease free, with straight stems, medium to thin branches that meet the trunk at a 90 degree angle, are good self-pruners, and possess a crown balanced in height and width. For timber purposes, avoid leaving trees with large diameter branches that may not be able to support snow loads and those with forked stems and irregular or misshapen crowns.

Two important aspects of stand density in even-aged stands are stocking level and uniformity. As the stocking level decreases towards a minimum, uniformity of distribution of trees increases in importance. The minimum stocking level for uniform stands of different (average) stand diameters was calculated using the maximum amount of growing space required by trees of different diameters (stocking chart). Minimum stocking for stands averaging 5 inches in diameter is about 400 trees and 60 squre feet per acre. In stands averaging 15 inches in diameter, minimum stocking is about 80 trees and 100 square feet per acre.

The stocking chart suggests the recommended upper limit of stocking for managed stands averaging 5 inches in diameter is about 1100 trees and 150 square feet per acre of basal area. For stands averaging 15 inches in diameter, the recommended upper limit of stocking is 175 trees and 215 square feet per acre of basal area. Contemporary management entails lower planting densities, plant equipment considerations, and access for pre-commercial thinnings.

Seedling and sapling stands (less than 5 inches average diameter) should have between 400 and 900 trees per acre. Fewer than 400 trees will not provide minimum recommended stocking by the time the stand reaches pole-timber size and more than 900 trees will exceed the upper limit of recommended stocking before the trees reach pole-timber size and can be thinned commercially.

Stand density guides for uneven-aged red pine stands have not been formalized but in general, seedling density needs to be higher than sapling density, which in turn needs to be higher than pole density, which should outnumber sawtimber density. Losses in the smaller size classes are expected and considerable effort will be needed to assure survival and growth of enough trees in each class to replace those harvested, lost, or which have recruited into the next size class. The regeneration in uneven-aged red pine stands must be monitored for Sphaeropisis and Sirococcus shoot blights.

Precommercial Thinning: Precommercial thinning may be needed in young stands to meet management objectives. For example, in a naturally-regenerated stand of seedlings (less than 2 inches average diameter) having more than 2000 trees per acre, a minimum of 100 potential crop trees per acre should be given a minimum growing space of 25 square feet per tree. Densely stocked saplings stands (2 to 5 inches average diameter) with a basal area of 160 square feet per acre or more should be precommercially thinned. Crop trees in sapling stands should be given about 50 square feet of growing space per tree to maintain good diameter growth.

Commercial Thinning: One of the most important ways stand composition and development can be controlled is by periodic commercial thinnings. Stands should be thinned before they exceed the recommended upper limit of stocking for managed stands (see below). For production management, a uniform distribution of high quality trees with at least the minimum recommended stocking for the average stand diameter should be left, but not over half – and preferably less – of the basal area should be removed in any one thinning. Stands managed near the minimum recommended stocking will have the most rapid diameter growth. As a more general guide, pole stands (5 to 9 inches average diameter) should be considered for thinning when the basal area is 140 square feet or more per acre and they should be thinned to leave about 90-110 square feet per acre.

Stocking charts and Density Management Diagrams (DMDs) are popular tools for developing thinning prescriptions for even-aged stands. Their popularity stems from their ability to easily incorporate the ecological principle of self-thinning, or competition induced mortality, which provides the stimulus for thinning. The focus of management in the context of density control is to prevent stands from reaching the zone where self-thinning can occur. On the stocking chart self-thinning would occur when stands reach the A-line. Basal area is represented by the “Y” axis and trees per acre are represented by the “X” axis. Average stand diameter at breast height is represented by the lines radiating out from the origin of the two axes. Red pine stands are over-stocked above the A line – too high of a basal area, too many trees per acre, and too big. The stand is under-stocked below the B line – too few trees of any size.

Stocking charts and DMDs are similar management tools; their primary difference being the scale of measurement. DMDs use a logarithmic scale. A density management diagram for red pine and instruction for its usage.

Thinning will not usually result in an increase in stand volume at the end of the rotation. Rather, thinning will allow individual trees to grow larger and increase the relative rate of stand growth. Thinned volume plus volume at the end of the rotation (total yield) may or may not be higher than the total volume of an unthinned stand. Thinning, however, serves to capture volume otherwise lost to mortality. Thus, total volume removed over the life of a stand may be greater if a thinning program is implemented. Site quality, thinning intensity, and stand age at thinning are factors that affect volume removed during thinning and stand volume at the end of a rotation. Residual trees should have a live crown ratio of 30% or greater in order to receive the maximum potential growth response from thinning. Many organizations are emphasizing a 40% live crown ratio. Depending on stand age, trees with shorter live crown ratios may have a minimal growth response from thinning. Red pine crowns develop upward and outward. Thus, if a stand is near its maximum potential height and has a 10% live crown ratio, very little growth response to thinning would be observed. For red pine, post-thinning stand density has a greater impact on post-thinning stand growth (Gilmore et al. 2005) than the thinning method used (Smith 2003).

For income focused management, sawtimber trees should be thinned periodically to maintain uniform growth rates on the crop trees. For habitat focused management in mixed-species stands, red pine might be favored as crop trees at each thinning. However, other species should be left to achieve biodiversity goals, provide habitat requirements, and expand the species and type of timber products.

Insect and disease Warning

From a pest management standpoint, thinning is viewed as very beneficial. It generally maintains or increases individual tree health and vigor. Vigorous pine trees can better defend themselves from most insect and disease problems. However, the presence of many freshly cut stumps and logs can, on occasion, attract or provide conditions suitable for some significant pests.

• Annosum root disease can be introduced into stands through fresh stumps. This disease causes expanding pockets of dead and dying pine. Annosum is present in lower Michigan and it has been expanding in Wisconsin. It is likely to be found relatively soon in Minnesota red pine. The link above provides detailed management recommendations that can reduce the likelihood of introducing or spreading Annosum.

• Freshly cut logs left in a stand in the spring and early summer can provide breeding material that produces large Ips bark beetle populations. Population increases of 10-fold can occur in one growing season, with three generations of Ips pini possible. Thus, a local population of 1,000 can increase to 1,000,000 individuals in one season. Management suggestions revolve around prompt removal of freshly cut logs in spring and summer. This includes removal in the early spring of logs cut during the winter months.