- (Figure 1) Map of Silvicultural Systems
Employed in the SFMA
"Silviculture" is a basic element of forest management and is often described as the art and science of managing the forest to produce products and services for societal benefit. Forest managers use harvesting, just one of many silvicultural tools, to influence the development of individual parts of a forest in order to derive benefit from the forest. Foresters practice silviculture at the "stand" level. A "stand" is defined as, "An area of the forest that is relatively uniform in species composition and/or age and can be managed as a single unit." A variety of silvicultural systems have been employed in the SFMA as depicted in figure 1. The SFMA uses both multi-age and even-age approaches to accomplish a broad set of management goals. More information about these systems can be found below.
A multi-age silvicultural system is designed to foster multiple age classes of trees in the same general area or stand. Generally there must be more than 3 age classes, about 20 years apart in age, for a stand to be considered "multi-aged." The different aged trees can be arranged in a vertical structure with younger trees growing beneath older trees in a layered fashion. This type of stand structure is generally achieved through a specific silvicultural system called the "selection system." Another approach to achieve multiple ages classes is to arrange the different age groups of trees in small groups across the whole stand. A harvest operation creates openings in the forest canopy, commonly called "gaps", where new groups of trees will establish and grow to maturity. The size of these groups can vary depending on the stand conditions and operational requirements. The map in figure 2 shows the arrangement of a set of groups like this. Figure 3 shows the same stand, 20-30 years in the future, when a second set of openings (labeled as #2’s) have been created to establish yet another age class or cohort. Eventual after 100 years or more the entire stand will have been covered with these openings and the gaps created in the first harvest will be ready to be cut again to start the cycle over again by establishing a new age group of trees (figure 4). The intention of this system is to enable a perpetual and regular harvest of trees from the same stand. There are many names for this type of silvicultural system one of which is an "irregular group shelterwood." A PDF with additional photos and information about this system can be downloaded here.
- (Fig 2)
- (Fig 3)
- (Fig 4)
Another silvicultural approach is termed "even-age". Even-age management seeks to develop a single age group of trees across an entire stand. This type of approach enables management of a stand of trees of similar age and size, across a large area, from the time those trees are first established as seedlings until they grow to maturity and are ready to be harvested to start a new stand of young trees again. The most common example of an even age silvicultural system used in the SFMA is called the shelterwood system. In keeping with the name the system starts with a lite harvest to create openings in the canopy that allows sunlight to reach the forest floor (figure 6). This light combined with seed dispersed from the overstory trees, results in the establishment of new tree seedlings. These new trees will grow in height and at some period after this "establishment harvest", 5-25 years, the overstory trees will be removed, termed an "overstory removal", to allow the young trees to have full sunlight conditions so they will grow rapidly to larger sizes (figure 7).
- (Figure 6) Shelterwood Establishment
harvest in spruce fir stand
- (Figure 7) Stand after an overstory removal harvest
- (Figure 8) Types of Retention Trees
Trees come in many shapes, sizes, species, and quality and each have different values from economic and ecological stand points. Straight trees without defects are desired as high value sawlogs and as seed sources for the future forest. Dead trees, called "snags" when standing, and "down/dead wood" when on the ground, serve as important habitat for birds, mammals, insects and amphibians. Depending on the type of silvicultural system being used there may be more or less of one type of tree than another. When conducting harvest operations in the SFMA managers strive to retain some of each category of tree when the harvest is completed. Standing dead trees can pose a serious safety hazard to harvest crews and good judgment must be used when harvesting around such trees. Figure 8 shows examples of different types of trees they may be considered for retention during a silvicultural operation.