Two-Aged Methods

Two-aged silvicultural systems involve a planned sequence of treatments designed to maintain and regenerate a stand with two age classes. Two-age methods generally involve removal of all but 10 to 40 ft²/acre of basal area. The residual overstory trees are termed reserves or standards. The limited number of reserve trees allows abundant light to reach the forest floor and provides for the rapid growth of the understory. In contrast to the shelterwood method, the reserve trees will be left standing for a second rotation, thus maintaining two predominant age classes. Generally, the treated area is subjected to site preparation treatments similar to those used after clearcutting (Stringer 2000).

In two-aged systems, stands continue to develop through all successional stages with residual trees in place. In later successional stages, the younger age class will begin to merge vertically with the older age class. Depending on the kinds of trees initially retained, later successional stages may contain trees much larger than would normally be found in mid- or late-successional stands. Therefore, at least some of the attributes of much older stands can be provided in stands managed with this system (SAMAB 1996).

After 40 to 60 years, several options exist for future treatment of residual trees in two-aged systems, depending on management objectives: (1) the older trees can be retained into the future along with the younger age class, (2) the older age class can be removed, leaving the younger age class as an even-aged stand, or (3) the regeneration process can be initiated again by removing the older age class and some of the younger age class, perpetuating the two-aged stand condition (SAMAB 1996).

• Clearcutting with Reserves: A clearcutting method in which varying numbers of reserve trees are left standing to attain goals other than regeneration. The overstory trees retained, called reserve trees, may be small or large trees, or combinations of small and large trees. They may be retained for future growth, certain species components, current or future den trees, future sources of snags or coarse woody debris, or some level of visual quality.
• Coppice with Reserves: A method of regenerating a stand in which the majority of regeneration is from stump sprouts or root suckers, and in which reserve trees are retained to attain goals other than regeneration. The conditions created with coppice with reserves are the same as with clearcutting with reserves or shelterwood with reserves. Conditions depend on the number of reserve trees retained.
• Seed-Tree with Reserves: A seed-tree method in which some or all of the seed trees are retained after regeneration is established to attain goals other than regeneration. The conditions created in a seed tree with reserves are identical to those created by clearcutting with reserves. The only difference between the two systems is that in the regeneration period, the trees retained have the specific function of producing seed to regenerate the stand.
• Shelterwood with Reserves: A variant of the shelterwood method in which some or all of the shelter trees are retained well beyond the period of normal retention to attain goals other than regeneration. Initial conditions created are identical to those for the even-aged variant of this method-- a micro-environment moderated by retention of residual trees. However, retaining overstory trees beyond 20 percent of the rotation creates a distinct two-storied condition that persists for 20 to 40 years (SAMAB 1996).

As in other systems, the choice of residual trees in the two-aged system is dictated by management objectives. Choosing residual trees for cavity trees, mast producers, growth, future snags or coarse woody debris provides the values associated with those trees (Stringer 2000).