This system involves making two or three cuts in a stand to stimulate advance regeneration before a final clearcut. In a three-cut system, the first is a heavy thinning that removes undesirable species and poorly formed or diseased trees while leaving the best trees with plenty of growing space to expand their crowns, grow vigorously, and produce seed. This cut can be eliminated if intermediate thinnings have achieved the same results. The second cut is a thinning made when there is a good seed crop. It usually leaves 50 to 70 percent crown cover, but allows enough sunlight to reach the forest floor that seeds from shade-tolerant species can germinate and survive.
Site preparation (prescribed fire, mechanical, or herbicide) usually occurs just before or after this harvest to expose mineral soil for the seedbed and set back undesirable shrubs and trees. The final cut is made three to ten years after the previous cut, when advance regeneration is well established. It clears remaining mature trees, releasing the young stand to grow in full sunlight. It also may result in stump sprouting or root suckering to supplement the established seedlings. There are several variations in the pattern of trees cut (Figure 4-9).
The shelterwood system is used to develop advance regeneration before a final harvest. It is most appropriate for species that are intermediate to tolerant of shade, where residual trees are not subject to wind damage or epicormic branching, where logging damage to residual trees can be minimized, and where the increased cost of several partial cuts is acceptable. If naturally developing advance regeneration is not adequate, supplemental planting can be used.
Figure 4-9. Shelterwood system.