In continuous cover forestry, the forest is not regenerated and grown as a single, even generation of trees, but there are trees of many sizes and ages. When felling, part of the forest is removed by selection cuttings and small-scale clearcuttings. The felling cycle is repeated in the forest approximately every 15–20 years. Small trees account for the largest share of the trunk. A prerequisite for continuous cover forestry is that new seedlings are born naturally and new trees develop from the undergrowth reserve. The trees must be healthy.