Tamm Review: Selection system reduces long-term volume growth in Fennoscandic uneven-aged Norway spruce forests

- Damage to regeneration and small trees increase with increasing harvest strength.
- Annual volume increment is positively correlated with residual standing volume.
- Long-term volume growth is lower than expected in single-storied plantations.
- Moderate harvests appear to result in 10-20% growth loss.
- Heavy harvests may result in 50% growth loss or more.

The review gives a short general history of the selection system, suggests a way to systematize silviculture and summarizes research on the selection system in Fennoscandia. The review is restricted to Norway spruce, Picea abies (L.) Karst, being the only tree species having documented examples of being managed with selection system in boreal Fennoscandia. It covers four main subjects: (i) the dynamics governing the stand structure and the possibilities to maintain a full-storied structure over time; (ii) how ingrowth is affected by stand density and structure, and by internal dynamics within the seedling-sapling layer, and to what extent it is possible to get sufficient ingrowth to perpetuate the selection system; (iii) the key characteristics governing volume growth at stand level; and (iv) the maximum long-term growth compared to the rotation-forestry system.The residual standing volume is crucial to the long-term growth level. A moderate harvest strength at relatively short intervals, focusing on the largest trees and leaving behind a large standing volume, results in a high sustainable volume growth, low requirement of ingrowth, large average stem volumes and low levels of damage to the remaining stand. Long-term volume growth can then be approximately equal to the site productivity expressed by site characteristics, which is lower than the level expected in even-aged plantations managed with rotation-forestry. The review ends with suggestions for future research.