Regularly Planted Rather Than Natural Understory of Norway Spruce (Picea abies H. Karst.) Contributes to the Individual Stability of Canopy Silver Birch (Betula pendula Roth.)

Abstract

Forest plantations, particularly high-density planted stands, are considered to be more prone to wind damage compared to naturally regenerated stands. The wind resistance (mechanical stability) of plantations can, however, be improved by close-to-natural management, for example, combining pioneer and shade-tolerant species. Presumably, the stability of such stands would be enhanced by the reduced competition of canopy trees and stronger root contacts provided by understory trees, which depend on spatial distribution. In the hemiboreal forest zone, silver birch (Betula pendula Roth.) and Norway spruce (Picea abies (L.) H.Karst.) form such a combination naturally. In this study, the static tree-pulling tests were performed to estimate the mechanical stability of canopy silver birch growing with random Norway spruce understory in naturally regenerated (post-clear-cut) and regularly planted bi-species mixed stands. The regular mixing of the high-density bi-species stand significantly improved the loading resistance of canopy silver birch compared to the naturally regenerated stands of similar composition and age. Such an effect might be related to the stratification of the canopy space between pioneer birch and shade-tolerant spruce, which improved the individual stability of the canopy trees. Further, a regular rooting network of the planted stands likely contributed to the stability by reducing weak spots. Accordingly, the wind resistance of trees in regularly planted bi-species stands might be improved, avoiding additional management.