Regeneration of oaks (Quercus robur/Q. petraea) and three other tree species during long-term succession after catastrophic disturbance (windthrow)

In broadleaved temperate forests in Europe, oak (Quercus robur/Q. petraea) regeneration is reported to be weak or absent. However, most work concern seedlings or saplings, studied relatively few years. We studied a Picea abies stand in Sweden, windthrown and logged (all stems harvested) in 1969, testing the hypothesis that oaks regenerate in the long term among competing tree species after catastrophic disturbance. In 2011, after 40 years/growth seasons, we recorded live and dead trees in the new stand and investigated the surroundings, competition, and succession. The following trees, up to 26 m tall, colonized: Sorbus aucuparia, Betula pendula/B. pubescens, Fagus sylvatica, Q. robur/Q. petraea, and Corylus avellana (a shrub). Betula dominated, and only Fagus was regenerating in 2011. Sorbus had produced most of the dead trees, mainly or partly through intraspecific competition. In the stand, compared to the surroundings, Quercus, Picea, and Alnus glutinosa were under-represented, and Sorbus, Betula, and Fagus were over-represented. Yet, the density of Quercus was far from negligible; 48 large trees/ha. Most of the oaks (74 %) were co-dominant trees and many grew near Sorbus. Thus, oaks can survive and grow fast among pioneer trees and browsing animals, a conclusion which is supported by the literature (nine studies identified). High mortality of Sorbus is part of a long-term succession, where Fagus might come to dominate. However, Quercus likely will persist, in low density. We propose three key traits contributing to long-term persistence of Q. robur/Q. petraea in European temperate forests: long life span, ecological plasticity, and resistance to disturbances.