Accumulation of dead wood in abandoned beech (Fagus sylvatica L.) forests in northwestern Germany

Abstract

Currently, there is much debate about what strategy is most suitable for increasing old-growth attributes in forests that have been managed intensively for wood production in the past. Passive restoration, i.e. cessation of forestry interventions, should be considered when the old-growth attributes desired can be restored within a feasible period of time.Our study focuses on standing and lying coarse dead wood (≥20cm diameter) in beech-dominated forests in northwestern Germany. We analyzed monitoring data of 545 sample plots (sized 500–1000m2) from 12 strict forest reserves (SFRs). The SFRs had been without forestry intervention for up to 28 years.Both, number of dead objects and volume of dead wood (m3ha−1) increased significantly with ongoing time since abandonment from forestry interventions. The mean amount doubled from 9 to 18m3ha−1 within 10 years. The proportion of standing dead wood was about 40% of the total dead wood pool ≥20cm diameter.With mixed linear modeling we showed that dead wood increased by a mean net rate of about 1m3ha−1a−1. Therefore, after three decades critical values for restoring the dead wood pool could be reached. We hypothesized that the rate of dead wood input is mainly determined by disturbance driven tree mortality such as oak decline, bark beetle infestations and storms.A comparison with primeval forests or reserves abandoned more than 100 years ago showed that the SFRs studied are at the beginning of a long process of dead wood accumulation.Based on our results, the abandonment of forest activities in harvestable pure and mixed beech stands is an effective strategy for restoring the dead wood pool.