Success factors for high-quality oak forest (Quercus robur, Q. petraea) regeneration

Background: Within the framework of close-to-nature forestry, oak forest (Quercus robur, Q. petraea) regeneration techniques that consider both silvicultural and nature conservation demands have become a very important issue. While there are many experimental and local studies that aim at disentangling the relationships between different environmental and silvicultural factors and the success of oak regeneration, systematic supra-regional studies at the greater landscape level are missing so far. Against this background, the first objective (a) of this study was to present an efficient and sufficiently accurate sampling scheme for supra-regional forest regrowth inventories, which we applied to young oaks stands. The second, and major, objective (b) was to identify the crucial success factors for high-quality oak forest regeneration in northwest Germany. Results: Objective (a): Factors that have been identified as potentially crucial for the success or failure of oak regeneration were either included in a field inventory procedure or extracted from forest inventory databases. We found that the collected data were suitable to be analyzed in a three-step success model, which was aimed at identifying the crucial success factors for high-quality oak forest regeneration. Objective (b): Our modeling procedure, which included a Bayesian estimation approach with spike-and-slab priors, revealed that competitive pressure from the secondary tree species was the most decisive success factor; no competition, or low competition by secondary tree species appeared to be particularly beneficial for the success of high-quality oak regeneration. Also fencing and the absence of competitive vegetation (weeds, grass, bracken) seemed to be beneficial factors for the success of oak regeneration. Conclusions: Trusting in biological automation was found to be mostly useless regarding economically viable oak forest regeneration. To efficiently organize oak regeneration planning and silvicultural decision-making within a forest enterprise, it is strongly recommended to initially evaluate the annual financial and personnel capacities for carrying out young growth tending or pre-commercial thinning and only then to decide on the extent of regenerated oak stands. Careful and adaptive regeneration planning is also indispensable to secure the long-term ecological continuity in oak forests. Oak regeneration should therefore preferably take place within the close vicinity of old oak stands or directly in them. The retention of habitat trees is urgently advised.