Fifty years of natural succession in Swiss forest reserves: changes in stand structure and mortality rates of oak and beech

Question What are the drivers of structural changes and mortality in oakbeech forests over 50 yrs of natural succession? Location Twelve unmanaged forest sites, comprising a large environmental gradient in the Swiss lowlands. Method By using repeated inventory data from more than 17 600 individually tagged trees, the dynamics of oakbeech stands over the past 50 yrs were analysed. Generalized linear mixed-effects models were fitted to quantify annual mortality rates of oak and beech based on DBH, stand basal area, precipitation and slope. Results Stand basal area increased, whereas tree density decreased over time. At most sites, the relative importance of oak decreased gradually compared to beech. Mortality increased over time for both oak and beech, but the increase was stronger for oak. Oak and beech mortality decreased with increasing DBH and tended to increase with precipitation. Additionally, oak mortality increased with stand basal area, whereas no such trend was found for beech. Conclusion Our study indicates that mortality in Central European oakbeech forests is driven by a combination of stand structures (i.e. tree size and stand basal area) and climate. However, the influence of climate on oak mortality is comparably low. Increasing oak mortality with stand basal area is a plausible consequence of its lower relative competitiveness and higher demand for light. Thus, in forests developing towards higher stand basal area, the ecologically important oak is increasingly outcompeted by beech, unless competition is reduced through management or disturbances.