Crown size-growth relationships of European beech (Fagus sylvatica L.) are driven by the interplay of disturbance intensity and inter-specific competition

• We examine changes in stem and crown characteristics along an anthropogenic disturbance gradient.
• Crown size becomes less important for radial growth of Fagus sylvatica in undisturbed tree communities.
• Inter-specific competition enhances crown efficiency of beech in unmanaged stands.
• Considering the continuity of ecological processes might refine our understanding of tree-tree interactions.
• Minimizing anthropogenic disturbances is recommended to favour higher structural complexity and stand productivity.

Crown size is considered one of the most important traits that affect radial tree growth, but it remains unclear how (anthropogenic) disturbance intensity affects crown size-radial growth relationships. This knowledge, however, is crucial for a better comprehension and prediction of community dynamics, and thus to support management decisions. We analyzed changes in stem and crown characteristics of dominant canopy European beech (Fagus sylvatica) trees along an anthropogenic disturbance gradient based on the duration of non-forestry use. We further investigated the dependency of basal area increment on crown surface area and linked this relationship to growth efficiency. Crown efficiency (basal area growth per unit crown surface area) was used as an indicator for the effectiveness of tree growth. Further stand attributes included stand density and tree species composition. Changes in crown efficiency with tree and stand attributes were assessed using generalized additive models (GAMs). Tree morphology sensitively responded to disturbance intensity. However, the indicative value of crown surface area for basal area increment decreased with increasing duration of non-forestry use and stand density. We found that the interplay between disturbance intensity and species composition modulates crown efficiency of dominant beech trees. Inter-specific competition enhanced crown efficiency in unmanaged stands, whereas managed stands showed an opposite trend. Consequently, crown efficiency significantly increased with decreasing disturbance intensity and intra-specific competition. Thus the widely accepted close correlation between crown size and radial increment needs reconsideration for trees growing under (near-) natural conditions. We hypothesize that carbon allocation in densely stocked stands can be adapted to an efficient trunk-crown relation, which in turn weakens crown size-radial growth relationships as known from managed stands. The importance of continuity in tree-tree interactions therefore imposes significant constraints on the generality of crown traits as radial growth determinants in beech forests. Our findings indicate that a higher structural complexity and stand productivity might be achieved in managed stands by a wider variety of crown size classes and tree species assemblages. Hence, stand dynamics can benefit from lowering anthropogenic disturbances and favouring self-regulation, which would be a further step towards near-natural forest management.