Lack of large-diameter living trees and low structural diversity characterise managed dry-mesic oak forests in the Hungarian Carpathians

Oaks are the dominant species of a wide range of forest types and are of great ecological, economic and cultural importance. Forest structure and composition have a major influence on forest function and biodiversity. It is a core determinant of naturalness, carbon storage and can be an important goal for forest management. We investigated the composition and structure in five age classes of dry-mesic oak forests in the Hungarian Carpathians. We collected standardised data from different stands and compared the structure (density, basal area, regeneration, dead wood, size distribution) and composition of oak forests among different age classes (40–165 years) and by management type (managed – abandoned). The effect of age class on the derived variables was analysed using general linear mixed models with age class as a fixed effect and region as a random effect. Oaks dominated all age classes in the canopy layer, with the combined relative basal area of oaks ranging from 95 % to 99 %. The composition of the understorey was very different from that of the canopy, with oak not dominating either the sapling or seedling size classes in any of the age classes. The diameter at breast height (DBH) range for age classes younger than 120 years old exceeds 28 cm on average, while in the oldest age class it reaches 45 cm. It is notable that large trees (DBH > 60 cm) are practically absent from the managed dry-mesic oak stands. The mean number of large trees per hectare was found to be 0.2 for all managed stands, while in the abandoned stands (with an age of over 120 years old) it was close to 5 trees per hectare. The diameter distribution of managed age classes exhibits a bell-shaped curve, whereas stands exceeding 120 years of age display a double-peak pattern, with the lower secondary peak occurring within the 35–40 cm diameter range. In order to facilitate the transition of dry-mesic oak forests towards a more natural state, it is crucial to ensure the preservation of intact areas and habitat trees, in addition to implementing appropriate techniques for their regeneration following felling. It is recommended that retention patches cover a percentage of the total area that is between 5 % and 25 %. Without leaving such immature patches, large trees will continue to be absent from dry-mesic oak forests in Hungary.