Gap disturbance patterns in an old-growth sessile oak (Quercus petraea L.)–European beech (Fagus sylvatica L.) forest remnant in the Carpathian Mountains, Romania

Abstract In recent decades, natural forest remnants have become increasingly important as reference objects for maintaining or restoring old-growth characteristics in managed forests. Canopy gaps play an important role in forest regeneration, particularly for the establishment and development of tree species with different ecological recruitment patterns. Yet quantitative descriptions of such patterns are still scarce, particularly for oak-dominated forests. The old-growth sessile oak–European beech forest remnant in the Runcu-Grosi Natural Reserve provided a unique opportunity to study natural disturbance regimes with minimal human influence in an ecosystem type rarely investigated. The study site comprised the best preserved part of the Reserve. Its 32.3 ha are dominated by sessile oak. A complete gap survey was carried out. The size, shape, spatial pattern and traits of the gapmakers of all 321 gaps were recorded. Additionally, the gap age as well as the structure and composition of gapfillers were investigated in 70 randomly sampled expanded gaps. The canopy gaps and the expanded gaps covered 12.8% and 28.5% of the study site, respectively. The frequency distribution of the canopy gap sizes corresponded to the negative exponential distribution, with most of the gaps (60%) smaller than 100 m2, 34% between 100 and 300 m2 and only 2% larger than 500 m2. Canopy gaps smaller than 300 m2 were responsible for 71% of the total gap area, suggesting a dominance of small and intermediate gaps in this forest. The pattern of the canopy gaps is characterized by a pronounced soft-core effect, and in one part of the study area a tendency towards regularity. Most of the gaps (84%) were caused by more than one gapmaker and seemed to be created in more than one disturbance event since, in 72% of the gaps, gapmakers of at least two different decay classes were found. The disturbance regime was driven by the mortality of sessile oaks, the main gapmaker species, caused mostly by uprooting. The other main canopy tree species was European beech, which died often by snapping. It was less common as gapmaker (20%), but was the main gapfiller (91%). In contrast sessile oak was almost absent among the gapfillers. These results suggest that the current small-scale disturbance pattern dominating this old-growth forest is more suitable for shade–tolerant species such as European beech, accentuating the already steady decline of oaks in mixed sessile oak–European beech stands.