Overstory succession in a mixed Quercus petraea–Fagus sylvatica old growth forest revealed through the spatial pattern of competition and mortality

Old-growth forests are often looked at as a reference for naturalness. Analyzing their successional pathways can provide valuable information on the main drivers of succession in the absence of anthropogenic influence. This knowledge could then prove particularly useful for the maintenance of mixed species stands as a management option to counteract potential changes in forest functions under foreseen climate scenarios. The study was conducted in an old growth Quercus petraea - Fagus sylvatica forest in the Runcu Grosi Natural Reserve, western Romania. Main goals of the research were to assess the structure of the old growth forest and understand how the spatial pattern of the two species is shaped by their competitive relationship. We wanted to figure out if the spatial relationship between Q. petraea and F. sylvatica is different between trees belonging to different layers, and how a possible difference can influence their successional dynamics. To accomplish these tasks, living and dead (both standing and lying) trees were mapped in four 0.56 ha plots. The relationship between trees, considering species, vegetative and social status, was analysed through univariate and bivariate point pattern analyses. Spatial structure based on tree size was investigated with Moran’s correlograms and a local indicator of spatial association. The spatial relationship between understory and canopy trees was also assessed using crown cover data to test for differences among the species. The upper layer is dominated by Q. petraea, whereas F. sylvatica was the only species in the suppressed one. The high amount of deadwood was mostly due to oak contribution. F. sylvatica living trees presented a clumped structure, while Q. petraea exhibited a random pattern. F. sylvatica dead trees showed instead repulsion to living ones. Suppressed trees were clumped at all distances, whereas trees belonging to dominant and intermediate strata were randomly distributed. Repulsion was found between the two species in the dominant layer. Beech recruitment occurred more frequently outside the canopy of dominant trees. Those beeches established under dominant tree showed a preference for oak cover. The succession from oak-dominated to beech-dominated stands has already reached an advanced stage in this old growth forest. The two species competitive interactions are mostly driven by their different shade tolerance and crown plasticity. Quercus-Fagus mixed stands like the Runcu-Grosi one should be preserved to maintain forest diversity and enhance forest resilience. Oak decline could be reduced by the occurrence of natural disturbances, together with active management that should be promoted to reduce beech competitive pressure.