Broad Variation in Tree Mortality Processes in Primary European Mountain Forests Dominated by Fagus Sylvatica

Abstract

Tree mortality, driven by natural disturbance agents and climate, represents a crucial process shaping forest dynamics also influencing forest structure. The mortality rates and composition of mortality agents in primary mixed-species forests dominated by Fagus sylvatica L. were examined on different spatial scales using an extensive set of sample plots located in the Carpathian Mountains (i.e., 13,915 living trees on 227 plots within 16 stands). The determined overall annual mortality rate (i.e., 0.89%) was well within the previously published range of background mortality. Species-specific annual mortality rate was 0.76% for Abies alba, 0.88% for Fagus sylvatica, and 1.1% for Picea abies. Although the annual mortality rates below 2% were detected on 90% of plots, the mortality rates of 5% per year were exceeded only for conifers on 5% of plots. Plot-level mortality varied; lower rates were observed in the western Carpathians, compared to other regions, and significant differences were found in species-specific mortality between coniferous species and F. sylvatica. Most trees that had recently died were found in the smallest diameter classes with their counts continuously decreasing towards the bigger size classes. Individual mortality agents had different distributions across the size classes, highlighting the complexity of the mortality process in the primary forests. Most study stands experienced drought stress; however, the climatic extremes were recorded as the major mortality agent only in a limited number of cases. While competition and various forms of abiotically caused physical damage were the major mortality agents for F. sylvatica and A. alba, insect infestation was the most frequent mortality agent for P. abies suggesting insects to have a crucial role in tree species dynamics emphasising the importance of structural and compositional heterogeneity even in the stands with low overall mortality and relatively low abundance of P. abies. Conservation of these remaining European beech-dominated primary forests is critical due to their significance in preserving local biodiversity and avoiding irreversible losses.

Keywords: Mixed-severity natural disturbances, annual mortality rate, mortality agents, primary forests.