Drivers of tree-related microhabitats along a management gradient in mature beech and pine forests

Tree-related microhabitats (TreMs) are valuable indicators and tools for the integrative conservation of forest biodiversity. However, research on their response to gradients of forest management intensityremains limited. This study examined TreM richness on two economically important tree species with contrasting ecological characteristics – Fagus sylvatica (beech) and Pinus sylvestris (pine) – across four forest management intensity categories: regular and naturalness-promoting management, and recently and long-term unmanaged stands in northeastern and eastern Germany. We surveyed 78 plots in mature forests and used generalized linear mixed models and generalized linear models to identify drivers of TreM occurrence. At both tree and plot levels, long-term unmanaged plots supported the highest TreM richness, whereas regularly managed forests showed the lowest values for both beech and pine. Across recently unmanaged and naturalness-promoting managed plots, species-specific differences emerge, with higher TreM plot-level richness in recently unmanaged and regularly managed beech compared to pine plots. Tree-level characteristics, especially diameter at breast height (dbh) and the presence of standing dead trees, were the strongest predictors of TreM occurrence. Interaction effects revealed that the influence of dbh decreased with increasing tree age. Nonetheless, based on models, forest management type had a significant influence independent of tree attributes. Our results emphasize that a combination of tree traits, stand structure, and management history shapes TreM tree- and plot-level richness. Promoting large and dead trees is crucial, but nuanced management strategies are needed to enhance TreM availability. Nevertheless, long-term unmanaged forests remain irreplaceable for supporting forest biodiversity in both beech and pine forests.