Effects of management and plant traits on tree-related microhabitats in second-growth stands of the Białowieża Forest

Abstract: The study presents research conducted on Tree-related Microhabitats (TreM) in managed stands and nearly unmanaged stands in the Białowieża Forest (BF). A TreM is an above-ground structure that creates substrate, site or place for forest-dwelling species, and could be an important tool for assessing the level of biodiversity. The studied area has the same origin and comes after naturally regenerated clearcuts of the British company – “The Century European Timber Corporation” from 1924-1929, however it differs in management approach. Four plots were established in stands unmanaged over the last 30-40 years located in the Władysław Szafer’s Landscape Reserve and another four in commercial forests managed by the Polish State Forests Enterprise. The main hypothesis of the thesis is that trees in unmanaged forest are more abundant and richer in TreMs in comparison to managed stands with the same origin. Another assumption is that abundance and richness of TreMs increase with increasing DBH and depend on plant traits (light requirements and lifespan). Based on these traits the species were grouped into three categories: light-demanding, plastic and shade-tolerant species. Data analysis was carried out in the R Studio Software 1.3.1073 and Microsoft Excel 2016. Main drivers of TreM abundance and richness were identified using generalized linear mixed models (GLMMs) with plot identity regarded as a random factor. Abundance was defined as the total number of TreMs per tree, while richness was calculated as a total number of TreM types per tree. The differences in abundance and richness overall and at TreM group level between managed and unmanaged stands were tested using nonparametric tests. The results showed that TreM abundance and richness differ significantly between managed and unmanaged stands. Higher TreM abundance and richness occurred in unmanaged stands compared to stands of similar origin but with a longer history of silvicultural practices. In addition, the GLMM confirmed that the main drivers of TreM abundance and richness in the studied stands were the tree DBH and its plant trait category. TreM abundance and richness increased with the increase of tree DBH. Moreover, light-demanding trees were associated with the highest predicted values of TreM abundance and richness, regardless of the stand management. The feasibility of the method used suggests that it could be widely used in forest management by locating and preserving trees containing TreMs. Retention of trees with higher TreM abundance and richness, notably large and light-demanding trees such as birch and aspen, is likely to be an important conservation action in managed forests.