Red-listed species under threat: Unexpectedly high deadwood thresholds for their presence in managed boreal forests

Red-listed species form an important basis for conservation action, yet their specific habitat requirements often remain elusive due to their rarity and cryptic nature. We address this gap by testing whether commonly used indirect measures of forest biodiversity can also indicate the presence of red-listed species and yield informative threshold values. We conducted a fieldwork campaign covering 120 plots along a gradient of structural availability and forest management intensity in Sweden. In these plots, we inventoried epixylic fungi and bryophytes, epixylic and epiphytic lichens, and tree-related attributes. Using logistic regression, we tested whether taxon-specific structural thresholds related to deadwood, broad-leaved trees, and large trees could be identified for the probability of presence of an increasing number of red-listed species. We found that deadwood volume, which strongly correlated with deadwood diversity, indicated a significantly increased probability of presence of red-listed epixylic fungi and bryophytes. The threshold values, for a 50% probability of presence of one red-listed species ranged from 36 to 63 m³/ha of total deadwood for fungi and 31 to 58 m³/ha of lying deadwood for bryophytes. For lichens, we were unable to find significant predictors of their presence. Synthesis and applications. We show that deadwood volume is a reliable, taxon-specific indicator of the presence of red-listed bryophytes and fungi but not lichens. By explicitly focusing on threatened species and deriving statistically defined thresholds from managed boreal forests, our study provides empirical benchmarks that go beyond commonly used biodiversity indicators. The threshold values identified here substantially exceed widely cited recommendations, demonstrating that deadwood requirements for red-listed species in contemporary managed landscapes are higher than previously assumed. This underscores the strong context dependence of structural thresholds and highlights a critical mismatch between current deadwood availability and the levels needed to sustain threatened epixylic species, reinforcing the importance of protecting deadwood-rich forests.