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 exact habitat requirements often remain elusive due to their rarity
and cryptic nature. Their occurrence is therefore often assessed
indirectly, and we explore whether common indirect measures of forest
biodiversity can also indicate the presence of red-listed species and
allow for detection of environmental threshold values. To this end, 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 investigated whether taxon-specific structural thresholds (relating to
deadwood, broadleaved trees, and large trees) could be ascertained for 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, for 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. With this study, we show that deadwood volume can be used as an
informative indicator for the presence of red-listed species of bryophytes
and fungi but not lichens. Deadwood amount has previously been found to be
a valuable single-variable-indicator, and our results further strengthen
this notion. However, within the context of our study area and design,
threshold values substantially exceeded previous suggestions for critical
volumes of deadwood, emphasizing the context-dependent nature of threshold
dynamics. Our findings nevertheless corroborate previous studies,
highlighting the conservation issues posed by current low levels of
deadwood provision in managed forests and underscoring the importance of
protecting areas with high deadwood quantities