Links between species richness of bryophytes and lichens and tree-related microhabitats on retention trees

Retention forestry is a commonly used approach to include biodiversity conservation in forest management of production forests. Tree-related microhabitat (TreM) assessment is suggested as a good practice for the selection of habitat trees to retain. TreMs are small structures on living or dead trees that provide living substrates, shelter, breeding places, or food sources for various groups of organisms, therefore, they can be a good indicator of the presence of forest-dwelling species. However, the knowledge linking bryophyte and lichen species diversity with TreMs is still scarce. In this study, TreMs and bryophyte and lichen species composition were assessed on living retention trees in 39 young forest stands (10–20 years post-cut) in Latvia to test whether the richness of epiphytes on retention trees up to 2 m height is related to the number of TreM types. There was a strong positive correlation between the total number of TreMs and lichen richness on Betula pendula (Rs = 0.66; p < 0.001), a moderate positive correlation on Alnus glutinosa (Rs = 0.56; p = 0.040), but no such link was found for other evaluated species. Trees with TreMs—foliose and fruticose lichens and dead branches–hosted more lichen species than trees without these TreMs. The number of bryophyte species on a tree was explained mainly by tree species, and bryophyte richness did not significantly correlate with the total number of TreMs. These findings suggest that retention trees of diverse species and with a high number of TreMs can contribute to maintaining greater species richness of epiphytic bryophytes and lichens.