Tree and stand characteristics jointly predict tree-related microhabitats on retention trees in production forests

Retention forestry emerged as a means of ameliorating the biodiversity impacts of clearcutting 30 years ago and has become an integrated part of forest management in many countries. Nowadays, there is still an increasing need to assess to what extent retention trees contribute to biodiversity in production forests. We used tree-related microhabitats (TreMs), as an indicator of the potential of the forest to host taxonomic biodiversity, to better understand the effect of broadleaf retention trees, and surrounding conditions derived from stand management, on biodiversity. We inventoried TreM types on 114 retention trees of four broadleaf tree taxa (Betula spp., Fagus sylvatica, Quercus spp., and Populus tremula) located within 20 even-aged conifer-dominated production forests (Picea abies) in southern Sweden. We evaluated the effect of retention tree attributes (species and age) and of the surrounding environment (production tree density and distance to the retained trees) on the diversity of TreM types and groups. We found that retention tree species with different characteristics and physiological niche (light-demanding vs. shade-tolerant and pioneer vs. late-successional) developed distinct TreM assemblages. TreM diversity increased significantly with increasing retention tree age and surrounding tree density. Higher surrounding tree density is particularly related to some TreMs either positively (crown deadwood, bryophytes) or negatively (buttress-root concavities, lichens). Overall, the extent that retention forestry potentially contributes to forest biodiversity will depend on promoting different broadleaved retention tree species and managing surrounding trees accordingly to allow retention trees to become older and maintain TreMs in the long term.