Long-term Effects of Retention Trees on Epiphytic Lichens

Approximately 8% of the world’s forests are planted and managed for wood production, and these often lack the structures necessary to support a diverse range of forest species. Retention forestry is a common practice where living and dead trees are left during the final harvest to maintain biodiversity. Many short-term studies have examined the extent to which retention trees are habitats for epiphytic lichens, but long-term studies (>10 years) are scarce. Therefore, the aim of this essay is to summarize existing evidence on the long-term (>10 years) persistence of epiphytic lichens on retained living trees and to identify areas for further research. Additionally, to better understand the longterm habitat availability for epiphytic lichens, the aim is to review studies on tree mortality, epiphytic lichens in naturally regenerated forests, and regeneration after clearcuts. Existing long-term studies on retention trees suggest that the richness of epiphytic lichens is maintained on retention trees for longer timescales (>10 years). However, an important aspect is that lichen species can respond differently to retention forestry, and some do not survive on retention trees. There were few long-term studies, and only two were performed on epiphytic lichens in the same forest stand at different times. There was also a study bias toward boreal forests. The importance of tree characteristics for lichen richness were contradictory, with some long-term studies indicating that tree species are more important for lichen richness, while others suggest that tree diameter has the strongest effect. Research on regeneration after clearcuts and natural disturbance events has shown that old trees are important, as they can maintain and disperse epiphytic lichens to younger trees. Young trees can also host lichens, particularly when older forests are nearby. There is a need for more long-term studies to gain a better understanding of how retention trees influence the survival of epiphytic lichens over time, which may not be apparent on shorter timescales, and also how retention trees can maintain landscape connectivity.