Winter - the optimal logging season to sustain growth and performance of retained epiphytic lichens in boreal forests

Epiphytic lichens, comprising an important biodiversity component with significant functions in old boreal forests, are susceptible to logging. Leaving retention trees may partly compensate for the adverse effects of logging, but the impact of logging season on retained lichens is unknown. To identify the least harmful logging season seen from an epiphytic lichen perspective, we simulated logging events by transplanting two old forest model species - Lobaria pulmonaria (cephalolichen) and Lobariascrobiculata (cyanolichen) - in summer, autumn, winter and spring, respectively. Lichens were collected in intact old forests and transplanted in clear-cuts. They were placed on five aspects (N, E, S, W, and top) on artificial stands covered with bark. They all gained biomass and area in the following year. However, logging in January resulted in subsequent higher relative growth rates than in lichens being exposed to clear cuts in other seasons, consistent with clear seasonal logging effects. Aspect impacted lichen growth less than logging season, despite significant bleaching and chlorophyll contents in the most exposed aspects. The high annual growth rate and the low level of damage after transplantation to clear cuts during winter suggest that green tree retention (GTR) logging practices in winter can support life-boating of susceptible old forest lichen epiphytes. Logging simulation during other seasons than winter resulted in lower growth, implying higher risk of permanent damage. Successful use of GTR cannot compensate for the loss of epiphytes at timber harvest, but supports local retained lichen populations that subsequently may boost re-colonization of the young stand.