Ash dieback can rapidly eradicate isolated epiphyte populations in production forests: A case study

The ongoing European ash (Fraxinus excelsior) dieback can pose a threat to the biodiversity dependent on ash trees, but the field evidence is scarce. We monitored epiphytic lichens (notably Lobaria pulmonaria) confined to ash trees in an Estonian old-forest remnant affected by the dieback. About half of ash trees were killed in five years, during which even the abundant L. pulmonaria (ca. 730 thalli on 74 trees) only produced <20 new thalli on alternative hosts. One epiphyte species disappeared and three more will probably go locally extinct in the near future. These results highlight that biodiversity risks from ash dieback can be substantial under certain conditions – when the distribution of large ash trees and their related biota have already been seriously reduced by intensive forest management. As a short-term response, we call for distinguishing and monitoring local ash-dependent epiphyte populations of broader significance and improving retention forestry practices. In the long term, silvicultural diversification should be a major tool for mitigating biodiversity risks of forest tree dieback.