Lichen communities on treefall mounds depend more on root-plate than stand characteristics

Large uprooted trees are typical old-forest structures that provide distinct microsites for sessile organisms. While the habitat value of treefall pits for plants is relatively well known, the characteristic exposed root-plates of treefall mounds, which may require different management, have received less attention. The aim of the current study was to explore key factors of lichen incidence and their community characteristics on root-plates. We mapped and described treefall mounds in 24 stands (2 ha each) in a balanced design representing mixed stands of two site types (mesotrophic; drained swamp) and two management types (old growth, mature commercial forest) in hemiboreal Estonia. First, the occurrence of any lichens and of a specialized pin lichen, Chaenotheca furfuracea, was recorded on all 1207 mounds found. According to multi-level logistic regression models, their incidence depended on every mound-scale variable tested: there were independent positive effects of root-plate area and treefall age, spruce treefall, and the absence of exposed pit. In contrast, weak stand-scale effects only appeared when their interactions with mound-scale variables were included or some main effects were eliminated. C. furfuracea also occurred more frequently where other inhabited root-plates were nearby, and such dispersal limitation interacted with habitat quality. Secondly, full lichen communities were described in detail on a subsample of 84 root-plates of Norway spruce (Picea abies). Altogether 48 lichen species were recorded, including eight red-listed or nationally rare species. The communities on large fresh root-plates and in mature drained stands had distinct species composition, while communities of older mounds and in old-growth stands converged in species composition. We conclude that lichen incidence on root-plates is mostly determined by colonization probability, and commercial forests can support a diverse lichen biota when large treefall mounds, particularly of P. abies and of >10 years age are provided. However, given some differences of lichen communities in old growth, and because treefall issues conflict with other management objectives in commercial forests, optimised management of treefall areas there should be complemented by their strict protection in reserves.