Epiphytic lichen diversity in pine-dominated boreal forests: community dynamics in relation to snag dynamics and disturbances

In my thesis I studied the diversity patterns of epiphytic lichens on dead and living Scots pines (Pinus sylvestris) in boreal Europe. My main intent was to address how epiphyte diversity is affected by forest dynamics, namely snag dynamics and disturbances. I surveyed lichens in near-natural forests and in experimental sites where forest dynamics had been emulated in forest management. A 275-year chronosequence showed that time since tree death is a primary determinant of alpha and beta diversity patterns of lignicolous lichens on standing deadwood. Snags that have been dead for at least 90 years are crucial for lignicolous lichen diversity. However, modeling snag fall rates in managed forests, specifically snags originating from trees retained during final harvests, revealed that maximal snag longevity in managed forests is 50 years. Thus, deadwood restoration faces two challenges. First, it takes several decades or even a century until restored (standing) deadwood hosts high lichen diversity. Accordingly, I found relatively low lichen diversity on restored deadwood when 20 years had passed since restoration. Second, restoration may fail to facilitate the formation of high-longevity snags. Inventories of lichens on living retained trees showed that residual structures may uphold lichen diversity in post-disturbance sites. High-severity fires changed habitat structure significantly by removing previously available substrates through tree mortality. However, they also produced new substrates in the form of deadwood, and thus the species richness of epiphytic lichens was only slightly reduced. Still, the freshly formed and homogeneous deadwood hosted relatively low lichen diversity. Low-severity fires had similar effects on community composition as high-severity fires, but smaller in magnitude. The emulation of natural dynamics should be effective in improving the epiphyte diversity of managed forests. The availability of heterogeneous substrates, but particularly old trees (or “old deadwood, i.e., high-longevity snags), should be a focus.