Community dynamics of lignicolous lichens on standing deadwood in a 275-year chronosequence

Dead trees provide discrete habitat patches in which patch quality changes
gradually due to wood decomposition. Although in most cases these patches
persist for not more than a few decades, in some ecosystems deadwood
decomposition and the consequent change in habitat patch quality can be a
centuries-long process, potentially leading to dynamics of
deadwood-associated communities taking place over a similar time scale. We
studied the dynamics of lignicolous lichen communities on snags in
near-natural European boreal forests. We surveyed lichens on 331 Pinus
sylvestris snags and obtained their year of death with dendrochronological
methods. The snags comprised a chronosequence of 275 years in time since
tree death. Most lichen species (86 %) displayed an association with
either fresh, intermediate or long-standing snags, with only few species
occurring at a similar rate throughout the chronosequence. Thus, the
habitat patches provided by snags were indicated to be characterized by a
sequence of colonizations by and extinctions of lichen species with
differing niches. Accordingly, community composition was closely connected
to time since tree death. Associations with long-standing snags were most
frequent (46 % of species), and consequently, species richness increased
with time since tree death – approximately a 2.5-fold increase from 20 to
90 years from death – but increased no further beyond 90 years since
death. Several obligate lignicoles occurred only on long-standing snags
indicating their dependence on this specific type of deadwood. Our
findings show that time since tree death is a primary determinant of the
species composition and richness of lignicolous lichens on snags. Thus,
lignicolous lichen communities are intrinsically linked with snag dynamics
that occur over centennial time scales in boreal forests. Lignicolous
lichen diversity is dependent on long-persisting standing deadwood that
typically occurs only in natural forests.