Question: Populus tremula (Aspen) is a post-fire successional tree with a large number of host-specific lichens. One, Ramalina sinensis, will colonize very young aspen stands. The apothecia of this species are attacked by a parasitic fungus, Abrothallus suecicus. The incidence and severity of disease caused by this parasite was studied in nine stands between 22 and 180 years old.
Location: Central Sweden.
Method: Thalli of R. sinensis were randomly sampled from nine stands. Each thallus was weighed and the number of diseased and non-diseased apothecia were scored for each thallus.
Results: The data show (1) an increase in both disease incidence and disease severity with increasing stand age; (2) that smaller thalli show an increasing probability of being diseased in older stands; (3) that high disease levels prevail in older aspen stands; and (4) a broad variation in disease severity for thalli of similar size in the four older stands.
Conclusions: The studied chronosequence is too short to elucidate whether the Abrothallus-Ramalina system is driven by disease escape in space and time, or co-existence. However, the high disease levels in older stands suggest that conservation programs aiming to sustain this system cannot only focus on preservation of isolated old-growth stands, but also need to incorporate continuous creation of young stands. If succession is ignored, conservation of organisms originally adapted to, now vanishing, large-scale disturbance regimes is likely to fail. We point out the need to assess the role of disease in conservation.