Interaction between a lichen and a fungal parasite in a successional community: Implications for conservation

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.