Growth and acclimation dynamics in reintroduced Erioderma pedicellatum (Hue) P.M.Jørg., a critically endangered cyanolichen in boreal rainforest

This study focuses on the critically endangered Erioderma pedicellatum, a sexually reproducing cyanolichen. The species’ decline is primarily attributed to habitat loss and extinction debts. The aim is to understand the possibilities of reintroduction via transplantation on Picea abies (L.) H.Karst. and gain insights into the species' growth response to environmental gradients in its boreal rainforest habitat. The source population of E. pedicellatum found on open, sun-exposed spruce branches within a waterfall spray zone, underwent a significant environmental shift when transplanted to the rainforest. The shift induced shade acclimation, which was marked by an increased maximal photosystem II efficiency (F_V/F_M), reduced thallus thickness and reduced apparent electron transport rate (ETR_app). However, stochastic fragmentation impacted 57 % of thalli, causing significant losses, particularly in sections with numerous apothecia, suggesting a trade-off between growth and reproduction. Unfragmented thalli with positive relative growth rate (RGR) exhibited increased growth correlated with base cation availability (high soil and bark pH) and light availability. Thalli that were already shade-adapted at the start (low ETR_app at high light) displayed the fastest growth in the rainforest. The best subset regression model of unfragmented thalli accounted for 72.8 % of the variation in RGR. The reintroduction of E. pedicellatum into the boreal rainforest is possible, but challenging, given that well-lit spruce branches with high base cation availability in humid canopy layers are rare.