Rapid legacy‐dependent succession of lichen assemblages after forest fires: Insights from two boreal regions

Abstract Questions (i) What is the impact of legacy structures on lichen assemblage development up to two decades post‐fire? (ii) How does that impact vary among ecological groups? (iii) Are charcoal‐inhabiting lichen assemblages regionally distinct? Location Hemiboreal pine forests, Estonia, and middle‐boreal pine forests, eastern Finland. Methods We performed standard lichen surveys (4 hr effort per 2 ha plot; on all substrates 0–2 m from the forest floor) and measured forest structure in 18 burned sites (nine in Finland and nine in Estonia). The sites included both old (15–21 years) and recent (9 years) fires, and half of the latter had been harvested for timber. We analysed lichen assemblages (full assemblages and different ecological groups) in relation to site factors (GLM for species richness; multivariate techniques for assemblage composition). Results Lichen assemblages on burned sites (altogether 187 species recorded) were regionally distinct and, additionally, significantly affected by the large variation in legacy and regeneration abundance among sites. Species richness was negatively related to abundance of fallen trees 9 years after fire but the relationship was positive 15–21 years post‐fire; this pattern indicated a change from the initial damage effect to a substrate‐providing effect of the fire. Microlichens and vegetatively dispersing lichens appeared more sensitive to disturbance (including harvesting), whereas macrolichens were more responsive to substrate provision. The main structural influences on lichens inhabiting charred substrates (67 species recorded in total) were similar to the factors affecting the composition of full lichen assemblages. Conclusions Within 10 years post‐fire, initial disturbance‐related damage to forest lichen assemblages was replaced by the dominance of substrate‐providing factors (legacies, regeneration). These damage and recovery phases differ among lichen groups, but are consistent between hemi‐ and middle‐boreal regions. The availability of post‐fire legacies, including charred surfaces, is of critical importance for the management of burned areas in modern landscapes.