Effective conservation of the iconic old forest lichen Lobaria pulmonaria depends on understanding its life history bottlenecks. To address this, reproductive dynamics were evaluated in 302 juvenile and early-reproducing thalli transplanted across environmental gradients—including elevation, canopy openness, and bark pH—in pristine British Columbian forests over a 14 months period. Soralia cover increased significantly but at a slow rate, with marginal soralia appearing earlier and more frequently than laminal soralia. The intrinsic trait of initial total soralia cover emerged as the strongest predictor of the change in reproductive effort. Additionally, thalli with low specific thallus mass at start (STMStart) and rapid growth rates exhibited reduced reproductive activity. Environmental stressors, such as high-light-induced chlorophyll bleaching and elevated altitude, weakly inhibited soralia development. Sexual reproduction was rare, with apothecia present in only 2% of transplants and no new apothecia forming during the study. The findings highlight the slow pace of soralia formation and, coupled with the species’ long generation time, suggest that current forest rotation cycles in managed forests may be insufficient to sustain viable L. pulmonaria populations. Conservation strategies should therefore account for both slow reproductive onset and sensitivity to environmental conditions. Keywords Epiphytic lichens · Specific thallus mass · Reproductive investment · Environmental stressors · Forest management.