Lichen bleaching as a response to long‐term experimental warming in the High Arctic

Abstract Lichens are an important component of Arctic ecosystems. Studies have indicated a decline in the abundance of Arctic lichens during recent decades, which is often attributed to competitive pressure from vascular plants. However, the direct effects of warming on the lichen symbiosis have rarely been explored, particularly in the High Arctic, where shrubification is less pronounced than in the Low Arctic, and where warming might hence have more direct effects on the growth and survival of lichens. Here, we investigated the physiological responses of Cetrariella delisei , a widespread circumpolar lichen species, to almost a decade of experimental warming with open‐top chambers. We found that C. delisei physiologically deteriorated in response to the warming treatment, with the myco‐ and photobiont differing in their sensitivities to warming. Symbiotic deterioration was manifested in a 2.4‐fold reduction in net photosynthesis, an approximate doubling in maximum respiration rate, and a loss of 60% of the algal photobiont and a 30% decline in chlorophyll concentration, which caused bleaching of the lichen thallus. These changes resulted in a negative thallus carbon balance and a loss of photosynthetic capacity at low temperatures. Our findings, which advance mechanistic understanding of symbiotic breakdown and lichen bleaching, shed new light on the physiological impacts of warming on polar lichens. We show that photobionts have high sensitivity to increases in mean and maximum daily summertime air temperatures of 1 and 4°C, respectively. These are early warning signals that the mat‐forming lichen C. delisei is under threat in the rapidly warming Arctic. Read the free Plain Language Summary for this article on the Journal blog.