Do the expected heatwaves pose a threat to lichens?: Linkage between a passive decline in water content in thalli and response to heat stress.

Being poikilohydric, lichens are inherently exposed to alternating desiccation and hydration cycles. They can exhibit extraordinary resistance to extreme temperatures in a dehydrated state but thermal thresholds for hydrated lichens are lower. The ability of the lichen Cetraria aculeata to recovery after high temperature treatment (40°C, 60°C) at different air humidity levels (relative humidity [RH]: <15%, 25%, 50%, 75%, ≅100%) was examined to find a linkage between passive dehydration of the lichen and its physiological resistance to heat stress. The response to heating was determined by measuring parameters related to photosynthesis and respiration after 2- and 24-h recovery. A higher RH level resulted in a slower decline in relative water content (RWC) in hydrated thalli. In turn, the stress resistance of active thalli depended on the ambient humidity and associated RWC reduction. Elevated temperature had a negative impact on bioenergetic processes, but only an unnatural state of permanent full hydration during heat stress resulted in a lethal effect. Hydrated lichen thalli heated at 40°C and 50% relative humidity (RH) tended to be least susceptible to stress-induced damage. Although atypical climatic conditions may lead lichens to lethal thresholds, the actual likelihood of deadly threat to lichens due to heat events per se is debatable.