Lichen symbiotic stress as a precursor to biodiversity loss – rapid assessment using Evernia prunastri as a bioindicator for nitrogen pollution

Excess reactive nitrogen (Nr) emitted from farming (NHx) and fossil fuels (NOx) is a major global threat to biodiversity and ecosystem function. Environmental Nr is often monitored using bioindicators such as lichens, which provide valuable insights in the absence of instrumental monitoring stations. As key bioindicators, lichen responses to Nr have been widely studied using either short-term highly controlled laboratory experiments or field sampling, linking functional aspects of lichen biology with real-world outcomes. However, a missing component in the available evidence base could be provided by field-scale experiments to isolate the response of lichens to contrasting Nr levels over longer time periods. Here, we investigated the response of the bioindicator lichen Evernia prunastri to contrasting ammonia (NH3) concentrations within a novel field-scale experiment and over a 12-week period. We measured fungal cell membrane damage and algal chlorophyll content as markers related to lichen tissue nitrogen accumulation, revealing impacts on the fungal and algal symbionts and explaining net outcomes on lichen relative growth rates. We compared the results of the field-scale experiment to trends observed in the real world. Our results suggest that E. prunastri tissue nitrogen content becomes saturated at 1.3% with long-term NH3 concentrations of c. 2 μg m-3, beyond which the species experiences unmitigated physiological damage. This response is however critically dependent on the exposure duration, which interacts with atmospheric NH3 to constrain acclimation through increased chlorophyll content, while causing accumulative damage to fungal cell membranes that compromises growth and leads to eventual mortality.