Lichen responses to nitrogen and phosphorus additions can be explained by the different symbiont responses

Responses to simulated nitrogen (N) deposition with or without added phosphorus (P) were investigated for three contrasting lichen species - the N-sensitive Alectoria sarmentosa, the more N-tolerant Platismatia glauca and the N-2-fixing Lobaria pulmonaria - in a field experiment. To examine whether nutrient limitation differed between the photobiont and the mycobiont within the lichen, the biomass responses of the respective bionts were estimated. The lichenized algal cells were generally N-limited, because N-stimulated algal growth in all three species. The mycobiont was P-limited in one species (A. sarmentosa), but the growth response of the mycobionts was complex, as fungal growth is also dependent on a reliable carbon export from the photobiont, which may have been the reason for the decrease of the mycobiont with N addition in P. glauca. Our findings showed that P availability was an important factor when studying effects of N deposition, as P supply can both mitigate and intensify the negative effects of N on epiphytic lichens.