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.