Conspicuously, some host trees (in temperate regions as well as in the tropics) are covered by epiphytic lichens, others by bryophytes, or we find bryophytes at the trunks and lichens in the crowns of trees. To determine the reasons for the dominance of bryophytes or lichens, humidity data loggers were installed over a period of one year on nearby lichen and bryophyte tree in the Vosges Mtns, France, as well in the Lower Rhine area, Germany. Since bryophytes and lichens are poikilohydric, only periods in which the plants are in wet state ( rH > 80% rH) or in wet state during day light (assimilation phases) were taken into account. The results show clearly that the different relative humidity is at least in part responsible for the abundance of bryophytes or lichens. Bryophyte habitats are characterized by 20-30% longer wet phases and accordingly shorter phases of desiccation. The comparison of regions with different air humidity reveals that the lichen tree at the Lower Rhine had 27.5% less wet phases than the lichen tree in the Vosges Mtns., which possibly counts for the different cover (5% viz. 90%).The results of calculations of air quality based on the cover or frequency of epiphytic lichens from differently humid areas can therefore not be compared.