Lichen diversity on steppe slopes in the northern Mongolian mountain taiga and its dependence on microclimate

Isolated steppes on sunlit southern slopes are a characteristic feature of the northern Mongolian mountain taiga. Lichen diversity of such steppe slopes was studied in the western Khentey Mountains, located between the Mongolian capital Ulan Bator and Lake Baikal. Functional types of lichen species were analyzed and measurements of microclimate were carried out as a first approach to figure out relevant site factors that control lichen distribution in the different habitat types of the steppe slopes. Such habitats include meadow steppe, siliceous rock and savanna-like Ulmus pumila open woodlands. Dominance of chlorolichens with either yellow or orange lichen substances (usnic acid, parietin, pulvinic acid derivatives) or melanin in the cortex, which absorb both ultraviolet radiation (UVR) and visible light usable for photosynthesis (PAR) suggests that effective protection against high solar irradiation is a crucial selective factor for lichens on the steppe slopes. The slopes are subject to high irradiation because of their steepness and because of the ultracontinental climate. In Betula platyphylla–Larix sibirica light taiga forest bordering the steppe on northern slopes, lichens with and without effective photoprotection co-occur. A second important factor, apparently influencing lichen distribution of the study area is water availability. Low precipitation, which falls in numerous small rain showers, combined with frequent dewfall in summer favors chlorolichens and explains the limitation of cyanolichens to particularly moist microsites, such as water runoffs on rocks or moss cushions on inclined or horizontal tree trunks. Ecophysiological measurements to prove these hypotheses on the significance of sunlight and water availability for lichen performance in the study area have not yet been carried out.