Succession of Lichens in Front of Retreating Glaciers in Sweden and Norway.

Climate change is accelerating the melting of glaciers to create new habitats colonized by pioneer organisms. Lichens are adapted to extreme conditions and, together with cyanobacteria and algae, are generally among the first organisms to appear in primary succession. The exact mechanism of lichen community formation and the factors influencing species turnover are still poorly understood. The aim was to determine the time zones of succession of terricolous and saxicolous species after glacier retreat, and to identify common ecological traits among pioneer species, including thallus type. Additionally, lichen species were categorized into time periods based on their colonization rates. Finally, differences among dated plots in the glacier forelands of Midtdalsbreen (Norway) and Storglaciären (Sweden) were compared. Taxon: Lichens. Location: Arctic. A total of 27 plots (135 subplots) were delineated across five successional age classes (9-264 years postglaciation). Lichen abundances and environmental variables (e.g., orientation, substrate composition) were recorded. Canonical correspondence analysis (CCA) and detrended correspondence analysis (DCA) were used to analyze species composition gradients and the effect of successional age. Monte Carlo permutation tests were performed to determine statistical significance (499 permutations). Lichen species richness increased rapidly in the early succession (15-25 years), but stagnated or slightly declined in older stages (> 100 years), probably due to competitive displacement. Early successional pioneer species included , , and -fruticose lichens that disperse easily by wind through thallus fragments. Late successional species, such as and , thrived under stabilized and nutrient-enriched conditions. CCA showed that successional age explained 12.9% of the variability in species composition, whereas geographic differences contributed 7.9%. Species turnover was influenced by substrate characteristics (fine-particle sediment vs. coarse rocky substrate), competition, and erosion, which was still shaped by environmental stability. Lichen succession follows the expected pattern, primarily shaped by terrain microtopography, climate, and substrate type. To refine the successional dynamics of lichen communities and other groups of organisms involved in the colonization of newly deglaciated habitats, more long-term studies from different regions will be needed.