Montane coniferous forests in Europe affected by pollutant-caused forest dieback often have a well-developed epiphytic lichen vegetation, including pollution-sensitive species. This phenomenon, apparently contradicting the assumption that acid air pollution is the cause for forest dieback, has been discussed controversially since the 1980s. Studies in Picea abies forests of the Harz Mountains, northern Germany suggest that the high lichen diversity on dieback-affected trees is due to reduced pollutant levels in bark and stemflow. Pollutant-caused needle loss reduces the interception of pollutants from the atmosphere and thus, reduces their concentrations in stemflow. Lower concentrations in stemflow result in lower concentrations in the bark because of reduced absorption. In the Harz Mountains, lower S concentrations in the stemflow of dieback-affected trees are thought to be the main cause for an even higher epiphytic lichen diversity on affected versus healthy trees. In one out of three spruce stands, high Cu concentrations in bark apparently have an additional limiting effect on lichen diversity. In all spruce stands studied in the Harz Mountains, high Mn concentrations in bark and partly also in stemflow limit lichen abundance. In contrast to S and Cu, Mn in bark and stemflow is primarily soil-borne. Lower Mn concentrations found in bark and stemflow of damaged and dead trees are attributed to reduced or lacking element uptake by the tree roots. Experiments to Mn uptake and toxicity in epiphytic lichens support the hypothesis that high Mn levels limit the abundance of epiphytic lichens. Decreasing lichen abundance with increasing Mn concentrations in bark or stemflow found in polluted and remote areas of western and eastern North America suggests that Mn may be an important site factor for epiphytic lichens in coniferous forests in general. Microclimate (including light and water conditions) is apparently of subordinate importance for the high epiphytic lichen diversity in dieback-affected spruce forests of the Harz Mountains. On dieback-affected Picea rubens on Whiteface Mountain, Adirondacks, New York, epiphytic lichen diversity was also found to be higher than on healthy trees. This cannot be attributed to S concentrations in stemflow, as the atmospheric deposition is much lower than in Germany. While Mn concentrations are apparently relevant for a limited number of lichen species on Whiteface Mountain, it is unclear whether lichen diversity might be limited by toxic effects of NO3− from stemflow. On dieback-affected Acer saccharum in Québec more acidophytic and less subneutrophytic lichen species were observed. This suggests that chemical site factors also control epiphytic lichen abundance in this case, however, the mechanisms leading to this effect are not known.
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