The significance of precipitation and substrate chemistry for epiphytic lichen diversity in spruce-fir forests of the Salish Mountains, northwestern Montana

The relevance of chemical site factors for the abundance of epiphytic lichens was studied in Picea engelmannii-Abies lasiocarpa forests of the Salish Mountains, northwestern Montana, USA. The Salish Mountains are an area with relatively low atmospheric pollutant load and low precipitation. Canonical correspondence analysis (CCA) suggests that cover of several lichen species was limited by high Mn concentrations of bark or by high ratios of Mn to Ca, Mg and Fe. Mn in the bark is known primarily to derive from the soil. An effect of Mn concentration or Mn/Ca and Mn/Mg ratios was not found on A. lasiocarpa. This suggests that A. lasiocarpa deposits Mn in the bark in a physiologically inactive form as already known from A. balsamea. Precipitation chemistry was apparently less relevant for epiphytic lichen distribution in the Salish Mountains, as no correlations between element concentrations in stemflow and cover values were found and as amounts of stemflow were small. However, precipitation in the study year was less than average. The lacking significance of precipitation chemistry is probably the cause why epiphytic lichen vegetation differed less between living and dead trees in the Salish Mountains than in highly polluted coniferous forests studied by our group in Germany; in Germany, the difference between living and dead trees was attributed to reduced interception of pollutants from the atmosphere by trees with reduced crown surface. The result of the present study that small-scale variation of epiphytic lichen abundance is only partly explainable by chemical parameters gives rise to the assumption that microclimate (e.g., moisture), which has not been systematically explored, could be an important site factor for epiphytic lichens in the Salish Mountains. Furthermore, tree age was identified by CCA as a relevant site factor for lichens on P. engelmannii.