Abstract top ↑
In both stands, more species grew on dieback-affected trees than on healthy ones. Especially pollution-sensitive species had higher mean cover or frequency in the dieback-affected plots than in the healthy ones. Conversely, the extremely toxitolerant crustose lichen Lecanora conizaeoides covered larger areas in the healthy plots than in the dieback-affected ones. Stem flow of the dieback-affected spruce trees of both stands contained significantly less SO42-, NO3-, and NH4+ than stem flow of the intact ones. Moreover, stem flow content of PO43-, H+ and Fe and conductivity were significantly lower on the dieback-affected trees than on the healthy ones in one of both stands. The concentrations of SO42- and NO3- decreased linearly in stem flow from healthy over dieback-affected to dead trees. Therefore it is assumed that a smaller canopy surface of the dieback-affected trees, which results from needle loss, causes a reduced interception of the atmosphere, and, thus, results in smaller concentrations in stem flow especially of dryly deposited substances.
For testing the impact of the factor altitude, a stand in an altitude of 1000 m was compared with a stand in 550 m. Epiphytic lichen vegetation in the more highly elevated stand consisted of 22 species, whereas only 13 species occurred at 550 m. Significantly larger areas of bark were covered on trees at 1000 m. Moreover, mean cover of 15 species was higher in 1000 m, but differences were significant for only seven taxa. At 550 m, six species had a trend for higher mean cover. Epiphytic lichen vegetation of both stands was dominated by Lecanora conizaeoides, making up more than 50 % of the total area covered by lichens. Element content in incident precipitation did not differ significantly between the stands. However, Na content in stem flow was significantly higher in 1000 m than it was in 550 m. Stem flow content of Mn and the ratios of Mn / Ca and Mn / Mg were higher in the more lowly elevated stand. Mn content, Mn / Ca and Mn / Mg ratios were also higher in the bark of the latter stand. These high Mn concentrations are assumed to be a result of significantly higher concentrations of extractable Mn in the soil.
A comparison between only the healthy trees of both stands revealed significantly higher doses of Na, SO42- and PO43- in stem flow of the more highly elevated stand. However, SO42- concentration in stem flow was higher for the trees of the more lowly elevated stand. Amounts of stem flow were significantly higher on trees of the stand in 1000 m. Therefore, it is assumed that despite higher deposition rates at higher altitudes, stem flow concentrations of pollutants such as SO42- and NO3- might be more unfavourable to lichens at lower altitudes because of a lower dilution in these stands.
Strong correlation was found between SO42- content of stem flow and lichen cover. In both stands, cover of Hypogymnia physodes and the number of species per tree declined with increasing concentrations. Cover values of Lecanora conizaeoides showed an optimum curve.
In the low elevated stand, high Mn concentrations in stem flow are considered to have a strong impact on lichen vegetation. However, the Mn / Ca ratio of stem flow is more important, as strongest correlations occurred both with cover of Hypogymnia physodes and the number of lichen species per tree. Mn content and the Mn / Ca ratio of the bark was also correlated with a decrease in cover of Hypogymnia physodes and the total number of species per tree. Positive correlations of cover of Lecanora conizaeoides with mean NO3- content in stem flow occurred in both stands. Furthermore, in one of both stands this was the only parameter with which L. conizaeoides was correlated. Therefore, a promotional effect of NO3- content in stem flow could not be ruled out. Other correlations with stem flow and bark parameters are considered coincidental, although they cannot be ruled out, as experimental knowledge is not sufficient in many cases.
Summarizing, there is strong evidence that forest-dieback promotes epiphytic lichen abundance due to reduced concentrations of atmospheric pollutants in stem flow. This applies to SO42- in particular. Furthermore, the importance of the factor soil is highlighted. Differences in epiphytic lichen vegetation between both stands are probably a result of between-stand differences in the concentrations of extractable Mn in the soil.