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Monitoring ammonia pollution with epiphytic lichens in the Netherlands.
Epiphytic lichens are monitored in the province of Overijssel in The Netherlands since 1989. This report deals with the results of the 6th round, carried out in 2020. All lichen species abundance changes over this period are calculated and discussed. Maps are provided, showing changes of the abundances of nitrophytes (expressed as NIW). Trends are calculated over all investigated years. Changes of NIW indicate that ammonia air pollution (NH3) has decreased since 1999, especially in agricultural areas due to clean air policy. However, very recently (> 2015) the NIW has increased again slightly, indicating a stagnation of the improvement. In built-up areas the NIW has increased over the whole period, probably due to car exhausts (ammonia emissions from catalytic converters and the application of AdBlue). Species with a sensitivity to ammonia (many acidophytes, expressed as AIW) are decreasing all the way since the start in 1989 due to a continuing deacidification of tree bark (ammonia has basic properties). This is a cumulative process, and will at its best only slow down at lower ammonia levels. Return of acid of bark properties (and acidophytes) at low ammonia levels is unlikely. Climate change is another important driving force for species abundance changes. Many atlantic species have increased. Indicator values for temperature have increased over the whole period, and similarly values for continentality have decreased.
In the Netherlands a monitoring programme is in operation to map the effects of ammonia pollution with epiphytic lichens. The method is presented here and the results are statistically correlated with abiotic data. The abundance of nitrophytes on Quercus robur appears to be a useful parameter. Detailed spatial patterns of ammonia pollution can be obtained with lichens. To avoid interference, it is important to consider other influences, for example dust, climate, exposure, age of the trees and other pollutants.
The lichen composition on wayside Quercus robur in the Netherlands was related to bark properties (pH, EC, NH4+, SO42-, NO3-) and levels of air pollution (SO2 and NH3). The pH of the bark and the susceptibility to toxic substances appear to be the two major primary factors affecting epiphytic lichen composition. These factors have independent effects on the lichen composition. Most of the so-called nitrophytic species appear to have a low sensitivity to toxic effects of SO2; their only requirement being a high bark pH. An increased bark pH appears to be the primary cause of the enormous increase in nitrophytic species and the disappearance of acidophytic species over the last decade in the Netherlands. Measurements of ambient NH3 concentrations in the air show that there is a nearly linear relationship between the NH3 concentration and the abundance of nitrophytes on Quercus. The abundance of nitrophytes was not correlated with SO2 concentrations. Most of the acidophytic species appear very sensitive to NH3 since in areas with concentrations on 35 microgr. m-3 or more, all acidophytic species have disappeared. Current methods using species diversity to estimate or monitor SO2 air pollution need some modification, otherwise the air quality may be erroneously considered to be relatively good in areas with high NH3 levels.