Popular Science: Where are Czech forests under potential risk due to the highest O3 exposures and N deposition?
Despite a reduction in emissions in Central Europe since the 1990s, the nitrogen deposition and ambient ozone exposure still exceeds the limit concentration. It has a major negative impact on vegetation and can be considered to be one of the causes of decreasing forest health and increasing forest loss.
Anthropogenic sources are mainly excessive nitrate fertilization, livestock and the formation of nitrogen oxides during all combustion processes, e.g. car traffic, which has a continuously increasing trend.
The authors have focused on the area covered by forests - an area of over 28 thousand km2, i.e. 34% of the Czech Republic, which is located mainly in mountainous areas, where there is usually higher wet nitrogen deposition due to higher precipitation amounts and higher concentrations of ambient ozone, which usually increase with altitude mainly due to higher global radiation, occasional stratospheric intrusions or a temperature inversion layer. The evaluation was based on continuous ozone concentration data and the total nitrogen deposition calculation using data from the Czech Hydrometeorological Institute for the period between 2000–2015. This period was chosen due to the relatively stable state of the concentrations of the investigated substances compared to the previous period and due to no changes in measuring methods.
While nitrogen deposition concentrations changed only minimally from year to year, ambient ozone exposures showed significant changes, as many other factors affect its formation and occurrence - not only the presence of ozone precursors, i.e. nitrogen oxides and volatile organic compounds, but also meteorological conditions, especially intense solar radiation, high temperatures, low air humidity and total precipitation. Meteorology often determines the occurrence of ambient ozone more than changes in emission concentrations.
Because the identification of potential risk areas with high loads is important for investigating the potential environmental impacts of high nitrogen and ambient ozone concentrations and for suggesting appropriate countermeasures, the authors have identified these areas.
To assess ozone exposure, the AOT40 exposure index for forests, which indicates the long-term critical level for ozone expressed as a cumulative exposure above the threshold of 40 ppb (parts per billion) and is calculated for each hour of the entire growing season, i.e. from April to September each year, was used. The index is currently widely used to determine the impact of ambient ozone on forest ecosystems and crops. The total nitrogen was calculated as a sum of wet (NH4 +, NO3- in precipitation) and dry (NOx) deposition.
An interesting result was the spatial distribution of areas with high nitrogen deposition and ozone exposure, which were different in individual parts of the Czech Republic. This is surprising news. While high atmospheric nitrogen deposition was observed mainly in the northern parts of Czechia near the border with Germany and Poland (Krušné hory, Krkonoše, Jizerské hory, Orlické hory, Jeseníky, Beskydy), high ozone exposure occurred mainly in the southern parts of our territory (especially Šumava, the southern part of the Českomoravská vrchovina, southeast Moravia). Only a small part of the areas (4.6%) were overlapping areas with a risk due both to high ambient ozone exposure and nitrogen deposition (especially the peaks of the Krušné hory, and Jeseníky). The results can be explained by the fact that the main sources of nitrogen oxide emissions are found mainly in the northwestern and northeastern parts of the Czech Republic and ambient ozone exposure corresponds to the map of global solar radiation.
Excessive loads of nitrogen and ambient ozone are dreaded factors that endanger the environment around the world in many different ways. From negative effects on soil microbiome composition and function, along with disruption of the natural biological function of the soil through degenerative plant changes resulting in yellowing and reddening of leaves to reducing photosynthesis and death. The results of the recent study thus represent essential information for a more detailed analysis of the impacts of high nitrogen concentrations and ambient ozone exposure on the health of Czech (and other) forests.