Decreasing N deposition leads to significant decrease in foliar N concentrations in forest trees

Despite reduction of nitrogen emissions, deposition in German forests remain high. Eutrophication of ecosystems thus remains an important issue of scientific and socio-political interest. Here we analyse data from 78 intensive forest monitoring (Level II) sites operated by the forest research institutes of the German federal states as part of the ICP Forests network (International Co-operative Programme on Assessment and Monitoring of Air Pollution Effects on Forests). In the 2013-2022 period, mean annual bulk open field (inorganic + organic) deposition was between 4.4 and 13.5 kg N ha^-1 a^-1. Over the past twenty years, N deposition decreased by about 40 % which corresponds to a decrease of 2.5 % per year compared to the deposition in 2010. The decrease in N-NO₃ (-3.1 % per year) was slightly higher than the decrease in N-NH₄ (-2.7 %). Organic N deposition decreased by only 0.7 % per year. Canopy budget models show that N deposition (wet + dry + occult) to forest sites was between 10 and 31 kg N ha^-1 a^-1 over the same period.

The deposition data is used for reporting duties such as the German federal states’ core indicators of environmental quality (LIKI) and for scientific research e.g. to evaluate changes in biodiversity, dynamics of nutrient cycles and ensuing vulnerability of ecosystem services, or effects on tree vitality. We used the data to assess the impact of N deposition on foliar N concentrations, an import indicator of tree nutrition status. Tree nutrition influences vitality and trees’ resilience to climate extremes. A deterioration of foliar nutrients has been observed in forest ecosystems across Europe. At the German Level II sites, all main tree species (European beech, Norway spruce, Scots pine, sessile and pedunculate oak) show a significant decrease in foliar N concentration of 0.2-0.3 % per year. Besides nitrogen deposition, the reduction has been linked to various environmental factors, including increasing temperatures and changing precipitation patterns, as well as, the increase in atmospheric CO₂ concentrations. At the spatial scale, nutrient availability can be explained by various site conditions such as parent material. Nonetheless, weak positive but significant relationships between mean foliar N and total N deposition for beech, oak, and pine for the 2013-2022 time period show that atmospheric deposition can explain part of the spatial variability between forest sites. The results indicate the importance of assessing deposition, trophy classes, and climate conditions at the same sites to fully understand their interaction.