Pinus sylvestris and Picea abies canopy effects on deposition of air pollutants

Tree canopies influence atmospheric pollutant depositions depending on type, ecosystem characteristics, and local climatic conditions. This study investigated the impact of Pinus sylvestris L. and Picea abies (L.) H. Karst., and a mixture of both, on the chemical composition of precipitation. Three permanent plots within the ICP forest level II monitoring network in Lithuania were selected to illustrate typical hemiboreal coniferous forests. The study analysed (1) the concentrations of NO₂, NH₃ and SO₂ in the ambient air; (2) the concentrations of SO₄²⁻, NO₃⁻, NH₄⁺, Na⁺, K⁺, Ca²⁺ and Cl^– in throughfall beneath canopies and in precipitation collected in an adjacent field, and (3) S and total N, Na⁺, K⁺, Ca²⁺ and Cl⁻ depositions in throughfall and precipitation over 2006–2022. Results show a significant decrease in SO₂ emissions in the ambient air; NO₂ and NH₃ emissions also decreased. The canopies reduced the acidity of throughfall, although they led to notably higher concentrations of SO₄²⁻, NO₃⁻, Na⁺, and particularly K⁺. During the study, low variability in NO₃^– deposition and a decrease in NH₄⁺ deposition occurred. Deposition loads increased by 20–30% when precipitation passed through the canopy. The cumulative deposition of S, Cl, Na, K, Ca, and N was greater under P. abies than under P. sylvestris. However, K deposition in throughfall was considerably lower under P. sylvestris compared to the P. abies or mixed stand. Throughfall S depositions declined across all three coniferous plots. Overall, there was no specific effect of tree species on throughfall chemistry.