Long-term forest monitoring allows to document developments which would otherwise remain undetected. Observed changes need to be interpreted by disentangling the various influencing factors. In addition to robust measurements, the selection of sites and the coverage of important ecological gradients are crucial for epidemiological data analysis. The significance of monitoring has been increasing in recent years, as the environment has undergone rapid changes and climate change has become the predominant factor influencing tree health. Four decades of monitoring European beech (Fagus sylvatica), Norway spruce (Picea abies), and oak species (Quercus robur, Q. petraea, Q. pubescens) in 247 sites of the intercantonal long-term forest monitoring in Switzerland show that direct or indirect effects of air pollution, such as nitrogen (N) deposition, causing eutrophication and soil acidification, continue to exert a significant influence on forest health. Evidence of soil acidification was provided through the measurement of soil solution samples on a monthly basis at 47 sites over a 25-year period, encompassing both topsoil and subsoil layers. The ratio between base cations and aluminium (BC/Al ratio), which is an indicator for soil acidification, revealed an increasing trend in soil acidification. N deposition, as the main driver of soil acidification in Switzerland, is causing nutrient losses in the soil and tree nutrient imbalances. The mycorrhiza communities are affected as well, impacting drought and pathogen resistance, as evidenced by the repeated summer drought years (2018–2023). High N deposition is linked to substantial growth reduction in beech and Norway spruce, further exacerbated by drought. Additionally, ozone was found to reduce growth and to increase water uptake. The hot summer 2018 was a tipping point for the vitality of European beech in Switzerland with the occurrence of acute cavitation and subsequent dieback of branches and whole trees, especially in lower elevations. In beech, the proportion of strongly damaged trees increased during the 2019–2024 period compared to 1984–2018 by a factor of 6.9 and the mortality by a factor of 4.2. Our data show that the cumulative impact of the consecutive drought years from 2015 to 2022 is a significant factor contributing to the exacerbated damage observed after 2018 in comparison to previous drought years, such as 2003. The observed vitality changes in Norway spruce in the last decades have accelerated in recent time. During the drought years 2018, 2019, 2020, and 2022, mortality increased by a factor of 9.9 compared to the years 1985–2017. For Norway spruce, we found evidence of increased mortality under high N deposition. The oaks performed better, even though pubescent oak was also affected by the drought, especially on dry sites. In pubescent oak, the number of strongly damaged trees increased by a factor of 16.6 during the period 2019–2024, and mortality increased by a factor of 15.5. Stem volume increment per ground area has recently decreased in all observed tree species. This is due to changes in forest structure as a result of mortality and wind damage, and, in the case of beech, also due to a decrease in growth at the tree level. The rapid changes caused by climate change, in combination with N eutrophication effects and the appearance of new parasites, emphasize the importance of intensive forest monitoring with cause-related data analysis in the future.