Nitrogen deposition effect on tree growth depends on climate, tree size, and leaf habit

The increase of nitrogen (N) deposition is a human-induced process associated with industry and agriculture that disrupts the nitrogen biogeochemical cycle. N deposition has been associated with environmental impacts such as land carbon sink increase, loss of biodiversity, and risk of eutrophication and acidity but further understanding of how N deposition affects trees of different environmental conditions, size, and leaf habit is missing. In this study, we use the ICP forest inventory data and the EMEP N deposition data to track how N deposition affects tree growth in interaction with temperature and precipitation, tree size classes, or leaf habits in Europe since 1990. We use linear mixed models to describe the interaction between mean annual temperature (MAT) mean annual precipitation (MAP) and N deposition in tree growth. In addition, we use gam models to track the different saturation points. We found contrasting interactions between N deposition and temperature in conifers and broadleaves. Conifers living in colder environments have a more positive response to N deposition than conifers living in warmer environments. On the other hand, broadleaves living in warmer environments had the most positive response to high N deposition levels. Interestingly, broadleaves showed lower saturation points than conifers, being around 25 kg ha yr and 30 kg ha yr respectively. Nonetheless, factors such as tree size and species can modulate such relations, being especially relevant for secondary forests or restoration processes. In conclusion, our findings point out climate, tree size, and leaf habit as strong modulators of N deposition impacts in tree growth that should be considered in future assessments and policy-making, especially in Europe. Further research is needed to certify these relations in other regions of the world.