Combination of multiple stable isotope and elemental analyses in urban trees reveals air pollution and climate change effects in Central Mongolia

The Ulaanbaatar area in Mongolia has become one of the most polluted regions worldwide due to the rapid increase in urbanization, industrial activity and traffic. However, we critically lack knowledge on the impacts of air pollution on surrounding forest ecosystems that may be further amplified by the ongoing climate change. Here, we apply a novel combination of multiple stable isotope analyses (nitrogen: 815N, carbon: 813C, oxygen: 818O, hydrogen: 82H) in foliar and tree-ring samples from different tree species, including deciduous, broadleaf species (poplar and birch), a deciduous conifer (larch) and needle evergreen species (spruce and Scots pine). This was complemented by trace element analysis, to study the influence of air pollution and climate on urban, suburban and more remote forests in and around Ulaanbaatar. We found indications of pollution effects in urban and suburban sites in foliar material, particularly in 815N, with unusually high values of > 10 & PTSTHOUSND;, that could be related to tree uptake of NOx. Results were similar for all species, but with a smaller effect for Scots pine. The tree-ring 815N values were found to be clearly enriched in recent years compared to 50 years ago at the urban sites, consistent with a pollution signal. Leaves and needles at suburban and urban sites had accumulated higher concentrations of various trace elements including Al, B, Ba, Ca, Cr, Cu, Fe, Mg, Na, S and Zn compared to the more distant sites, confirmed by Principal Component Analysis. Our data on 813C, 818O and 82H enabled us to infer possible physiological effects induced by air pollution. Consistently increasing tree-ring 813C values over recent decades for all investigated species indicated increasing plant stress, like hampered stomatal conductance and photosynthesis, but this was found for all sites, suggesting climate change rather than air pollution effects. In summary, we show that our multi-isotope and -element approach provides new insights into the threats to forests in urban areas, where the occurrence of more frequent droughts acts together with air pollution.

Automated summary

The Ulaanbaatar area in Mongolia has become highly polluted due to urbanization, industrial activity, and traffic. However, the impact of air pollution on surrounding forests is not well understood. This study uses stable isotope analyses and trace element analysis to investigate the effects of air pollution and climate change on forests in and around Ulaanbaatar. The results show pollution effects in foliar material and increased concentrations of trace elements in leaves and needles, indicating the threat to forests in urban areas.