Tree rings record fossil-fuel CO2 emission reduction and reveal structural responses in street trees during the COVID-19 lockdown.

Air pollution ranks among the top causes of premature death worldwide. In cities, air pollution mitigation depends on curbing emissions and fostering ecosystem services, particularly through tree planting. Whereas the role of urban trees in air pollution mitigation is well established, trees' responses to reduced air pollution stress remain poorly quantified. Air pollution can limit trees' benefits by constraining stomatal conductance, photosynthesis, and xylogenesis. This study assessed the impacts of acute air pollution reduction during the COVID-19 lockdowns on Tipuana tipu trees growing in a park and on a main avenue in São Paulo, Brazil. To test whether air pollution-driven physiological and structural limitations were reduced during the lockdowns, we assessed high-resolution tree-ring stable carbon isotope composition (δ13C) and wood density (WD) across 15 evenly-spaced intra-annual segments from 2018 to 2021. Avenue trees exhibited lower δ13C values than park trees, indicating greater exposure to fossil-fuel CO2 emissions, and the higher WD was consistent with water stress. The most restrictive lockdown (based on pollution and CO2 concentration data) at the end of the 2020/2021 growing season coincided with a shift in the typical decreasing intra-annual δ13C trend in the avenue trees to an increasing trend at the end of the tree ring, a pattern not observed in the park trees. This increasing shift was not readily explained by improved weather conditions or leaf-physiology responses. Instead, the δ13C fractionation model simulations suggest a strong source signal, reflecting an increase in the atmospheric δ13C signature during the lockdown, more so than the reduction in CO2 concentration. Although the intra-annual WD trends remained consistent, increasing from earlywood to latewood, avenue trees displayed a steeper increase in WD across the 2020/2021 tree ring, possibly associated with increased stomatal conductance. These results suggest that avenue trees recorded changes in fossil-fuel emissions and showed structural responses.