A tale of two cities: Impacts of the COVID-19 lockdown on growth and wood chemistry of urban trees

The COVID-19 pandemic led to widespread lockdowns, significantly reducing traffic emissions and improving city air quality. Urban forests and parks recorded these abrupt pollution changes in their annual tree rings. However, no detailed study has yet quantified how COVID-19 lockdowns impacted tree-ring wood chemistry. Such dendrochemical analyses are very relevant because: (i) they provide a temporal and integrative framework to assess changes in air pollution, and (ii) represent a benchmark to use tree-ring wood as a biomonitoring tool. We used dendroecological techniques to quantify the impact of COVID-19 lockdowns during March-April 2020 on the sapwood concentrations of chemical elements in big (Madrid) and mid-sized (Zaragoza) Spanish cities. We compared the growth patterns, growth responses to climate and dendrochemical data (period 2018-2022) of three widely planted conifers (Pinus halepensis, Pinus pinea, and Cedrus atlantica) sampled in three sites near areas with high traffic density. No abrupt growth change was observed in 2020 and the growth increases detected in some sites were related to wet spring conditions, which enhanced growth. The lockdowns reduced air pollution as shown by the reduction in NO₂ concentrations from March to July 2020 in both cities. We detected significant decreases in wood concentrations of some elements in all sites and species (Pb) or in some sites (Cr, Fe, Si, Sr and Ti). Dendrochemical data serve as proxies for air pollution, but careful selection of sites, tree species, and chemical elements is essential for effectively using them as biomonitoring tools. Sudden socio-economic crises triggering drastic reductions in traffic and air pollution offer unique settings to assess the value of biomonitoring proxies, including urban forests.