Evidence of declining trees resilience under long term heavy metal stress combined with climate change heating

Annually resolved tree rings enable us to distinguish past pollution and climate change effects on chronic tree decline, which affects significant areas around the world. The aim of the current study was to establish a pattern for the assimilation of heavy metals in regular and declining Quercus robur L. trees. Using laser ablation-inductively coupled plasma mass spectrometry (LA-ICP-MS), ICP-MS, and atomic absorption spectroscopy (AAS), the present and historical changes in element levels were investigated. Twenty different environmental species, including water, soil, sediments, mushrooms, acorn, leaves, branches, bark, and wood, were analyzed. Time profiles of elements in earlywood tree rings were investigated. We demonstrated higher HM contamination in the present background with increasing Pb and Cd levels. The presence of Li and Sr in groundwater was reported for the first time. We found significant differences (p < 0.001) between declined (D) and normally (N) growing trees only in the 1960–2019 period (Levene's t-test), which was sustained by a significant negative trend in D‒ chronology (MK-test, two-tailed). The bootstrapped correlation showed a weak positive relationship between D‒ chronology and soil moisture and a negative relationship with temperature and evapotranspiration. Declined trees were less resistant to environmental stress factors than N‒ growing trees at present (2012 compared to 1953). Mn, Cu, Cd, Pb, Zn, and Ni contents in tree rings correlated with climate-induced drought conditions, explaining the increasing elemental contents after 1980. A significant relationship between earlywood tree rings, climate and HM contamination explained different response patterns for normal and declining trees. Metal availability is increasing with climate change effects, leading to higher rates and future concerns, especially considering the high environmental health risk of some elements, such as Pb and Cd.