Hazardous metals coupled with increasing summer diurnal temperature range contributed to growth decline of two oak species in a heavily industrialized area

The chemical elemental profile of annual tree rings represents a biological proxy for reconstructing past pollution history. Furthermore, changes in the heavy metals content in wood can be used to understand possible early signs of growth decline caused by the joint effects of climate change and pollution. In the present study, we assessed interactions between radial growth, heavy metal content, and climatic variables and documented their effects in changing trends of Quercus robur and Quercus pubescens during 19722019 in an industrialized area of Romania. The LA-ICP-MS technique was used to quantify the elemental profile of Cd, Hg, Pb, Ni, Sr, Cr, Cu, Fe, Mn, Mo, and Zn in tree rings with annual resolution. Growth of both oak species negatively responded to the summer diurnal temperature range. Species-specific patterns indicated that Q. robur showed higher metal levels in wood than Q. pubescens. Contrary to our expectations, radial growth increased after industrial activity cessation for a decade, after which it declined. The heavy metal trend in Q. robur ascended for the entire study period, particularly in the case of Cd, whereas it increased after 1990 for Hg and Pb when industrial activities ceased. Our findings show that tree species differ in their growth resilience capacity after pollution ceases. The diurnal temperature range documented a notable negative effect on tree growth associated with uplifted content (> 70 µg g-1) of non-toxic metals. Warmer summer temperatures triggered growth decline in the drought-sensitive Q. robur. Tree species subjected to high pollution levels may be more adversely impacted by hotter summer conditions. We argue that pollution-climate interactions should be more widely considered when assessing forest decline, particularly in heavily industrialized areas.