Impact of pollution on the temperature sensitivity of multiple Norway spruce tree-ring parameters in Central Europe

Central European forests experienced high rates of air pollution in the second half of the 20th century, especially along the borders of Czechia, Germany and Poland. Consequently, tree-growth declines were detected in heavily polluted forests. However, information about how pollution has influenced growth-climate responses beyond tree-ring width (RW) in pollution-affected forests remains sparse. In this study, we investigated the impact of high-level pollution during 1960s-1980s in Central Europe on the climatic signals of various tree-ring parameters of Norway spruce, including RW, latewood Blue Intensity (LWBI), and maximum cell wall thickness (CWT), to understand how tree growth and climatic sensitivity were affected. Tree-ring cores were collected from six temperature-limited high-elevation sites within four pollution-affected regions in Czechia and northern Slovakia. RW and LWBI were measured for all samples and CWT was produced from two sites with contrasting pollution impacts. Distinct pollution-related RW growth suppression was detected in 1970s to 1980s at several sites. LWBI and CWT chronologies were highly correlated (r LWBI = 0.52-0.75; r CWT = 0.63-0.68) with growing season (April-September) temperature and did not exhibit clear signs of distortion by pollution compared to RW (r RW = 0.28-0.58). Pollution stress seemed to reduce tree growth by decreasing cell numbers and made RW less sensitive to climate. This study reveals that impacts of pollution on different tree-ring parameters varied which can further influence their climatic sensitivities. It provides valuable insight in improving the utility of pollution-affected tree-ring chronologies by choosing appropriate parameters, which can ultimately contribute to substantially improving the calibration of climate reconstructions from heavily polluted regions.