Forest Dieback in Drinking Water Protection Areas—A Hidden Threat to Water Quality

For centuries, forests have been considered a safeguard for drinking water quality. However, unprecedented pulses of forest dieback globally caused by the rising frequency and intensity of droughts may jeopardize the forests' crucial role in protecting water quality, potentially even turning forests into sources of contamination. To underscore the critical importance of the topic, here we provide the first comprehensive assessment of forest cover, type, and dieback across drinking Water Protection Areas (WPAs) in Germany, one of the countries hit by the unprecedented Central European drought of 2018–2020. Our findings reveal a high forest cover of 43% in WPAs, from which a substantial amount of 4.8% canopy cover got lost within only 3 years. Spruce-dominated forests were particularly susceptible, but other dominant tree species also experienced anomalously high mortality rates. Combining this assessment with exemplary records of nitrate concentrations in the groundwater of WPAs revealed that forest dieback can significantly impair drinking water quality. On average, nitrate concentrations more than doubled in WPAs with severe forest dieback, whereas concentrations did not significantly change in undisturbed WPAs. However, we also found pronounced differences between WPAs affected by forest dieback, underlining the need for further data and research to derive a generalizable understanding of the underlying mechanisms and controls. Based on this assessment, we deduce critical data and knowledge gaps essential to developing well-informed adaptation and mitigation strategies. We call for interdisciplinary research addressing the hidden threat forest dieback poses for our drinking water resources.