Forest dieback in drinking water protection areas – a hidden threat to water quality

For centuries, forests have been considered a natural safeguard for drinking water quality. We challenge this view in light of the rising frequency of climate extremes, such as droughts coinciding with high temperatures, which have caused unprecedented pulses of forest dieback globally. Drought-induced forest diebacks may jeopardize the crucial role of forests 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 (assessed as canopy cover loss) across drinking Water Protection Areas (WPAs) in Germany, one of the countries hit most severely 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 5% canopy cover got lost within only three years as a direct or indirect consequence of the drought. Spruce-dominated forests, constituting 28% of all forests in WPAs, 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 quality. On average, nitrate concentrations more than doubled in WPAs with severe forest dieback, whereas nitrate 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 prediction, adaptation, and mitigation strategies. We call for interdisciplinary research addressing the hidden threat forest dieback poses for our drinking water resources.