Forest Dieback Poses a Hidden Threat to Drinking Water Quality

For centuries, forests located in drinking water protection areas have been regarded as a natural safeguard for maintaining high drinking water quality. However, the growing occurrence and severity of droughts increasingly threaten the forests’ protective function. An important event is the severe drought from 2018 to 2020 in Germany, which induced an unprecedented pulse of forest dieback. Using this event as a showcase, we provide evidence that forest dieback might jeopardize the forests’ essential role in protecting drinking water quality. Initially, we have compiled the first comprehensive overview of forest cover, forest type, dominant tree species, and canopy cover loss in all drinking water protection areas in Germany. Our results show that forests cover a substantial area of around 43% of all drinking water protection areas. During the multi-year drought of 2018-2020, an excessive fraction of approximately 5% of the canopy cover was lost. We further analyzed a sample of groundwater nitrate concentration records in drinking water protection areas with and without severe forest dieback. We show that 7 out of 13 sites with severe forest dieback showed a significant increase in groundwater nitrate concentrations. On average, nitrate concentrations in the forest dieback sites have more than doubled. In contrast, we did not observe significant changes in sites without forest dieback. Nevertheless, the variable responses in sites affected by forest dieback underscore the necessity for further research to understand the underlying mechanisms controlling resistance to nitrate leaching. Our assessment serves as an initial effort to underscore the hidden threat forest dieback might pose to our drinking water resources. This assessment highlights the need for intensified and collaborative research into how forest dieback affects water quality.