Forests are often associated with clean water, but a study of Douglas-fir plantations in France and Luxembourg has found that the trees can be linked to much higher nitrate levels in nearby streams and groundwater. Researchers examined paired headwater catchments with different land covers, including Douglas-fir, spruce, deciduous forests, grassland, and clearcuts. The catchments were monitored for three to five years to understand how Douglas-fir affected water chemistry. The study published in ‘ Environmental Pollution ’ found that streams draining Douglas-fir catchments had nitrate concentrations two to ten times higher than streams draining other forest and grassland areas. Groundwater beneath Douglas-fir also recorded nitrate concentrations reaching 50 milligrams per litre.
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