Long lasting nitrate leaching after bark beetle attack in the highlands of the Bavarian Forest National Park

During the past decade bark beetle (Ips typographus) attacks killed nearly all of the Norway spruce [Picea abies (L.) Karst.] stands in the unmanaged zone in the highlands of the Bavarian Forest National Park. This study was conducted to predict if and how long the catastrophic event might cause elevated nitrate (NO3-) concentration in seepage water, and if the presence of ground vegetation may reduce NO3- leaching. A chronosequence approach was used to investigate NO3- leaching before and after the death of trees. Additionally, the impact of ground vegetation coverage on NO3- leaching was determined. Flux weighted yearly NO3- concentrations were significantly elevated in the first 5 yr after the dieback compared with intact stands (27 mu mol(c) L-1), with highest concentrations in the fifth year after the dieback (579 mu mol(c) L-1). Lowest NO3- concentrations were observed 17 yr after the dieback (10 mu mol, L-1). Suction cups in places without ground vegetation showed significantly higher NO3- concentrations of 163 to 727 mu mol(c) L-1 (Year 2-5 after the dieback) than suction cups without vegetation. However, net uptake of N by ground vegetation observed during the first 7 yr after the dieback was low on a plot scale. Compared with other severe disturbances in forests, NO3- concentrations were elevated for a longer period. Due to high rates of precipitation, NO3- dilution occurred and concentrations remained mostly below the European critical level for drinking water. Part of the observed heterogeneity in NO3- concentrations could be attributed to different patterns of ground vegetation coverage.