Change, stability, and atmospheric pollutant effects in European forest vegetation

Abstract: While the “acid rain” of the 1980s caused widespread damage to forests, the sulphur emissions responsible have been much reduced since. However nitrogen emissions remain at concerning levels, and there are also questions about how well ecosystems recover even after air quality improves. By using data from long-term monitoring projects, I investigated how understorey vegetation communities respond to disturbances including atmospheric pollutant effects, and how the concepts of ecological stability and resilience can help us understand this.  First, the natural experiment of extreme natural disturbances at a monitoring site showed that surviving refuge areas act as “ecological memory” and contribute to resilience. Then I focused on lichens and bryophytes, which are known to be sensitive to air pollution. For lichens in Sweden, only limited recovery was found despite improved air quality, which may be due to a lack of nearby source populations to act as refuges analogous to those in the first study. Using data from sites across Europe I found adverse effects of nitrogen deposition in bryophyte communities. Finally, I tracked the stability of vegetation communities over time and found that the extreme disturbances in the first study were clearly visible but that the specific effects of atmospheric pollutants could not be seen in the vegetation community as a whole, despite the effects earlier found in the most sensitive parts of the community (lichens and bryophytes). These results highlight the importance of looking at sensitive sub-groups when looking for atmospheric pollutant effects, and the importance of long-term monitoring data in investigating these questions.