Scheibler, D. (2015)

Nitrogen Deposition and Forest Clearings in Wood Warbler Habitats in Switzerland

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University of Zurich




Introduction: Atmospheric nitrogen deposition greatly affects ecosystems and especially forests. Critical loads of nitrogen deposition were exceeded in over 95 % of the forest area of Switzerland in 2007, and had been for several decades before. Forest inhabitants are exposed to chemical pollution as well as to other factors like forest management, which both change over time. The wood warbler Phylloscopus sibilatrix is a trans-Sahara migratory bird species which breeds on sparsely vegetated forest floors of middle-aged forests with a closed canopy in Switzerland. The populations of the wood warbler have declined in western Europe in the last 30 years. There are up to now only few possible explanations for this decline. This thesis addresses the possibility that excess atmospheric nitrogen deposition and forest clearings may have contributed to this decline.
Method: Swiss bird observation data was used from kilometer squares (KmS) which were monitored in 1993-1996 (period 1) and 2011-2014 (period 2). The KmS were grouped by occupancy: KmS occupied in period 1, but abandoned in period 2 (Case 0), KmS occupied in both periods (Case 1), and KmS occupied in period 2, but not in period 1 (Case 2). Thirty-eight kilometer squares per Case were selected and investigated for "mean nitrogen deposition" and "mean exceedance of the critical load" between 1990 and 2010. Further, the "proportion of forest clearings" and the "distance to the next forest clearing" in wood warbler breeding habitats which occurred before and between periods 1 and 2 were examined from orthophotos. Finally, the variables "elevation" above sea level and "site quality" (productivity of a forest stand) were also included in the analysis. All variables were tested for correlations and in MANOVAs.
Results: There were significant negative correlations between the variables "elevation" and "mean nitrogen deposition", between "elevation" and "distance to the next forest clearing" as well as between "elevation" and "site quality". Further, mean exceedance of the critical load significantly differed among the three cases and explained 14 % of the variation (R2 ad) in a follow-up ANOVA. Regarding nitrogen deposition, Case 2 KmS had the lowest mean nitrogen deposition and exceedance of the critical load, followed by KmS of Case 1 and 0. The variable "distance to the next forest clearing" also differed among the cases (R2 ad = 6 %) and was lowest for Case 1 followed by Case 2 and 0.
Discussion: These findings suggest that wood warbler breeding habitats at low elevations above sea level are at a higher risk of becoming unsuitable for the wood warbler because critical loads of the nutrient nitrogen are more often exceeded and forest clearings are more common than in habitats at higher elevations. Since wood warblers prefer habitats with good site quality (which have higher population densities than forest stands with a poor site quality) and such habitats are mostly located at low elevations, it is likely that the wood warbler suffered a loss of suitable habitats in Switzerland. Possible mechanisms of how nitrogen deposition and forest clearings could have affected wood warbler habitats in Switzerland are discussed, and management options are suggested to provide suitable wood warbler habitats in the future.
Keywords: Wood warbler, nitrogen deposition, forest management, critical loads of the nutrient nitrogen