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Shrub Expansion of Alnus viridis Drives Former Montane Grassland into Nitrogen Saturation

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Abstract

The N2-fixing shrub Alnus viridis is currently encroaching on montane grasslands in the Alps as a result of reduced land management and complete abandonment. Alnus introduces large amounts of nitrogen (N) into these formerly N-poor grasslands and restricts the succession to montane forests. We studied pools and fluxes of N and the associated C pools in pastures (controls) and adjacent Alnus shrublands at two elevations (1650 versus 1950 m a.s.l.) in three valleys in the Swiss central Alps. The total N and C pools stored in 50-year-old Alnus shrubland did not exceed those in adjacent pastures with a total of approximately 610 g N m−2 in phytomass plus soil (down to 30 cm) at both elevations. In Alnus stands, reduced soil N pools balanced the gain in phytomass N pools, a likely result of a faster turnover of soil N. The soil solution under Alnus was continuously enriched with nitrate, with a total N leaching of 0.79 g N m−2 season−1 (June–October) under 50-year-old stands at both elevations and the highest flux of 1.76 g N m−2 season−1 in 25-year-old shrubland at low elevation, clearly indicating an excess of available N in Alnus shrubland. In contrast, N leaching across all pastures was close to zero (0.08 g N m−2) throughout the season. At the catchment scale, streamlet water showed increased nitrate concentrations with typical flushing peaks in spring and autumn, provided more than one fifth of the catchment area was covered by Alnus shrubs. We conclude that the expansion of Alnus rapidly converts centuries-old, N-poor grassland into N saturated shrubland, irrespective of elevation, and it reduces the C storage potential of the landscape because the Alnus dominance constrains re-establishment of a natural montane forest.

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Acknowledgements

This study was funded by the Mercator Foundation Switzerland within a Zürich-Basel Plant Science Center PhD Fellowship program. The Korporation Ursern and the Alpine Research and Education Station Furka (ALPFOR) supported the fieldwork. A. Alaoui and S. Lagger (University of Bern) provided discharge data of the micro-catchments (interdisciplinary SNF project VALURsern, CR30I3–124809/1 to C. K. and others).

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    1. Department of Environmental Sciences, Institute of Botany, University of Basel, Schönbeinstrasse 6, 4056, Basel, Switzerland

      Tobias Bühlmann, Christian Körner & Erika Hiltbrunner

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    1. Tobias Bühlmann
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    Correspondence to Tobias Bühlmann.

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    The study was designed by Erika Hiltbrunner and Christian Körner, who also supervised Tobias Bühlmann. Tobias Bühlmann and Erika Hiltbrunner collected, prepared and analysed the data. All authors (Tobias Bühlmann, Erika Hiltbrunner, Christian Körner) contributed to the manuscript.

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    Bühlmann, T., Körner, C. & Hiltbrunner, E. Shrub Expansion of Alnus viridis Drives Former Montane Grassland into Nitrogen Saturation. Ecosystems 19, 968–985 (2016). https://doi.org/10.1007/s10021-016-9979-9

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