Treeline changes and soil organic matter cycling in the Ural mountains

Historic photographs document that treelines have shifted by 40 to 80 m during the last century. Our first results suggest that the upward shift in treeline has small net effects on C storage in soils, but speeds up SOM cycling and net N mineralization, which in turn might stimulate plant growth and thus C sequestration in tree biomass.

Treeline in the Southern Ural. Foto: Frank Hagedorn.

Historic photographs of treelines in the Southern Ural (1929, below 1999). Source: Stepan Shiyatov.

Historic photographs of treelines in the Southern Ural (1999, above 1929). Source: Stepan Shiyatov.

Rationale

Treelines are natural boundary ecosystems where dominant plant species, life forms and plant productivity change drastically within a small altitudinal gradient and a short distance. In the Ural mountains, historic fixed-point landscape photographs document that positions of treeline have moved up by 60 to 80 m in altitude and that the forest of the treeline ecotone has become denser during the last century (Moiseev & Shiyatov, 2003). These changes of the forest-tundra ecotone very likely result from a changing climate since treelines positions are thought to be limited by vegetation period temperature and the highest mountains of the Urals had never been impacted by humans. Similar up- and northward shifts of treeline ecotones have been reported from North America, Scandinavia and Siberia, showing that these climate-induced advances occur in large areas of the Northern Hemisphere.
The advancing forest will alter the cycling and sequestration of C. It will increase C storage in forest biomass. Its effects on soil carbon, however, are less certain, since the upward-shifting treeline ecotone will not only change the quantity and quality of C inputs into soils, but also lead to a more favourable microclimate, which may stimulate respiration losses from soils.

Aims

In an EU-INTAS-project we are investigating how the observed upward shift of the treeline ecotone affects biomass pools and soil C and N dynamics.

Methods

In our space for time-approach we determine biomass stocks, soil C and N pools as well as SOM quality along altitudinal gradients in the Southern Northern and Polar Urals, assuming that ecosystems at different altitudes reflect different stages of the upward shifting forest-tundra ecotone.

Partners

• Institute of Plant and Animal Ecology (IPAE), Ekaterinburg (S. Shijatov, P. Moiseev, V. Mazepa)
• University of Halle-Wittenberg (G. Guggenberger)
• ETH Zürich (H. Bugmann)
• Andreas Rigling, Niklaus Zimmermann (both WSL) 
  

Results

First results suggest that an upward shift in treeline has small net effects on C storage in soils, but speeds up SOM cycling and net N mineralization, which in turn might stimulate plant growth and thus C sequestration in tree biomass.