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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.

 

 

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; Devi et al., 2008). 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 soil microbial communities, and the carbon and nutrient cycling. 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 ERA.NetRus-project we are investigating how the observed upward shift of the treeline ecotone affects biomass pools, above-and belowground diversity and soil C and N dynamics.

Methods

In our space for time-approach we determine biomass stocks, plant diversity, microbial communities, 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.

Results

Upward expansion occurs along the 1500 km long Ural mountains. Results indicate a change in tree growth forms during the last century from creeping krummholz to vertical single, stem trees. The primary climatic change is winter climate with more snowfall. The upward shift in treeline leads to a slow increase in tree biomass and has small net effects on C storage in soils. However, SOM cycling and net N mineralization is speeded up, which in turn might stimulate plant growth and thus C sequestration in tree biomass.

 

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