Norway Spruce Shows Contrasting Changes in Below- Versus Above-Ground Carbon Partitioning towards the Alpine Treeline: Evidence from a Central European Case Study

Abstract

The marked change in above-ground forest stand structure with elevation towards the alpine treeline has been widely recognized, while studies on altitudinal effects on the root system are still scarce. We studied Norway spruce stands along a 700-m-long elevational transect at Mount Brocken (Harz Mountains, central Germany) to test the hypothesis that fine root dry mass partitioning shows an inverse response to elevation towards the treeline compared to above-ground biomass. Microclimate measurements revealed that thermal conditions at the treeline of Mount Brocken are closely comparable to other treeline sites around the world. Above-ground structure did not differ significantly among stands at lower and mid elevations, but tree height and stem biomass decreased strongly with elevation upslope. Fine root biomass increased with elevation by a factor of nearly two. Annual fine root production was found to be highest at mid elevation but was only 40% lower than this maximum at the treeline. Consequently, the ratio of fine root production to above-ground stem biomass increased by a factor of 2–3 with elevation, indicating a strong shift of below- versus above-ground carbon allocation towards the treeline. We hypothesize that the enlargement of the fine root system at cold sites represents an adaptation to the unfavorable soil conditions, such as impaired nutrient supply.

Acknowledgments

We are very grateful to the national park administration for the permission to conduct the study in the Harz National Park, and in particular to Gunter Karste for his excellent help to select the study sites. We thank Mechthild Stange and Marina Röderstein for their support of the field work. The contribution of Felix Norman to the analysis of the temperature measurement data is also gratefully acknowledged. We thank Astrid Rodriguez, Lars Köhler, and Stella Aspelmeier for helpful comments on the manuscript.