Nitrogen status of conifer needles at the alpine treeline

Abstract

Background and Aims: High elevation treelines occur worldwide at similar mean growing season temperatures. Does this result from direct impact of low temperature on growth or carbon metabolism, or does nutrient limitation, induced by low soil temperature, play a role? Similar treeline elevations at contrasting soil fertility argue against the latter, but the actual nutritional status of treeline trees (here addressed as foliage nitrogen concentration) has never been assessed systematically. Although needle nitrogen (N) concentration does not necessarily indicate growth limitation by N, the relative abundance of N would indicate obvious depletions at the treeline.

Methods: A central problem with any foliage nutrient assay is that the units for describing the element concentration are dependent on elevation themselves. Here we separate changes in N per unit tissue from changes in reference units.

Results: Needles of Pinus cembra and Picea abies in the Alps do not show elevational differences in N concentration per dry weight, water content, area or volume, thus, there is no N depletion near the elevational tree limit. Hence, nutrient supply is either unaltered, or growth is adjusted so that nutrient depletion in needle tissue does not occur.

Conclusions: Chronic N shortage at needle level is not an explanation for low tree vigour at the treeline.

Keywords:

• Alps
• high elevation
• nutrient limitation
• Picea abies
• Pinus cembra
• tree growth

Acknowledgements

We gratefully acknowledge the support provided to this project by Hermann Heilmeier (TU Freiberg), logistic help by Michael Bahn (University of Innsbruck, Austria), Gerhard Wieser (BFW, Innsbruck) and Christian Rixen (SLF, Davos, Switzerland).