G05_P09

Why do tree tissues have older radiocarbon ages than chronological ages?

Hilman B¹,  Solly E², Hagedorn F³, Kuhlman I¹, Herrera-Ramírez D¹, Trumbore S¹

¹Max-planck society, Jena, Germany, ²ETH Zurich, Zurich, Switzerland, ³Swiss Federal Institute for Forest, Snow and Landscape Research WSL, Birmensdorf, Switzerland

Tree tissues and especially fine roots (≤ 2mm) often have older ¹⁴C ages than chronological ages (¹⁴C excess). The common explanation is that stored and old non-structural carbon (NSC) contributes to new tissue growth. Accordingly, the ¹⁴C excess increases with the proportion of stored vs. recently fixed NSC in the growth substrate. Here we suggest that rather than this proportion, what determines the ¹⁴C excess is the turnover of the NSC pool. We demonstrate this using measurements of needles, branches, and fine roots of two coniferous trees in a treeline ecotone in Stillberg, Switzerland. In such ecotones the non-structural carbohydrates (NSCarb) stocks increase with elevation, probably due to the fact that low temperatures suppress growth more strongly than photosynthesis. In addition, we did not expect large variations in the proportion of stored NSC in the growth substrate. We expected initially that increasing NSCarb stocks with elevation is a proxy for slower turnover rates and therefore greater ¹⁴C excess. However, we observed the opposite trend in the fine roots with turnover intensification and smaller excess ¹⁴C towards the treeline, while ¹⁴C excess in aboveground tissues did not vary with elevation. Based on current and previous results we conclude that the greater surplus of photo-assimilates in the treeline increases fluxes to both NSCarb and to belowground. The greater flux belowground speeds the turnover rate of the roots NSC and lowers its ¹⁴C excess.