Tracing fresh assimilates through Larix decidua exposed to elevated CO₂ and soil warming at the alpine treeline using compound-specific stable isotope analysis.

How will carbon source-sink relations of 35-yr-old larch trees (Larix decidua) at the alpine treeline respond to changes in atmospheric CO(2) and climate? We evaluated the effects of previously elevated CO(2) concentrations (9 yr, 580 ppm, ended the previous season) and ongoing soil warming (4 yr, + 4°C). Larch branches were pulse labeled (50 at% (13)CO(2)) in July 2010 to trace fresh assimilates through tissues (buds, needles, bark and wood) and non-structural carbon compounds (NCC; starch, lipids, individual sugars) using compound-specific isotope analysis. Nine years of elevated CO(2) did not lead to increased NCC concentrations, nor did soil warming increase NCC transfer velocities. By contrast, we found slower transfer velocities and higher NCC concentrations than reported in the literature for lowland larch. As a result of low dilution with older carbon, sucrose and glucose showed the highest maximum (13)C labels, whereas labels were lower for starch, lipids and pinitol. Label residence times in needles were shorter for sucrose and starch (c. 2 d) than for glucose (c. 6 d). Although our treatments showed no persistent effect on larch carbon relations, low temperature at high altitudes clearly induced a limitation of sink activities (growth, respiration, root exudation), expressed in slower carbon transfer and higher NCC concentrations.