Altitudinal increase of mobile carbon pools in Pinus cembra suggests sink limitation of growth at the Swiss treeline

Low temperature driven carbon shortage is often assumed to explain slow growth and treeline formation at high elevations. To test this hypothesis, we analysed mobile carbon pools in Pinus cembra across the treeline ecotone in the Swiss Alps. Concentrations of non-structural carbohydrates (NSC) in needles, branches, stems and roots, as well as lipids (acylglycerols) in all woody tissues were measured throughout the growing season. Starch was the most prominent non- structural carbon compound in needles, whereas lipids represented 50-75% of the mobile carbon compounds in wood. The relative seasonal variation of the lipid fraction was very small, but due to the high absolute amount of lipids, the annual variability of carbon in lipids exceeded that of NSC in woody tissues. Mobile carbon compounds were highly abundant throughout the year and were never significantly depleted. Across a 110 m altitudinal transect from timberline to the uppermost site of tree existence, NSC and lipid concentrations generally increased. This trend became even more pronounced when the increasing structural density of tissues at higher elevations was accounted for. An estimation of the whole tree mobile carbon concentration (fraction of mobile carbon compounds within the whole tree biomass) also revealed an increasing trend of NSC and lipid pools with elevation. We therefore conclude that carbon limitation is unlikely to be responsible for reduced tree growth at the alpine treeline studied. Increased concentrations of NSC and lipids at the upper tree limit rather suggest that sink activity is limited. Hence, treeline formation is most likely the result of a direct thermal restriction of tissue formation (investment in structures) under otherwise sufficient carbon assimilation during the growing season.