Bark-associated yeasts and their potential role as tree carbon sink at the treeline of the southern Andes.

The upper elevation of the tree life form (treeline) is explained by temperature limitations in the carbon (C) investment in biomass (the growth limitation hypothesis, GLH). The GLH predicts that tissue concentrations of non-structural carbohydrates (NSC) increase with elevation. This prediction has received mixed support in deciduous species. In addition, other potentially relevant C sources (e.g., inner bark) and sinks (e.g., bark microbiome) have not been considered. We assessed the year-round NSC concentrations in the inner bark, roots and branches of Nothofagus pumilio at the treeline and 200 m below it in the southern Andes of Chile. We furthermore quantified the abundance of bark-associated yeasts and evaluated the relative importance of inner bark's starch and soluble sugars (SS), tree size and seasonality as predictors of the yeast abundance. NSC, starch and SS concentrations decreased in springtime in both elevations. In late spring, NSC, starch and SS concentrations were significantly lower at the treeline than below the treeline for all organs and most so for the inner bark. Bark's starch and SS concentrations were the best predictors of yeast abundance. At the treeline, yeast abundance was highly predicted by the month, peaking in spring. We found support for C limitation in the treeline formation of N. pumilio in late spring, when C reserves were at their minimum concentrations and yeasts' abundance was at their maximum. Given that yeast abundance was mostly predicted by starch and SS concentrations, our results suggest that yeasts act as a C sink, particularly at the treeline.