Role of storage reserves in new tissue growth quantified using bomb 14C at the alpine treeline

The bomb radiocarbon approach allows to estimate the time elapsed since carbon (C) fixation and thus to study the use of stored C. Previous studies show that fine roots (≤ 2 mm), a large and dynamic C pool in trees, are constructed from C that was fixed years to more than a decade previously. In comparison, aboveground tissues grow from C mostly < several years old. Our current understanding is that ¹⁴C ages of tree tissues mainly reflect the ¹⁴C content of the C substrate which supports their growth. We addressed the question – what explains the variations in ¹⁴C age of C supporting growth among tissues and between trees?

We compared tissue chronological ages determined from annual-ring count (roots) or observations (needles, branches) with ¹⁴C-based C ages along an elevational gradient near the Alpine tree line (Stillberg, Davos, Switzerland). Previous studies have shown that decreasing temperatures along this gradient limit tree growth. Carbon assimilation via photosynthesis is less sensitive to low temperatures, resulting in accumulation of storage compounds in tree line trees. We therefore expected stored C in trees at the tree line to have the slowest turnover rates and oldest C; i.e. that the age difference between ¹⁴C and chronological age would increase with elevation.

While needles and branch wood tissues were produced from 0-2 yr C, fine roots were produced from C fixed up to a decade ago. Contrary to our prediction, the C age in the roots decreased from 9.5 yr 150 m below the tree line to 4.5 yr at the tree line. Relatively unstressed trees in a site located 600 m lower used the youngest C to build roots (1.5 yr). But the old C ages cannot be explained by a complete decoupling from fresh C, as the roots contained a metabolically active pool with ages of 0-1.5 yr used for root respiration. Previous studies showed that root growth commences before shoot growth and mostly occurs earlier in the growing season. It is therefore plausible that during root growth the reliance on old stored C is greater than at the end of the summer when sampling took place. Yet, the timing of root growth alone cannot explain the elevation trend in the age of C used to build fine roots. The carbon allocation model we adopted suggested that roots are built from older C when the turnover time of the root storage declines. In agreement, the soluble storage compounds below the tree line had the oldest ¹⁴C ages and the slowest turnover times.

Overall, the oldest belowground C reserves were found at intermediate elevations, where growth limitation was slightly eased, and perhaps a larger proportion of fresh C assimilates were used for the growth of aboveground tissues rather than fine roots. Larger amounts of fresh C are allocated to the belowground when C availability is in surplus either when conditions are favorable (at the lowest elevation) or when growth rates are small compared to C assimilation (at treeline).