Weather and climate controls over the seasonal carbon isotope dynamics of sugars from subalpine forest trees

We examined the environmental variables that influence the Δ¹³C value of needle and phloem sugars in trees in a subalpine forest. We collected sugars from Pinus contorta, Picea engelmannii and Abies lasiocarpa from 2006 to 2008. Phloem and needle sugars were enriched in ¹³C during the autumn, winter and early spring, but depleted during the growing season. We hypothesized that the late-winter and early-spring ¹³C enrichment was due to the mobilization of carbon assimilated the previous autumn; however, needle starch concentrations were completely exhausted by autumn, and we observed evidence of new starch production during episodic warm weather events during the winter and early-spring. Instead, we found that ¹³C enrichment was best explained by the occurrence of cold night-time temperatures. We also observed seasonal decoupling in the ¹³C/¹²C ratios of needle and phloem sugars. We hypothesized that this was due to seasonally-changing source-sink patterns, which drove carbon translocation from the needles towards the roots early in the season, before bud break, but from the roots towards the needles later in the season, after bud break. Overall, our results demonstrate that the ¹³C/¹²C ratio of recently-assimilated sugars can provide a sensitive record of the short-term coupling between climate and tree physiology.