Global patterns of mobile carbon partitioning in mountain trees in response to elevation

Non-structural carbohydrate (NSC), a major substrate of primary metabolism, has been implicated in mediating cold-induced termination of tree growth, particularly in the alpine environment. Despite its functional significance, NSC pattern as well as its partitioning between sugar and starch in response to elevation across the globe remains unclear. Here, we compiled data from 67 studies encompassing 90 elevational transects worldwide and conducted a meta-analysis to evaluate the responses of NSC (including sugar, starch, and total NSC concentrations) as well as sugar:starch ratio in mountain trees to increasing elevation at organ scale. Our results showed that declining temperature with increasing elevation promoted NSC accumulation in aboveground organs, but did not affect the sugar:starch ratio in all organs analysed, implying that the remarkably significant role of NSC in both short-term cellular metabolism and long-term recalcitrant storage. Trees grown within warmer mountain regions (i.e., regions with relatively higher mean temperature during the growing season) stored more NSC in leaf in response to elevation than those grown within colder mountain regions. We also found that the observed increasing pattern of NSC with elevation occurred only in treeline tree species but not in non-treeline trees (i.e., tree taxa that do not reach the alpine treeline). Additionally, a significant negative effect of elevation on NSC revealed an unbalanced carbon supply in the roots of angiosperms tree species, indicating that the growth inhibition of belowground organ with increasing elevation occurred mainly in angiosperms rather than in gymnosperms. Our findings suggest that a sugar-starch partitioning pattern, which remains essentially stable over a wide range of climates and functional type species, predominately modulates the balance of carbon assimilates between supply and demand in responding to cold stress in the mountain trees worldwide.