Prioritized carbon allocation to storage of different functional types of species at the upper range limits is driven...

The upper elevational range limit of tree species (including treeline and non-treeline species) is generally considered to result from either carbon limitation or sink limitation. Some evidence also suggests that tree line might reflect preferential carbon allocation to NSC storage at the expense of growth. How might the importance of these potential mechanisms be determined? We used an elevational gradient to examine light-saturated photosynthesis (Asat) and NSC concentrations in plant tissues of three different functional types of tree species. We also examined the effects of consecutive 4 years of in situ defoliation on growth and NSCs at the upper elevational range limit. Declining temperature with increasing elevation did not reduce Asat in any of the species. We found NSC increased with elevation in major storage tissues (e.g., roots and twigs) but not in leaves. The defoliation showed that C storage took priority over growth. Such preferential carbon allocation, directly caused by growth decline, always existed in the deciduous tree species. In the evergreen tree species, however, growth decline resulted from preferential carbon allocation to storage was only detected in 2017 and then disappeared as the intensity of defoliation increased. Our results showed that trees prioritized sustaining stores of C more highly than allocation of growth, regardless of the trees' C or sink limitations. At the cold range limits, the prioritized carbon allocation to storage in deciduous tree species was in response to low temperature stress, while in evergreen tree species, the prioritization of carbon allocation was only a transient physiological response to defoliation disturbances.