Short-term carbon allocation dynamics in subalpine dwarf shrubs and their responses to experimental summer drought

Carbon (C) allocation is a key determinant of plant functioning and ecosystem processes. Its dynamic responses to environmental conditions and global change are still poorly understood. This holds particularly true for dwarf shrubs, which are an important component of high mountain vegetation. We studied the short-term C allocation dynamics in two dominant dwarf shrub species, Vaccinium myrtillus and Vaccinium uliginosum, at two subalpine sites, in the Pyrenees and in the Alps, and additionally performed an experimental rain exclusion to simulate summer drought at the site in the Alps. We compared net assimilation rates (Amax), stomatal conductance (gs) and shoot water potentials (ΨS) between the two species and carried out a 13C pulse labelling experiment to trace their C allocation dynamics, both under natural conditions and in the drought experiment. Under ambient conditions, V. uliginosum showed higher Amax and gs and used recent assimilates as a respiratory substrate more slowly than V. myrtillus. C allocation dynamics in current-year shoots differed significantly between the species, showing a progressive decrease after the pulse labelling in V. uliginosum shoots but a progressive increase in V. myrtillus shoots. In the drought experiment, a significant reduction of soil moisture caused predawn ΨS below −1.2 MPa in both species, which is considered close to the turgor loss point. However, the stomatal conductance and the speed of transfer of newly assimilated C to belowground organs decreased in V. uliginosum only. We found a strong coupling in C allocation dynamics between xylem and phloem tissues of the rhizome, which was maintained under drought. Although both species were largely tolerant to the drought imposed, our results indicate a higher drought sensitivity of V. uliginosum compared to V. myrtillus, with potential implications for community composition and ecosystem C cycling in a future climate.