Size-related decline in dryland shrubs is related to reductions in hydraulic efficiency and carbon assimilation and not nonstructural carbohydrate depletion.

Plant growth and survival are fundamentally constrained by water transport from roots to leaves, impacting carbon assimilation and associated labile carbon pools. However, physiological constraints on growth and survival vary with plant age, due to changes in metabolic sinks and increases in hydraulic path length from rhizosphere to canopy. We investigated crown dieback, growth, hydraulics, carbon assimilation and nonstructural carbohydrate (NSC) storage in relation to increasing basal diameter of two dominant shrub species (Caragana korshinskii and Artemisia ordosica) at the southeastern edge of the Tengger Desert, China. The aim was to identify mechanisms of decreased performance with plant size in dryland shrubs. Clear contrasts in stomatal regulation of leaf water potentials were detected between species. Despite these contrasts, radial growth, hydraulic transport efficiency (Ks), and carbon assimilation similarly declined in both species with increasing plant size, while NSC reserves remained unchanged. Xylem embolism (percentage loss of conductivity) increased with plant size, resulting in significant reductions in carbon assimilation in both species. Results indicate that hydraulic and potentially carbon assimilation constraints, rather than NSC depletion, govern growth-related dryland shrub decline. These findings improve our understanding of how population demography impacts dryland forest response to climate change.