Drought Increases Carbon Allocation to Root Respiration While Maintaining Storage in Qinghai Spruce Seedlings

Forests worldwide face escalating drought threats, with their resilience and survival hinging on effective carbon (C) allocation strategies. Yet, a comprehensive understanding of how trees allocate C to different physiological processes across organs during drought remains elusive. We examined the effects of drought on the trade-offs among growth, respiration, and non-structural carbohydrates (NSC) storage in Qinghai spruce, and used a 13CO2 pulse-labelling method to determine the allocation proportions of recently assimilated C released via respiration and retained in different tree compartments and soil during an 11-day chase period. Results showed that drought significantly reduced net photosynthetic rate and foliar biomass, while increased root respiration. However, NSC reserves (starch and soluble sugars) remained constant. The reduced photosynthetic supply under drought decreased the absolute amount of excess 13C allocated to roots and soil. Nevertheless, a greater proportion of excess 13C was allocated to roots under drought likely to support the increased root respiration. Furthermore, drought did not significantly alter the C transport velocity from needles to roots. Overall, our study identified that Qinghai spruce seedlings employ a conservative allocation strategy that prioritizes the maintenance of NSC storage by suppressing growth, and proportionally allocate more recently assimilated C to roots in response to water stress.