Hydraulic capacitance as a key trait regulating water potential thresholds and drought adaptation in woody species

Hydraulic capacitance reflects the ability of plant tissues to store and release water, thereby buffering plant water status during fluctuating transpiration demand. Despite its crucial roles in plant water relations, how capacitance varies across organs and biomes, and how it interacts with drought-related hydraulic thresholds, remains insufficiently understood. Here, we quantified leaf and trunk hydraulic traits, including hydraulic capacitance, turgor loss point (TLP), elastic modulus (ε), predawn and midday water potentials (Ψpd, Ψmd), as well as stem embolism resistance (P50, P88) in 13 common tree species from a northern tropical forest in China. We further integrated a global dataset of sapwood hydraulic capacitance and associated hydraulic traits for 255 woody species. Our results showed that, despite organ-level differences, hydraulic capacitance and saturated water content were significantly correlated between leaf and trunk sapwood, indicating integrated investment in water storage across organs. By contrast, leaf and trunk sapwood TLPs were decoupled, suggesting organ-specific optimization of drought adaptation. In trunks, Phase I hydraulic capacitance supplied nearly half of the total stored water for northern tropical trees, playing a critical role in buffering water potential fluctuations. At the global scale, stem sapwood capacitance increased with climatic water availability and was correlated significantly with Ψmd, TLP and P50, underscoring its role in underpinning drought adaptation across biomes. Our findings highlight that hydraulic capacitance is a key integrative trait linking organ-level water storage to whole-plant drought tolerance, representing an additional, fundamental strategy of plant adaptation to water stress across biomes.