Height effects on hydraulic and photosynthesis explain Hippophae rhamnoides decline on the Qinghai–Tibetan plateau

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BACKGROUND AND AIMS: Worldwide, climate change and human activities are accelerating the decline and mortality of adult trees, largely driven by hydraulic dysfunction. While the hydraulic limitation hypothesis (HLH) is well documented in tall trees, its relevance to the decline of tall shrubs remains poorly understood. Although widely distributed across China, the dioecious shrub Hippophae rhamnoides is experiencing widespread decline. However, the role of height-related hydraulic constraints in shrub decline, especially among dioecious species, is still unclear. METHODS: To evaluate potential hydraulic constraints, we investigated natural H. rhamnoides on the Qinghai-Tibetan Plateau (QTP), measuring stem hydraulic conductivity, leaf gas exchange parameters and xylem anatomy across individuals of different heights using standard hydraulic and microscopy techniques. This approach enabled assessment of height-related variation in hydraulic efficiency and carbon fixation. KEY RESULTS: Sapwood-specific hydraulic conductivity (Kₛ) and net photosynthetic rate (Pn) decreased with increasing height, which supports the HLH in shrubs. However, stomatal conductance (gs) remained stable with increasing height, a pattern inconsistent with the stomatal regulation predicted by the HLH. Moreover, the reduction in hydraulic conductivity correlates with increased water potential (Ψ), lower vessel density and larger hydraulically weighted mean vessel diameter (Dₕ). The structural equation model revealed a height-driven compensatory mechanism: reduced sapwood density resulting from height growth increased Kₛ and Pn, enabling H. rhamnoides to survive for decades after decay. During the decline stage, taller males exhibited lower Kₛ, leaf-specific hydraulic conductivity (Kl) and sapwood density, yet higher Ψ, than females, indicating sexually dimorphic hydraulic traits associated with height differences. CONCLUSIONS: Despite compensatory adjustments, height-related hydraulic constraints and reduced photosynthesis drive the H. rhamnoides decline, with hydraulic sexual dimorphism linked to height divergence. Our results provide a theoretical basis for tall shrub decline and hydraulic sexual dimorphism, supporting vegetation restoration and conservation strategies.

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