Patterns and drivers of intraspecific variation in xylem anatomy of <i>Reaumuria songarica</i> using inter-vessel pits as a case study

In the context of global warming and increasing drought, clarifying intraspecific variation in plant xylem hydraulic traits and their ecological adaptation strategies is crucial for predicting vegetation stability and functional maintenance in arid ecosystems. This study investigated Reaumuria songarica, a widely distributed and highly drought-tolerant shrub native to the desert regions of northwestern China. By integrating quantitative anatomical analyses of xylem vessels and inter-vessel pits with trait network modelling, we systematically assessed the variation patterns and environmental responses of its hydraulic structural traits along an aridity gradient. The results demonstrated that: (i) R. songarica exhibits substantial intraspecific variation in hydraulic traits, with individuals in hyper-arid regions possessing larger pit aperture areas and higher pit densities, indicative of an 'opportunistic' water-use strategy that facilitates rapid responses to episodic rainfall events; (ii) different traits exhibited distinct responses to temperature, precipitation and soil conditions, with tissue-level traits being more responsive to temperature, while pit-level traits were primarily influenced by precipitation and soil factors; and (iii) vessels and pits displayed significant coordinated variation at the anatomical level. Trait network analysis further revealed that the topological structure of hydraulic traits was substantially reorganized along the aridity gradient, transitioning from a highly coordinated strategy centered on embolism resistance in arid regions to a structure-stabilization strategy centered on vessel wall thickness in hyper-arid regions. This study provides clear evidence of intraspecific variation in pit structure in R. songarica and uncovers the coordinated regulatory mechanism between pit and vessel structures. These findings offer microstructural insights into hydraulic adaptation strategies in desert plants and contribute theoretical support for drought-tolerant shrub selection and ecological restoration in arid regions.