Hydraulic, Gas-Exchange and Leaf-Trait Variation Across Hydrologically Contrasting Sites and Tree Sizes in Populus euphratica.

Plant water status, hydraulic transport and carbon exchange can respond differently across natural hydrologic contrasts. We measured hydraulic, gas-exchange, carbon, morphological and anatomical traits in 33 Populus euphratica trees distributed across four fixed sites (D1-D4) and three tree-size classes. Site and size associations were evaluated with additive models, heteroskedasticity-consistent inference, false-discovery-rate control and a balanced D1-D3 sensitivity analysis. Midday leaf water potential became progressively more negative from D1 to D4, whereas branch xylem hydraulic conductivity was higher at D3-D4. The correlated instantaneous responses of assimilation (A), stomatal conductance (gs), and transpiration (E) peaked at D3 and declined at D4. Within the balanced D1-D3 subset, site effects for midday water potential, hydraulic conductivity and the A-gs-E response group remained supported. Larger trees had lower specific leaf area, soluble sugar and total non-structural carbohydrates, but higher leaf water content and palisade tissue thickness; starch showed a consistent site association. After adjustment for site and size, no anatomy-function association based on total palisade tissue thickness (PT) or epidermis thickness (Epi) retained false-discovery-rate support. Hydrologic context and tree size were associated with partly distinct functional dimensions, indicating multidimensional and context-dependent intraspecific variation in P. euphratica.