Intraspecific Variability of Xylem Hydraulic Traits of Calligonum mongolicum Growing in the Desert of Northern Xinjiang, China

Plant hydraulic traits are essential for understanding and predicting plant drought resistance. Investigations into the mechanisms of the xylem anatomical traits of desert shrubs in response to climate can help us to understand plant survival strategies in extreme environments. This study examined the xylem anatomical traits and related functional traits of the branches of seven <i>Calligonum mongolicum</i> populations along a precipitation gradient, to explore their adaptive responses to climatic factors. We found that (1) the vessel diameter (<i>D</i>), vessel diameter contributing to 95% of hydraulic conductivity (<i>D</i>₉₅), hydraulic weighted vessel diameter (<i>Dh</i>), vessel density (<i>VD</i>), percentage of conductive area (<i>CA</i>), thickness-to-span ratio of vessels ((<i>t/b</i>)²), and theoretical hydraulic conductivity (<i>K_th</i>) varied significantly across sites, while the vessel group index (<i>V_g</i>), wood density (<i>WD</i>), and vulnerability index (<i>VI</i>) showed no significant differences. (2) Principal component analysis revealed that efficiency-related traits (<i>K_th</i>, <i>D_h</i>, <i>D</i>₉₅) and safety-related traits (<i>VI</i>, <i>VD</i>, inter-wall thickness of the vessel (<i>t</i>)) were the primary factors driving trait variation. (3) Precipitation during the wettest month (PWM) had the strongest influence, positively correlating with (<i>t/b</i>)² and negatively with <i>D</i>, <i>D</i>₉₅, <i>D_h</i>, <i>CA</i>, and <i>K_th</i>. (4) Structural equation modeling confirmed PWM as the main driver of <i>K_th</i>, with indirect effects through <i>CA</i>. These findings indicate that <i>C. mongolicum</i> displays high plasticity in xylem traits, enabling adaptation to changing environments, and providing insight into the hydraulic strategies of desert shrubs under climate change.