Dryland herbs coordinate leaf protection, absorptive root morphology and lateral-root proliferation along aridity gradients.

Introduction: Dryland plants must balance atmospheric water demand with below-ground water and nutrient acquisition, yet it remains unclear whether increasing aridity drives coordinated whole-plant trait shifts or instead promotes modular adjustment of leaf and root functions. Here, we examined leaf-root functional coordination in herbaceous plant communities across a continuous aridity gradient in Xinjiang, northwestern China. Methods: We measured six leaf traits, four root traits and plotlevel environmental variables for 474 matched species-by-plot observations, and combined single-trait models, multivariate trait axes, leaf-root correlation networks, sliding-window coordination analyses and path modeling. Trait responses to aridity were selective rather than uniform. Drier conditions were associated with thicker leaves and higher specific root length, whereas wetter conditions supported greater lateral-root proliferation. By contrast, leaf dry matter content, leaf water content, root length and root diameter showed weak or no direct responses to the aridity gradient. Leaf-root coordination was also trait-specific: leaf size traits were positively linked to root length and root diameter, but negatively associated with specific root length, indicating a separation between below-ground structural support and absorptive efficiency. Environmental models further showed that variation in leaf and root trait axes was associated with aridity through direct and indirect statistical pathways involving soil water availability, microbial carbon processes, SOC-N nutrient status, elevation and life form. Results: Our results suggest that dryland herbaceous plants do not respond to aridity through a single synchronized whole-plant economic axis. Instead, they maintain function through coupled but modular adjustments involving leaf structural protection, absorptive root morphology and lateral-root proliferation. Discussion: This modular leaf-root coordination provides a trait-based framework for understanding how herbaceous plants persist along dryland moisture gradients and may improve predictions of plant functional responses to intensifying drought.