Distance- and Threshold-Conditioned Associations of Ecosystem Multifunctionality with Climate, Soil Moisture, and Biodiversity in Northwestern China’s Populus euphratica Forests

Dryland riparian forests depend on river-derived water in arid landscapes, but the relative associations of regional climate, local soil moisture, and biodiversity with ecosystem multifunctionality (EMF) remain uncertain. We analysed data from 516 plots in Populus euphratica forests across northwestern China. EMF was quantified from eight vegetation and soil attributes using averaging and multi-threshold approaches. We combined species richness, Rao’s quadratic entropy, community-weighted trait axes, climate principal components, and soil principal components in mixed-effects models, hierarchical partitioning and partial least-squares path models. EMF declined from near- to far-riparian plots, whereas soil water content did not differ significantly among distance bands. Hydroclimatic stress was negatively associated with EMF, especially in far-riparian plots and at higher thresholds, while soil moisture was positively associated mainly at low-to-intermediate thresholds. The relative explanatory contributions of biodiversity varied across riparian positions and predefined functional thresholds: RaoQ contributed most near rivers at lower thresholds, species richness was more important at intermediate and far positions, and community-weighted traits became prominent under selected high-threshold conditions. Species richness and community-weighted trait axes also changed with riparian distance, whereas RaoQ was comparatively stable. In the PLS-PMs, modelled direct abiotic associations were generally stronger than indirect associations involving biodiversity. Given the observational study design and correlational analyses, these findings indicate context-dependent associations among climate, soil moisture, biodiversity and ecosystem multifunctionality rather than causal relationships.