Biodiversity gains importance in sustaining ecosystem multifunctionality when beyond aridity thresholds in temperate grasslands

Ecosystem multifunctionality (EMF), integrating the multifaceted nature of ecosystem functioning, is increasingly used as a key indicator for assessing grassland health and guiding restoration strategies. It remains unclear whether biodiversity consistently enhances EMF across aridity gradients in water-limited ecosystems. Based on a 445-site survey spanning a broad aridity gradient in the temperate grasslands of northern China, we revealed threshold-type responses of EMF to aridity, with a critical value of 0.565. Below this threshold, EMF was strongly driven by water availability and declined sharply with increasing aridity. Above the threshold, intensified environmental filtering for drought-tolerant species weakened the influence of aridity but amplified the role of plant species diversity, likely due to their unique and irreplaceable contributions to EMF. Grazing exerted relatively weak direct effects on EMF compared with aridity. However, greater grazing intensity reduced plant species diversity, thus reducing EMF below the threshold, while above the threshold, it increased soil pH, reducing both plant diversity and EMF. These findings reveal the vulnerability of dryland ecosystem functioning to ongoing climatic drying and the strengthening influence of biodiversity in sustaining it, underscoring the necessity of conserving biodiversity to maintain ecosystem multifunctionality and resilience in arid regions. • Ecosystem multifunctionality exhibits a clear aridity threshold, marking a shift in ecosystem response patterns. • Biodiversity shifts from a supporting to a central role in sustaining ecosystem multifunctionality beyond the threshold. • Grazing reduced ecosystem multifunctionality indirectly by decreasing biodiversity, particularly under low aridity.