Phylogenetic and root-trait structure mediate species coexistence across meadow types along a drought gradient on the eastern Tibetan Plateau

Root functional trade-offs reflect plant resource investment strategies and provide insights into adaptation under water limitation. However, how belowground functional traits and phylogenetic structure shift along drought gradients remains insufficiently understood. We quantified eight root traits for 75 herbaceous species across four meadow types on the Qinghai-Tibet Plateau along a pronounced gradient of declining soil water availability (swamp, alpine, shrub, and steppe meadows). Phylogenetic structure shifted from overdispersion in swamp meadow to significant clustering in the three drier meadow types, indicating a transition from competition-dominated to environmentally filtered community assembly along the water-limitation gradient. Community-level root trait structure was dominated by functional clustering for most traits (≈75%), whereas trait divergence (≈25%) emerged under specific abiotic conditions. Rather than soil moisture alone, coordinated changes in water availability and nutrient status structured both phylogenetic and functional patterns of root communities. In resource-rich sites, species with contrasting root strategies co-occurred, whereas resource-limited sites favored finer root systems with higher specific root length, reflecting a shift toward more resource-acquisitive strategies under increasing water limitation. These findings provide mechanistic insights into belowground community assembly and offer guidance for dryland restoration under future climate drying.