Climate-driven asymmetric range shifts of montane birds along a subtropical elevational gradient in China

Species distributions in montane ecosystems are rapidly reorganizing in response to climate change, underscoring the need to understand species redistribution along elevational gradients for effective conservation planning. Although upslope shifts in avian elevational ranges are increasingly documented, the mechanisms underlying these patterns remain uncertain and contentious. Here, we conducted a 64-year resurvey (1957–2020) of bird distributions along a subtropical elevational gradient (500–3099 m a.s.l.) on Mount Emei, China—a UNESCO World Heritage Site that has undergone significant warming and drying. Using a resampling-based protocol to mitigate sampling bias and a null model to control for stochastic and geometric constraints, we quantified elevational range shifts and then examined how climate and functional traits mediated them. Bird species exhibited highly individualistic and asymmetrical range dynamics. Upper limits showed a mean expansion of 213.93 ± 572.47 m, while lower limits contracted by 34.18 ± 504.51 m. Multivariate analyses revealed that temperature and precipitation jointly mediated shift directions—warming promoting upslope movements and reduced precipitation driving downslope shifts. Trait-mediated responses varied by elevational context: upper limit shifts were associated with dispersal ability, reproductive potential, ecological generalism, and site fidelity, whereas lower limit shifts were weakly linked to dispersal ability, reproductive potential, and site fidelity. This study offers empirical evidence of climate-driven bidirectionally elevational redistribution of birds in the Asian subtropics, expanding our understanding of multi-directional range shifts in montane ecosystems. Our findings highlight the urgency of elevation-specific conservation strategies that account for asymmetric range dynamics in subtropical montane systems amid accelerating climate change.