Global elevational shifts and drivers of alpine treelines

• Produced the high-resolution maps of global observed and potential treelines (2000–2020) using multi-source remote sensing data. • Only 47% of observed treelines reached potential climatic positions, while 25% shifted downslope, with 38% of downslope shifts linked to fire events. • Temperature dominates observed treeline elevation, but anthropogenic disturbance causes deviations from potential treelines. • Observed treelines near potential positions with low disturbance shifted upslope faster, yet lag behind climate warming rates. Treelines are generally expected to respond to climate warming by shifting to higher elevations. However, significant variations in treeline shifts across different mountain ranges have been observed, and the underlying reasons for these differences remain unclear due to the lack of comprehensive global treeline mapping. This study provides a global analysis of treeline shifts using satellite-derived forest canopy height data at a 30-meter resolution, combined with a model of potential treeline positions based on climatic data. The elevations of both observed and potential treelines were mapped for the years 2000 and 2020, and the elevational shifts in treeline positions were quantified over this period. On a global scale, 42% of observed treelines shifted upslope and 25% downslope. However, these shifts generally lagged behind climate-driven potential treeline elevations. By contrast, the rate of treeline shifts was strongly influenced by anthropogenic disturbances. Fire disturbances contributed to 38% of downslope shifts, and regions with lower overall anthropogenic disturbance had faster upslope shifts of observed treelines and thus consequently smaller lags. These findings emphasize the importance of considering both climatic and anthropogenic factors when assessing treeline dynamics and suggest that historical and ongoing human impacts play a critical role in modulating treeline responses to climate change.