Dispersal dynamics along river corridors modulate impact of climate change on treeline shift in Hengduan Mountain

Abstract. Alpine treelines are expected to respond to climate warming, but the magnitude and direction of treeline shifts often vary across mountain landscapes. In the Hengduan Mountains, river corridors may shape how forest expansion is expressed as lateral and elevational treeline shifts, with important implications for future alpine habitat loss. Here, we modified and applied the spatially explicit individual based model LAVESI to simulate treeline dynamics along four major river corridors: the Dadu, Lancang, Nu, and Yalong rivers. Simulations covered the historical period from 1940 to 2020 and the future period from 2020 to 2100 under SSP1-2.6, SSP2-4.5, and SSP5-8.5.

During the historical period, simulated treeline shifts were gradual but river specific. Lateral changes were strongest in the Lancang and Nu rivers, intermediate in the Yalong River, and weak in the Dadu River. Elevational changes were more limited, with the Lancang River showing the clearest upward shift and the Nu River remaining close to stable. After 2020, lateral treeline shifts became stronger and more divergent among river corridors and scenarios. In most rivers, future lateral advance increased, with SSP2-4.5 generally producing relatively high cumulative expansion. Elevational shifts followed a related but not identical pattern: the Lancang River maintained the strongest upward trend, the Dadu and Yalong rivers showed moderate increases, and the Nu River remained weakly responsive. The relationship between lateral and elevational changes therefore varied among rivers, indicating that horizontal boundary reorganization did not always translate into comparable upslope shift.

Treeline invasion potential was also uneven. The Dadu River frequently reached the predefined upper limit, although this result should be interpreted in relation to the shorter simulation extent. In contrast, most simulations for the Lancang, Nu, and Yalong rivers remained below the upper limit by 2100, suggesting incomplete occupation of the available treeline tundra ecotone. Overall, our results indicate that future treeline shifts in the Hengduan Mountains are likely to remain spatially heterogeneous across river corridors, with different implications for alpine habitat vulnerability. This vulnerability should therefore be evaluated at the river corridor scale, where lateral expansion, elevational advance, and local topographic and ecological settings jointly shape the potential for future forest expansion.