Topography shapes the carbon allocation patterns of alpine forests

Topography plays a crucial role in determining the structure of alpine forests, as it restricts the availability of nutrients and water necessary for plant growth. Nevertheless, our information on how variations in forest carbon allocation patterns driven by fine-scale topography are influenced by broader-scale environmental contexts is limited. In the northern Tibetan Plateau, we combined field data from 89 forest plots with a high-resolution (1 m2) digital elevation model (DEM) and utilized a linear mixed-effects model to investigate how microtopography (characterized by slope, aspect, and topographic wetness index (TWI)) and broader-scale environmental context (characterized by elevation) and their interactions affect the carbon allocation patterns of alpine forest. Our results revealed that at low and high elevations with pronounced subsurface resource limitations, plants tend to allocate a higher proportion of carbon to the root system and have lower aboveground carbon stocks (ACS). Microtopographic heterogeneity significantly influenced the carbon allocation patterns of forest, with the intensity and direction of these effects varying across the environmental gradient. At low elevations, topographically wetter and northerly microhabitats had higher ACS and lower ratios of below- and aboveground carbon stocks (RBA); however, at high elevations, topographically drier and southerly microhabitats had higher ACS and lower RBA. TWI and aspect had the weakest effect on ACS and RBA in the mid-elevations. The relationship between slope and ACS and RBA was significantly positive but not evidently related to the broader-scale environmental gradient.