Beneath the Surface: How the Declining Health of a Keystone Shrub Species Impacts Soil Carbon in a Temperate Mountain Ecosystem

Mountain pine ( Pinus mugo Turra) is a key high-elevation species in the European Alps. It primarily establishes on exposed, erosion-prone sites and supports large soil organic carbon (SOC) stocks by sequestering carbon, stabilising soils, and preventing erosion. These functions are increasingly threatened as global change-related pressures like invasive pathogens impact mountain pine health. We investigated how declining mountain pine health affects SOC in Berchtesgaden National Park (BGNP), Germany. We studied two alternative vegetation states as potential outcomes of mountain pine decline, conifer-dominated forest and sparsely vegetated unstocked areas, and compared them to healthy and ailing mountain pine stands. We found that a transition from mountain pine to forest would have little effect on SOC ( p = 0.055). However, a trajectory to unstocked areas (with no woody plants establishing after mountain pine die-off) could result in 47.3% lower SOC compared to healthy mountain pine ( p < 0.001). To better understand the temporal development and landscape-scale consequences of such a potential loss of woody vegetation on areas currently stocked by mountain pine, we combined field data with carbon cycle simulations. Simulations showed that short-term SOC gains during mountain pine die-off shifted to long-term losses persisting over decades. A trajectory to unstocked areas on sites currently occupied by mountain pine could result in a loss of 28.3 kilotons of SOC for BGNP, whereof nearly 90% are in sensitive high elevation areas. We demonstrate that the global change-induced loss of a single keystone species could have long-lasting consequences on ecosystem functioning.