Abstract ID: 3.12506 | Accepted as Talk | Talk/Oral | TBA | TBA

Mountain pine (Pinus mugo) is a keystone species at the treeline ecotone of the European Alps. This pioneer shrub contributes to soil development, prevents erosion by gravitational processes, and contributes to carbon sequestration at high elevations. These ecosystem services are at risk as climate-related stressors and invasive species threaten mountain pine health. A widespread die-off could have serious consequences for alpine ecosystems and their role in the carbon cycle. We investigated 1. how declining mountain pine health influences topsoil carbon stocks; 2. whether changes in carbon concentration or soil volume drive these patterns; 3. when the carbon balance shifts from gains to losses upon mountain pine die-off; and 4. the potential impacts of a die-off on ecosystem-level soil carbon storage.
We collected soil samples from plots representing three conditions (healthy mountain pine, ailing mountain pine, no mountain pine) along an elevational gradient (1065 – 1810 m a.s.l.) in Berchtesgaden National Park, Germany. Organic carbon concentration was determined by elemental analysis. We analysed how mountain pine health and elevation influence topsoil carbon stocks using Linear Mixed Models (LMMs). Working with a simple model of soil carbon dynamics, we simulated how carbon stocks change over time upon mountain pine die-off and estimated the ecosystem-level impacts of a die-off within the national park.
Our LMM analyses showed that soil carbon stocks increased with elevation. In comparison to the healthy state, carbon stocks increased moderately under ailing mountain pines. However, carbon stocks in plots without mountain pines were considerably lower than those under healthy and ailing mountain pines. In our soil carbon simulations, the carbon gain from needle loss under ailing mountain pines lasted less than a decade before net losses from decomposition set in. This process slowed down as elevation increased. In Berchtesgaden National Park, our model estimated that loss of mountain pine would reduce soil carbon stocks above 1500 m a.s.l. by more than 15%.
Carbon stock decline upon mountain pine die-off is slower at higher elevations, yet the long-term trend shows a substantial net loss. Our results highlight the consequences of declining mountain pine health on the dynamics of alpine ecosystems through impaired soil carbon storage.