Carbon sequestration potential of high-altitude afforestation in the Eastern Central Alps

Changes in alpine land management have led to the abandonment of high-altitude pastures in the European Alps. At the same time, climate warming facilitates forests to expand upwards, creating opportunities for carbon (C) sequestration. However, the amount of C that can be captured remains uncertain, as the effects on soil C are unclear and knowledge about forest growth at high altitudes is limited. We quantified biomass and soil C stocks in forest stands planted on subalpine pastures (elevation 1600-2100 m a.s.l.) in the Austrian Alps, compared them with those of adjacent pastures, assessed vascular plant diversity and monitored timber line dynamics around the afforestation sites. Ecosystem C stocks at afforested plots succeeded those of pastures by 121 ± 49 Mg C per hectare, indicating a sequestration potential of 441 ± 179 Mg carbon dioxide per hectare within 42-65 years after tree planting. Carbon sequestration occurred primarily in tree biomass whereas soil C stocks remained largely unchanged. The diversity of vascular plants decreased significantly under closed forest canopies. But higher diversity in nearby mature forests suggests partial recovery at later stand stages. Despite regional warming, the upper forest boundary has remained stable over the past two decades. Our results suggest that afforestation can accelerate forest establishment around the current timberline and create local carbon sinks, while biodiversity impacts are mixed and context-dependent. Site-specific assessments are therefore crucial when considering afforestation as a climate mitigation strategy.