Monitoring climate and land-use change impacts on Alpine grassland vegetation dynamics and carbon sinks

Terrestrial vegetation represents one of the planet’s primary carbon sinks, playing a pivotal role for climate change mitigation. Enhancing carbon storage in natural and managed ecosystems requires a deeper understanding of vegetation dynamics. In this context, Alpine Mountain ecosystems, are dealing with two significant challenges increasing the vulnerability of their carbon sinks: firstly, the atmosphere in the Alps is warming up twice as fast as in other areas of the planet and droughts and heat waves are becoming more frequent; secondly, socio-economic changes have led to partial land abandonment, affecting the composition and distribution of plant species. Specifically, in the Aosta Valley region (Northwest Italian Alps), land-cover and land-use changes (LCLU) are reshaping vegetation dynamics, particularly through the abandonment of mountain pastures below the forest line (~1500 meters asl).

The goal of our research is to investigate how climatic and socio-economic shifts drive woody species encroachment into mountain grasslands, altering carbon sequestration patterns and contributing to ecosystem changes. The activities were carried out at the ‘Integrated Carbon Observation System’ (ICOS) associated site Torgnon (IT-Tor), an abandoned subalpine pasture dominated by Nardus stricta, located in the Aosta Valley region at about 2100 m asl. An area of 15000 square meters was selected in the pasture, which is undergoing recolonization by larches (Larix decidua) and shrubs (specifically Calluna vulgaris, Juniperus communis, Vaccinium myrtillus, V. uliginosum, Rhododendron ferrugineum). Since 2015, periodic surveys (2015, 2018, 2021, and 2024) were conducted to monitor vegetation dynamics. The area was divided into line transects using ropes for sequential monitoring. Employing a GNSS system with 20 cm positional accuracy, we mapped larch tree locations, measured trunk diameters, heights, and crown dimensions, and documented associated shrub growth. Shrubs were independently counted to quantify their spread across the study area.

Continuous measurements of CO2, water fluxes, and meteorological variables are available at the site since 2008. To further evaluate ecosystem fluxes, an additional eddy covariance station was installed in October 2024 in the encroached area.

Results show an increase in the number of larches, most significant in the 2015-2018 period. During that period the number of larches almost doubled. After 2018 growth rates were lower but highlight an ongoing shift from grassland to woody vegetation, that affect carbon and water dynamics. Preliminary flux measurements will be presented, providing first insights into different carbon dynamics in the transition area.

This research underscores the critical role of LCLU changes in shaping present and future global vegetation dynamics and carbon sinks, that need to be considered to improve our understanding and modelling of ecosystem carbon cycle.