Carbon Under Threat: Insights from Grazing Exclusion and Climate Impacts in Pyrenean high mountain peatlands

Peatlands are globally significant ecosystems with high organic carbon storage capacity due to slow litter decomposition in water-saturated soils promoting anoxic conditions. However, these ecosystems are increasingly threatened by climate change and land-use pressures. In the Pyrenees, mountain peatlands have become relict ecosystems, reduced to small isolated areas of just over one hectare. There, hydrological conditions and vegetation cover are severely impacted by rising temperatures, reduced water availability, and intense livestock activity from large animals (i.e. horses and cows). Effective management strategies to mitigate potential large greenhouse gas (GHG) emissions from these ecosystems are urgently needed.

The ALFAwetlands and Pyrepeat projects investigate the effects of grazing exclusion (i.e. enclosures installed in 2016) and hydrological variation (i.e. strong seasonality) on GHG fluxes in two Pyrenean peatlands, Rubió (42.41º N, 1.24º E) and Estanyeres (42.61º N, 1.05º E). Both peatlands are characterized by contrasting pH (5.97 ± 0.08 and 7.78 ± 0.21, respectively) and water saturation levels. Over two years, monthly measurements of carbon dioxide (CO₂), methane (CH₄), and nitrous oxide (N₂O) emissions were conducted across three plot types outside and within grazing exclusion zones: (1) low livestock trampling and soil disturbance, with barely vegetation gaps (mosses and sedges species); (2) pugged soils with wide exposed peat areas increasing CO₂ oxidation potential, and large vegetation gaps; (3) peatland margin areas with the driest condition, rarely flooded, and continuous vegetation cover of grass-like plants. Within the enclosure, these three types of plot were identified before fences were installed and nowadays they are partially recovered.

Preliminary results highlight the vulnerability of these ecosystems to climatic changes. CO₂ emissions were highest in dry plots of both peatlands, where reduced water content accelerated organic carbon oxidation. In the drier peatland, emissions were further amplified outside exclusion zones. CH₄ emissions were higher in the wetter peatland, consistent with anaerobic conditions that promote methanogenesis, while N₂O emissions remained consistently low across both sites due to nitrogen limitation.

These findings emphasize that climate-driven drying poses a significant threat to peatlands by increasing CO₂ emissions, a risk that is exacerbated by livestock disturbances. Management actions such as grazing exclusion are critical to maintain peatlands’ carbon storage capacity and mitigate GHG emissions from these vulnerable ecosystems. This research contributes to the growing body of knowledge needed to align peatland conservation and restoration with climate change adaptation.