Stable carbon sequestration in temperate mountain peatland despite shifting plant species composition

Peatlands are the largest and most efficient terrestrial carbon (C) storage ecosystems, with the potential to amplify climate warming by releasing large amounts of C into the atmosphere. Mountain peatlands are underexplored and particularly vulnerable to climate change. Their reduced resilience arises from strong water-carbon coupling, high sensitivity to environmental conditions, and the greater vulnerability of mountain regions. While undisturbed peatlands have remained C sinks over millennia, disturbances reduce C uptake often reflected in changing plant composition, indicating irreversible ecosystem changes. Indeed, peatland plant communities are expected to undergo substantial changes under future climate scenarios, and thus quantifying peat vegetation changes and their effects on the C cycle is a critical research priority.

Here, we examined vegetation changes of a well-preserved mountain peatland for over 15 years and assessed its current state using carbon dioxide (CO₂) and methane (CH₄) flux data from the past three years. Vegetation was monitored through biennial surveys at 27 permanent plots and annual aerial photographs. C fluxes were calculated using the eddy covariance method. The study was conducted in the Sonnenberger Moor peatland in the Harz mountains, Germany, which spans ~110 ha at 758–830 m a.s.l.

We observed a shift in plant composition towards woody species and declining Sphagnum moss coverage. Noticeable increases of woody species such as Calluna vulgaris, Andromeda polifolia, and Vaccinium oxycoccos were observed. Calluna coverage increased from 2.4% in 2009 to 15.4% in 2023, while Andromeda expanded from 7.5% to 16.8%. Small-scale vegetation changes, particularly the spread of Calluna, were inferred from aerial photographs. Summer CO₂ exchange comparisons over three years revealed that the peatland remained a stable C sink despite vegetation shifts and low summer precipitation. August data showed that ecosystem respiration (R_eco) increased with reduced precipitation (2023: 132 g C m^-2 month^-1 at 207 mm precipitation; 2022: 143 g C m^-2 at 31 mm). Gross primary production (GPP) increased under drier conditions but only marginally beyond certain thresholds. Accordingly, the driest August recorded the lowest net CO₂ uptake (2022: 23 g C month^-1), whereas the wettest August showed lower C uptake (2023: 37 g C month^-1) than the following drier August of 2024 (45 g C at 54 mm). CH₄ emissions decreased strongly during drought periods, but could not offset CO₂ uptake, so the peatland continued to act as a net C sink. This notwithstanding, CH₄ emissions amplified the effect that the peatland could not store maximum C under optimal wet conditions.

The observed shift from graminoids and mosses to woody species suggests reduced stability in waterlogging. Rising regional and global temperatures, reduced summer precipitation, and increased nitrogen deposition are the likely drivers of these observed changes. Increased woody species typically correlate with lower water levels, increased R_eco and reduced C uptake. The GPP rises even under drought conditions suggest that the investigated peatland ecosystem is shifting to a new stable level. Given the critical role mountain peatlands play in global C storage and climate regulation, our findings can support predicting C dynamics and developing effective management strategies.