Contrasting hydrothermal regimes shape carbon–water coupling and water use efficiency in alpine swamp meadows

Alpine swamp meadows on the Tibetan Plateau are integral to the regional carbon-water cycle. Hydrothermal variability co-regulates photosynthesis and evapotranspiration, thereby governing carbon-water coupling and water-use efficiency (WUE). Resolving these hydrothermal controls across contrasting regimes is essential for reliable carbon and water balance projections and improved ecosystem modeling. Using eddy-covariance measurements from two climatically contrasting sites—Dashalong (DSL) on the northeastern margin of the plateau and Longbao (LB) in the plateau interior—we quantified growing-season carbon and water vapor fluxes and their biophysical controls during 2017–2018. Results showed that DSL was a stronger carbon sink (–211.8 ± 20.2 g C m–2), with lower evapotranspiration (353.5 ± 100.8 mm) and higher WUE, whereas LB had greater evapotranspiration (408.9 ± 5.6 mm) but weaker net carbon uptake (–113.5 ± 7.4 g C m–2). Lower temperatures at DSL suppressed respiration, resulting in greater carbon sequestration than at LB. In both ecosystems, temperature and radiation act as the primary energy constraints and play a key role in driving variations in growing-season carbon-water exchange and WUE. Hydrothermal contrasts further modulated carbon sequestration and water consumption, resulting in distinct carbon-water coupling patterns between the two ecosystems. These findings improve our understanding of carbon-water coupling in alpine wetlands and provide a basis for assessing future changes in carbon sequestration and evapotranspiration.