Assessing Carbon Flux Variability in an Alpine Steppe: Insights from Dual-Height Measurements

Abstract. Future projections of climate warming on the Tibetan plateau (TP) imply a 4 °C warming in the next 100 years, the largest in the middle of the troposphere. Climatic variabilities of this magnitude are likely to trigger a cascade of climate-carbon (C) feedbacks within the TP ecosystems. However, a robust consensus of the feedback mechanisms and their drivers is scarce due to a lack of observations and unaccounted spatial heterogeneity. In the present study, we investigated how coarse-scale heterogeneity impacts the CO₂ fluxes using a dual eddy covariance tower system (3 m and 19 m) over an alpine steppe ecosystem near the Nam Co Station for Multi-sphere Observation and Research (NAMORS) on the central TP. The source area of the 3 m height is relatively homogenous. On the other hand, the source area of the 19 m height covers the steppe and part of the neighboring lake. The steppe acted as carbon neutral over the 10-month measurement period (August 2018–May 2019) at the 19 m footprint as opposed to the 10-month long-term (2006–2018) average (-78 g C m^-2) observed at the 3 m footprint. We found that the difference in the magnitude of CO₂ fluxes observed from the two towers was attributed to the combined effects of winter snow cover and lake-land interactions. The extreme snow accumulation over the period increased the ecosystem respiration thus elevating the emissions in winter, highlighting the role of extreme snow events in regulating carbon dynamics in high-altitude ecosystems. Additionally, the neighboring lake substantially influenced carbon fluxes over larger spatial footprints, serving as a natural buffer that mitigates land carbon emissions during critical periods. Fluxes measured from land-dominated areas at both tower heights were largely consistent, demonstrating the reliability of steppe-derived flux measurements across 3 m and 19 m footprints. The findings emphasize the critical need for adopting a landscape-scale perspective to better capture flux variability in heterogeneous environments.