Regime-dependent reversal in temporal asymmetry of ecosystem carbon exchange across global biomes

Abstract. Terrestrial ecosystems play a central role in regulating the global carbon cycle through exchanges of carbon between land and atmosphere. While ecosystem carbon exchange magnitude has been widely studied, no systematic comparison has examined whether the temporal asymmetry of typical and statistically extreme fluctuations in Net Ecosystem Productivity (NEP) differs across biomes. Here, we quantify the temporal asymmetry of daily NEP trajectories using eddy-covariance observations from 13 FLUXNET sites spanning seven biomes. Specifically, we compare the temporal asymmetry of the top 1 % most extreme NEP fluctuations with that of the full fluctuation distribution to investigate regime-dependent behavior in ecosystem carbon exchange. We find a consistent regime-dependent reversal in temporal asymmetry across all sites and biomes. Extreme NEP fluctuations exhibit positive asymmetry, characterized by a more gradual afternoon decline than morning rise, whereas the full distribution of fluctuations exhibits negative asymmetry, characterized by a steeper afternoon decline than morning rise. Positive asymmetry associated with extreme fluctuations increases with mean air temperature, while negative asymmetry associated with the full fluctuation distribution is most strongly related to vapor pressure deficit. These contrasting relationships indicate a shift in dominant physiological controls, from temperature-regulated photosynthetic capacity during extreme carbon uptake events to VPD-driven stomatal limitation under typical conditions. The consistent reversal in asymmetry observed across diverse ecosystems suggests that diurnal NEP asymmetry is a robust emergent property of ecosystem carbon exchange. Our findings reveal distinct regimes associated with typical and extreme NEP fluctuations and highlight the importance of accounting for regime-dependent ecosystem responses when interpreting and modeling terrestrial carbon dynamics.