Drivers of Extreme Carbon Sources and Sinks Across Diverse Ecosystems in the Western USA

Increasing climate variability is impacting the carbon cycle in unprecedented ways, demanding an understanding of the conditions leading to extreme carbon flux states. In this study, we sought to determine the strength and influential timescales of key environmental drivers governing unusually high daily gross primary productivity (GPP) and ecosystem respiration (R_eco) across diverse ecosystems in the western USA. We obtained CO₂ flux data from 14 AmeriFlux eddy covariance flux tower sites (11-22 years/site) across the western USA to understand the drivers of extreme CO₂ sinks (extreme GPP) and sources (extreme R_eco). To account for seasonality, we defined extreme sink and source states as those GPP and R_eco values exceeding a site-level 95th quantile spline regression. We assembled environmental covariates computed across multiple timescales (day, week, month, year), including temperature, vapor pressure deficit, short-wave radiation, soil moisture, and precipitation, along with site-level characteristics (e.g., mean annual precipitation [MAP]). We used random forest classification models to evaluate the importance of different covariates for identifying extreme versus "nominal" fluxes. Patterns of large, but infrequent precipitation events over the preceding month were key positive drivers of the probability of both extreme sinks and sources, and the importance of such precipitation "packaging" was 16.5 times greater under water limitation (low MAP). We also found climate timescales interacted hierarchically: historical, long-term climate and conditions over the past month and/or year establish baseline ecosystem resource limitation (water or light limitation), within which short-term interactions-such as compounding events and event duration-operate to influence extreme fluxes. Preceding-month temperature and preceding-year variation in shortwave radiation had climate-specific influences, particularly in high MAT sites and in Mediterranean sites, respectively. Our results indicate the evolution of extreme carbon fluxes with climate change, including the likelihood of increased occurrence of extreme carbon flux states in response to increasing hydroclimate volatility.