Distinct magnitude asymmetries of daily extreme anomalies in gross primary productivity between forests and non-forests

The magnitude asymmetry between positive and negative extreme anomalies in gross primary productivity (GPP) and the underlying mechanism have been investigated based on daily records from the data-driven and in situ datasets. Distinct magnitude asymmetries of daily GPP extreme anomalies (DGEAs) have been identified between forested and non-forested areas. Forested areas are dominated by negative asymmetries, manifesting as the magnitude of negative DGEAs significantly larger than that of their positive counterparts; while non-forested areas are dominated by positive asymmetries. These distinct asymmetric features are due to the different climatic constraints between forests and non-forests. We found that solar radiation is the dominant climatic controller of the daily variation in GPP of forests. Due to the existence of a light saturation point, vegetation productivity is less sensitive to extremely high values of solar radiation than it is to the extremely low, thus leading to the negative asymmetries. For the non-forested areas, however, the daily variation in GPP depends on the preceding 3‒6-day soil moisture conditions. Heavy precipitation is more effective at causing strong and persistent soil moisture anomalies, as soil water can penetrate to deeper soil layers. Therefore, strong positive asymmetry of precipitation anomalies results in the positive asymmetry of soil moisture anomalies and thus leads to the positive asymmetry of DGEAs. Due to the significant asymmetry in the DGEAs, the cumulative effect of DGEAs further rectifies the interannual variability of annual GPP, implying the importance of asymmetry in the DGEAs to changes of terrestrial carbon cycle.