Explaining the contrast responses of evergreen needle-leaved forest and grassland carbon fluxes during the growing season droughts over the North Temperate Zone

As climate change exacerbates, droughts have become more intense, causing significant, widespread, and enduring adverse effects on carbon dynamics within terrestrial ecosystems. Therefore, it is crucial to quantitatively assess how drought influences carbon fluxes in these ecosystems. This understanding is essential for improving predictions of ecosystem responses to water stress and providing critical information to mitigate the effects of drought. In this study, we collected daily observations from 2004 to 2020 at two eddy covariance sites: one in an evergreen needle-leaved forest (abbreviated as ENFTha) and the other in grassland (abbreviated as GRAGri), both located in the North Temperate Zone. We first quantified the impacts of growing season (GS) droughts (GSD) on gross primary productivity (GPP) and ecosystem respiration (RECO). Our results indicated contrasting impacts of GSD on the two ecosystems: GPP and RECO increased in ENFTha, while they decreased in GRAGri. To explain these contrasting effects, we developed XGBoost models for GPP and RECO in ENFTha and GRAGri during the GS using eight environmental variables. We then applied the TreeExplainer-based SHapley Additive exPlanations framework to assess the significance of these variables in regulating GS carbon fluxes and to analyze their contributions to changes in GSD GPP and RECO. During the GS, four environmental variables—downwelling shortwave radiation (SW), vapor pressure deficit (VPD), longwave radiation (LW), and soil water content (near-surface air temperature (TA), SWC, SW, and LW)—significantly influenced GPP (RECO) in ENFTha and GRAGri, although their importance varied between the two ecosystems. These variables and their interactions also had nonlinear effects on GS GPP and RECO, with distinct threshold effects observed. In ENFTha, the enhanced SW demonstrated greater interaction effects with VPD, leading to increased GPP during GSD. In contrast, the rise in RECO could be attributed to the direct effects of TA. On the other hand, the declined SWC determined the decreases in carbon fluxes in GRAGri.