###### Abstract  
It is projected that extreme drought events will become more frequent and more severe across many regions of the globe by the end of the 21 st century. Despite the substantial efforts that have been made to explore the impacts of droughs on terrestrial ecosystems, our understanding of the response of diverse ecosystems, including resistance and resilience, remains unclear. A total of 16 site years of eddy covariance-based carbon flux data were used to reveal the different responses of forest and grassland ecosystems to two extreme summer droughs. We found that the carbon fluxes of the forest, namely gross primary productivity (GPP), ecosystem respiration (Re), and the net ecosystem carbon exchange (NEE), exhibited distinct seasonal patterns with a single peak. However, GPP and NEE of grassland showed multiple peaks owing to hay harvesting throughout one year. Meanwhile, all climate factors jointly affected the seasonal dynamics in the NEE of the forest, whereas solar radiation only dominated the variability in the NEE of grassland. Moreover, the optimal response relationship was quadratic between the vapor pressure deficit (VPD) and the NEE, with the thresholds being 5.46 and 5.84 for forest and grassland, respectively. Owing to the large increase in VPD during the droughs of 2003 and 2018, the carbon sequestration of forest and grassland reduced sharply and even altered from carbon sink to carbon source. Compared with grassland, forest GPP showed stronger resistance with weaker resilience to droughs. However, larger resilience appeared for both forest and grassland NEE relative to their resistance. All analyses reflect the different adaptive strategies among plant functional types, which is crucial to evaluate ecosystem carbon sequestration to overcome future climate change. resistance; resilience; drought extremes; ecosystem carbon sequestration +
Footnote †: journal: Journal  
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storing it in biomass and soils, which accounted for 1.1 \(\pm\) 0.8 Pg C year\({}^{-1}\)[9]. In addition, grassland covers an area of 52.5 million km\({}^{2}\) of Earth's land surface and also acts as an important carbon sink, with the carbon stored as root biomass and soil organic carbon [10]. Nevertheless, extreme climate events, particularly droughts, seriously threaten terrestrial ecosystem structures and functions that could essentially weaken the role of forests and grasslands as carbon sinks and even make ecosystems become carbon sources [11; 12]. Recent studies have been conducted to explore the effects of droughts under different periods and severities on terrestrial ecosystem carbon cycling by affecting the balance between carbon uptake and carbon release [2; 13; 14]. Overall, the impacts of drought on ecosystem carbon cycles are complicated and depend on a variety of biotic and abiotic factors. For example, drought can result in decreased carbon uptake and productivity in forests and grasslands by reducing their stomatal conductance to limit water loss through transpiration [15; 16]. Additionally, drought can cause plant mortality, which can result in the release of a mass of stored carbon back into the atmosphere in a short period [5; 17]. During extreme drought, plant mortality owing to the hydraulic transport failure and the restriction of the carbohydrate supply to C-dependent metabolic, defence, or hydraulic functions, will lead to not only a sharp decrease in gross primary productivity (GPP) but also an increase in carbon emission from decomposition (Re), thereby altering the net ecosystem carbon exchange (NEE) [18]. The 2003 European drought was one of the most severe and prolonged heatwaves in recent history, with temperatures reaching record highs across most regions of Europe [19]. Subsequently, the 2018 drought even exceeded the hot drought of 2003 [20].