Widespread reduction in gross primary productivity caused by the compound heat and drought in Yangtze River Basin in 2022

Terrestrial ecosystems play a pivotal role in the global carbon sequestration process, and their photosynthetic capacity is highly susceptible to fluctuations in climate conditions. In 2022, the Yangtze River Basin (YRB) in China experienced an extensive and severe compounded heat and drought event. Compared with the past two decades, our results revealed that the temperature increased by approximately 0.78 ± 0.45 °C and precipitation decreased by about 45.20 ± 30.10 mm from July to October 2022 over the whole YRB. Region I (west from the Sichuan Basin and east to the easternmost of the basin) experienced a more severe temperature increase (0.98 ± 0.35 °C) and precipitation decrease (−60.27 ± 23.75 mm) compared to the other regions in the YRB. Changes in temperature and precipitation resulted in an increase of 0.14 ± 0.06 kPa in vapor pressure deficit (VPD) and a decrease of 5.28 ± 2.09 m3 m−3 in soil moisture, ultimately leading to a total loss of 26.12 ± 16.09 Tg C (about −6.08% compared to the 2001–2021 mean) in gross primary productivity (GPP) of July to October in 2022. It is noteworthy that broadleaf forests, which comprise 12.03% of the natural vegetation in region I, contributed only 6.46% of the GPP loss between July and October compared to other vegetation types, showing greater resistance to this climate event. Our findings from multiple linear regressions highlight that high temperatures and reduced soil moisture together contribute up to 94% photosynthesis loss in July–October in natural vegetation in region I, while the contribution of reduced VPD is minimal. In the future, we will further explore the impacts of compound heat and drought events on the coupled carbon and water cycles across different ecosystems, in order to better understand the ecosystem response mechanisms to extreme climates.


Introduction
In recent years, extreme climate events have become more frequent and severe (Jiang et al 2019, Bartusek et al 2022, Yin et al 2023), impacting the environment, economy, and society, and posing risks to ecosystem services and agriculture (Huang et al 2023).From heat waves to droughts, wildfires, extreme precipitation, and flooding (Walsh et al 2020, Dos Reis et al 2021), these extreme events have caused significant stress on ecosystems and may occur more frequently in the future (Li et al 2019, Qin et al 2022, Yao et al 2022), requiring comprehensive understanding and proactive strategies.
Heatwaves and droughts are among the most prevalent and quintessential extreme climate events (De Boeck et al 2011, Lesk et al 2021).In recent decades, the incidence of both heatwaves and droughts T Li et al has increased in response to global warming (Wolf et al 2016, von Buttlar et al 2018, Ahrens et al 2021).In particular, an increasing occurrence of drought and heat wave simultaneous extreme climate events (AghaKouchak et al 2020), i.e. warm drought events (Gampe et al 2021), has been observed from the 20th century to the 21st century.It is possible to cause more damage to ecosystems and socioeconomics as a result of the compounding effects of high temperatures and droughts than any single extreme climate event (Marchin et al 2022).Research has demonstrated that compounded extreme climate events, with both heat and drought, cause greater levels of leaf damage and more harm to ecosystems and socioeconomics than any single extreme climate event.Mazdiyasni and AghaKouchak (2015) reveal an increased frequency and expansion of simultaneous heatwaves and droughts across the United States.Hao et al (2013) quantified the changes in monthly temperature and precipitation extremes that occurred simultaneously during the observation period and observed a significant increase in the frequency of simultaneous occurrence of temperature extremes and droughts.Several studies have also used CMIP5 simulations and found that the positive correlation between high temperatures and droughts leads to a much higher frequency of simultaneous occurrence than isolation (Zscheischler et al 2014).However, questions remain as to the magnitude and variability of ecosystem photosynthesis anomalies and their underlying causes during these composite climate events (Pokhrel et al 2021, Sturm et al 2022).
Under the background of global warming, influenced by the persistent La Niña event, the subtropical high-pressure system in the western Pacific Ocean has expanded and intensified since July 2022 (Zhang et al 2023).This atmospheric phenomenon has affected the Yangtze River Basin (YRB) in China, leading to extensive and enduring heat and drought.This extreme climate event experienced in YRB had a significant impact on local vegetation and the region's socioeconomic functions, affecting various sectors such as agriculture and tourism.
Previous studies have demonstrated the adverse effects of heat and drought on vegetation, leading to reduced photosynthetic rates, increased water stress, and decreased plant productivity (Piao et al 2020, Smith et al 2020, Au et al 2022).However, the impact of these factors may vary depending on plant species (Huang and Xia 2019, Yan et al 2023), soil type, and other environmental factors (Zhou et al 2021, Yu et al 2022), making the interactions complex (Ciais et al 2005).Given the inherent complexity of how extreme climate events affect terrestrial ecosystems, the response mechanism of vegetation photosynthesis under specific compounded extreme climate events remains a current scientific inquiry (Huang et al 2017, Huang and Xia 2019, Li et al 2020).
This study aims to quantitatively assess the influence of environmental factors on vegetation photosynthesis in response to the compound heat and drought event in YRB.Specifically, we (1) evaluated the characteristics of this compound heat and drought event using 2 m air temperature, precipitation, vapor pressure deficit (VPD), and soil moisture as indicators, (2) analyzed the response patterns of gross primary productivity (GPP) and other proxies such as sun/solar-induced chlorophyll fluorescence (SIF) in YRB, and (3) further explored the effects of different environmental factors on the response of photosynthesis to compounded heat and drought event.The results of this study are expected to reveal the potential impacts of environmental factors on vegetation photosynthesis, and contribute to an enhanced understanding of how vegetation in terrestrial ecosystems responds to increasingly frequent climatic stresses, providing reliable insights for policy-makers to develop vegetation conservation and land management strategies.

Study area
The YRB is a crucial economic corridor in China, with a historically significant science and technology industry (Huang et al 2023).Spanning approximately 1.8 million square kilometers, this basin ranks as the third largest river system globally, crossing 19 provinces, municipalities, and autonomous regions within China (Yang et al 2021).
The YRB, exhibits distinct geographical features characterized by a significant temperature differential between land and sea, along with seasonal variations in atmospheric circulation.These characteristics classify the YRB as a typical East Asian Monsoon region (Pan et al 2022), rendering it highly sensitive and susceptible to climate change.Consequently, it represents an ideal region for investigating the responses of different topographic regions and vegetation cover types to compounded heat and drought events.To assess the response of different land cover types to the compounded heat and drought event, we divided the YRB into two regions according to elevations and severity (figure 1).This dataset seamlessly integrates model-derived data with comprehensive global observational records, culminating in a comprehensive and harmonized global meteorological dataset.We obtained daily 0.1 • meteorological data, including 2 m air temperature, precipitation, 2 m dew point temperature, relative humidity and soil moisture (averaged over depths of 1-100 cm) for 2001-2022 and aggregated to monthly scale.

FluxSat GPP data
FluxSat data (Gentemann et al 2020) are derived from FLUXNET eddy covariance tower site data and NASA Terra and MODIS instruments on the Aqua satellites with 0.05 • and daily spatial and temporal resolution.FluxSat GPP data are estimated through neural networks in good agreement with flux data (Joiner and Yoshida 2020) and have been validated in various studies (Wang et al 2023, Zhao et al 2023).We selected the FluxSat v2.0 from 2001 to 2022 and resampled it to 0.1 • .

GOSIF data
GOSIF data is a novel global SIF dataset developed with high spatial and temporal resolution (0.05 • , 8 d) using discrete OCO-2 SIF soundings (Li and Xiao 2019), MODIS remote sensing data, and meteorological reanalysis data.GOSIF proves invaluable for evaluating terrestrial photosynthesis and ecosystem performance, highly correlated with GPP from 91 FLUXNET sites.SIF reflects the fluorescence energy emitted by plant chlorophyll upon light excitation.We obtained GOSIF data spanning 2001-2022 and harmonized it to 0.1 • resolution to assisted in understanding changes.The agreement in anomalies for 2022 between FluxSat GPP data and GOSIF data is more pronounced in the region I.

Method 2.3.1. Calculation of VPD
According to Tetens' Formula, the relationship between temperature and the partial pressure of water vapor: where, ea is the actual vapor pressure or vapor pressure at dew point temperature, es is the saturation vapor pressure or vapor pressure at air temperature and Rh is the relative humidity, all calculated from ERA5 Land 2 m air temperature and 2 m dew point temperature.

Calculation of the contribution
In this study, we first eliminated outliers in various variables, such as 2 m temperature month by month from 2001-2022.Then, we de-trended these variables by considering their trends over the same period in 2001-2022 to remove human management and CO 2 fertilization effects.This study specifically examines the distribution of monthly anomalies concerning long-term levels during the extreme climate event.As a result, we only removed interannual trends while not accounting for seasonal and diurnal trends.We also standardized the variables to ensure scale consistency.The variance inflation factor values within different vegetation cover types and regions were all less than 10, indicting low covariance between independent variables.
Finally, we used a multiple linear regression model, a well-established approach in research, to quantitatively determine the contributions of temperature, VPD and soil moisture to GPP and SIF anomalies across various regions and vegetation types.The specific analyzing equations are as follows: where y represents photosynthesis (GPP or SIF) anomalies, after de-trending.∆T, ∆VPD, and ∆SM also represent anomalies of variables after detrending.β T , β VPD , and β SM are the parameters of the respective variables after multiple linear regressions for the whole region, and ε is the residual.Thus β i ∆i (i = T, VPD and SM) represents the contribution made by different environmental factors to the anomaly.

Anomalies of climate and photosynthesis in YRB during 2022
Influenced by the persistent La Niña event, the anomalously wide-ranging and high-intensity subtropical high pressure in the western Pacific Heatwaves can exacerbate the abnormality of terrestrial water vapor transport conditions, intensifying the impact of drought on vegetation growth and photosynthesis (figures 2(b) and (c)).Consequently, during the period of July-October, the VPD exceeded the long-term trend by a significant margin (0.14 ± 0.06 kPa).High temperature accelerated surface moisture loss, resulting in decreased soil moisture (−5.28 ± 2.09 m 3 m −3 ), potentially contributing to vegetation damage and wilting.Furthermore, the interactions among temperature, precipitation, VPD, and soil moisture may become more pronounced during extreme climate events.This complex interplay of extremes led to a substantial photosynthesis reduction (figure 2(c)), with a 26.12 ± 16.09 Tg C decrease in GPP from July to October compared to the 2001-2021 long-term trend across region I.

Spatial distribution of anomalies in the YRB
In July-October 2022, widespread compound heat and drought event in the YRB, generating notable heat stress and water stress, which affected the rate of photosynthesis and carbon sequestration (Wang et al 2023).The response of GPP to this compounded heat and drought event exhibited significant spatial heterogeneity (figure 3(e)), with notable variations among different regions.Both GPP and SIF data confirmed that this extreme climate led to a considerable reduction in photosynthesis, with 26.12 ± 16.09 Tg C of GPP reduced from July to October relative to the 2001-2021 long-term levels across region I.
Regarding the spatial and temporal distribution of environmental factor and photosynthesis data anomalies, the Sichuan Basin in the middle reaches of the YRB encountered pronounced high temperatures and drought in July.These anomalies extended to encompass the entire basin in August but were limited to the southeastern basin in September.The spatial and temporal patterns of the VPD anomalies closely mirrored those of the high temperature and drought, while low soil moisture persisted until the end of October (figures 3(c) and (d 1-4 )).Meanwhile, the GPP and SIF in the Sichuan Basin only showed significant anomalies in August, indicating a roughly one-month time lag in the photosynthetic response of vegetation in this region.
In the YRB, the low-elevation region I endured a more severe and prolonged compound heat and drought stress.Conversely, region II, which also encountered high temperature and drought, exhibited smaller anomalies in VPD and soil moisture, which might be due to the fact that the high temperature melted the permafrost in the Tibetan Plateau region, alleviating moisture stress resulting from reduced precipitation, and the GPP were almost no change.
Photosynthesis anomalies mainly occurred in region I, and different vegetation types exhibited varying responses (table 1).During the compound extreme high-temperature and drought event in 2022, the GPP anomaly of broadleaf forest was the smallest (−8.20 ± 14.31 g C m −2 mon −1 in July-October average), and large in coniferous forests and scrub (−17.41 ± 13.95 g C m −2 mon −1 and −16.80 ± 14.20 g C m −2 mon −1 in July-October average).Additionally, the GPP anomaly of broadleaf forest reached its peak in July (−10.56 ± 21 g C m −2 mon −1 ), whereas all other vegetation types peaked only in August and September, indicating that the resistance of photosynthesis to climate extremes varied among different vegetation types.

Contribution to the photosynthetic anomaly
We employed a multiple linear regression model to assess the impact of individual environmental factors, including 2 m air temperature, VPD, and soil moisture, on GPP and SIF anomalies across different vegetation cover types in YRB (figure 4).The coefficient of determination (R 2 ) ranges from 0.60 to 0.82.High temperatures at 2 m and declining soil moisture together contributed to the decrease in productivity.Notably, our results indicated that elevated VPD had a minimal impact on photosynthesis.
In coniferous forests, broadleaf forests, and shrubs, high temperature had the largest effect on photosynthesis changing, followed by soil moisture, which was much larger than the contribution of VPD.In grassland, both GPP-based and SIF-based contribution analyses revealed that decreased soil moisture contributed more to photosynthesis changes than high temperature.

Development and impacts of compound heat and drought event
In 2022, the YRB experienced a nearly four-monthlong extreme climate event that significantly affected vegetation GPP in the region.We further analyzed the development and impact process of extreme climate in the severely affected region I (figure 5).In July, region I exhibited a 4.01% increase in temperature and a 33.58% decrease in precipitation compared to the July average across 2001-2021, marking the early onset of climate extremes, resulting in an increase in VPD and a slight decrease in soil moisture, and consequently a small decrease in GPP.The compounding effects of persistent high temperatures and drought, as well as suppressed vegetation growth in August-September, resulted in a significant increase in VPD and a decrease in soil moisture and GPP.Although temperature and precipitation had returned to normal gradually in October, soil moisture and GPP required a longer duration to return to their normal ecological state, probably due to the effects of prolonged extreme climate.This phenomenon emphasizes the long-term effects of extreme climatic events on ecosystems, especially on vegetation photosynthesis.By November, changes in all variables compared to the previous two decades fell within three standard deviations, indicating that this compound heat and drought event is essentially finished.
The response of vegetation to environmental change is a complex process involving numerous physiological and ecological mechanisms.Unlike the compound heat and drought events that occurred in Europe in 2003 (Ciais et al 2005) and North America

Heat and soil drought lead to the decline in photosynthesis
Simulated by multiple linear regression, high temperatures greatest impact on the reduction of GPP during July-October 2022, followed reduced soil moisture, with VPD contributing minimally.
High temperatures accelerate transpiration, causing vegetation to lose water and inhibit photosynthesis, as water is necessary for this process.High temperatures can trigger heat stress, which impairs photosynthetic enzymes, further reduces diminishing photosynthetic efficiency.
Under elevated VPD, the stomatal conductance of most species decreases to minimize water loss, with the cost of reduced photosynthesis (Grossiord et al 2020).Meanwhile, VPD-driven enhanced evaporation leads to soil moisture loss, further increasing the risk of exposure to drought stress (Duan et al 2014).Earlier studies have shown that photosynthesis is strongly but lagged correlated with VPD (Wu et al 2017), which is best correlated with VPD three months earlier.Consequently, the multiple linear regression models showed a relatively weak contribution of VPD to the GPP reduction.These findings emphasize the water regulation strategies of vegetation under elevated VPD, highlight the potential role that VPD may play in long-term ecosystem response, which is instructive for further research on the complex impacts of extreme climatic events on ecosystems in the future.
To be noted, the results of the attribution analysis may be influenced by various factors such as vegetation type and meteorological conditions.For instance, in grasslands, unlike other vegetation types, soil moisture contributes slightly more than high temperatures.Forests will adapt to drought through a stronger increase in water use efficiency than grasslands (Schulze et al 2005, Wolf et al 2013).In contrast, grasslands maintain evapotranspiration as long as soil moisture is available (Teuling et al 2010).
Compound heat and drought event in the YRB in China in 2022 leading to a substantial GPP reduction was also supported by Zhao et al (2023) and Wang et al (2023).Due to different study scopes, time periods, climate factors, and modeling methods, there are some differences in the contributions of different climate factors to GPP reduction.During compound heat and drought extreme climate events, the effects of environmental factors on photosynthesis are complex, demanding further research into the interactions and coupling between environmental factors.

Less reduction of photosynthesis in broadleaf forests
Broadleaf forests, with their superior water regulation strategy (Lee et al 2021), stronger leaf stomatal regulation (van der Molen et al 2011) and characteristic water uptake depth of the root systems (Wang et al 2013), have been shown to efficiently access water and sustain photosynthesis during drought conditions.In contrast, coniferous forests, shrubs and grasslands exhibit lower resistance to this extreme event, primarily due to their limited biodiversity (Wang et al 2021, Liu et al 2022, Renard et al 2023).These analysis of the ecological characteristics and resistance of different vegetation types deepens our insights into how they respond to climate change, facilitating improved ecosystem conservation and T Li et al management decisions, such as the implementation of reforestation and fallowing strategies in vulnerable areas to minimize the reduction of photosynthesis during compound heat and drought events.
Heat and drought are not uncommon in YRB.Therefore, certain plant species have developed adaptive mechanisms, like deeper root systems or reduced leaf transpiration, in response to repeated heat and drought.This adaptation enhances their resilience to extreme climates compared to other species.

Conclusion
From July to October 2022, the YRB in China experienced an extensive extreme climate event.Region I experienced a temperature increase of 0.98 ± 0.35 • C and a precipitation decrease of 60.27 ± 23.75 mm compared to the previous two decades, significantly affecting carbon sinks.Our study comprehensively assessed environmental variables, analyzed photosynthesis responses, and investigated the reactions of various vegetation types to these challenges.
(1) Our findings identify July to September as the peak period of high temperature and drought, with anomalies in soil moisture and GPP persisting into October, indicating a delayed environmental impact.
(2) The spatial heterogeneity of GPP and SIF anomalies is highly noticeable, with a substantial GPP loss between July and October in region I, amounting to 26.12 Tg C less than the 2001-2021 average.(3) Notably, broadleaf forests exhibited higher resilience to extreme events with minimal GPP anomalies (approximately 6.46%) and quicker recovery than other vegetation types.(4) We found that high temperatures and reduced soil moisture combined to cause decreased photosynthesis during this compounded heat and drought event.
In conclusion, our study examines the effects of the 2022 high-temperature and drought event in the YRB, emphasizing its influence on ecosystem photosynthesis.We uncover the role of various environmental factors and the distinct reactions of vegetation types to extreme climate events, providing valuable insights for ecosystem management and climate change adaptation strategies, such as targeted conservation policies for different vegetation types, sustainable land-use planning, and the promotion of interdisciplinary collaborative research to continuously monitor and assess the impacts of climate change on ecosystems.

TFigure 1 .
Figure 1.Spatial distribution of the Yangtze River Basin: (a) geographic location, elevation range, distribution of main stream and lakes, and two regions divided according to topographic: region I indicates the low-altitude area and region II indicates the high-altitude area (b) major natural vegetation cover types in YRB, including coniferous forest, broadleaf forest, shrub and grass.

Figure 4 .
Figure 4. Contribution of 2 m temperature, VPD and soil moisture to (a) GPP and (b) SIF anomalies in different vegetation types, gray bars denote unexplained variations.

TFigure 5 .
Figure 5. Changes of GPP and environmental variables in region I of YRB during the compound extreme high temperature and drought from July to October.T, P, VPD, SM and GPP denote the de-trended anomalies in 2 m air temperature, precipitation, vapor pressure deficit, soil moisture and gross primary productivity, respectively, for that month relative to the 2001-2021.

Table 1 .
Monthly and average GPP (g C m −2 mon −1 ) and SIF (mW m −2 nm −1 sr −1 ) detrending anomalies from July to October for different vegetation cover types in region I.
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