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

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).

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2.2.1. Climate data

2.2.2. FluxSat GPP data

2.2.3. GOSIF data

2.3.1. Calculation of VPD

According to Tetens' Formula, the relationship between temperature and the partial pressure of water vapor:

$$\begin{equation}\begin{array}{*{20}{c}} {VPD = es - ea = es - \left( {Rh \times es/100} \right)} \end{array}\end{equation} \tag{ 1 }$$

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.

2.3.2. 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 CO₂ 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:

$$\begin{equation}\begin{array}{*{20}{c}} {y = {\beta ^T}\Delta T + {\beta ^{VPD}}\Delta VPD + {\beta ^{SM}}\Delta SM + \varepsilon } \end{array}\end{equation} \tag{ 2 }$$

where $y$ represents photosynthesis (GPP or SIF) anomalies, after de-trending. ΔT, ΔVPD, and ΔSM also represent anomalies of variables after de-trending. ${\beta ^T}$, ${\beta ^{VPD}}$, and ${\beta ^{SM}}$ are the parameters of the respective variables after multiple linear regressions for the whole region, and $\varepsilon$ is the residual. Thus ${\beta ^i}\Delta i$ (i = T, VPD and SM) represents the contribution made by different environmental factors to the anomaly.

Influenced by the persistent La Niña event, the anomalously wide-ranging and high-intensity subtropical high pressure in the western Pacific Ocean has stabilized in areas such as the YRB in China since July 2022 (Zhang et al 2023), leading to extensive and compounded heat (0.78 ± 0.44 °C) and drought (−45.20 ± 30.10 mm) in July- October. Among them, the high temperature (0.98 ± 0.35 °C) and precipitation reduction (−60.27 ± 23.75 mm) in region Ⅰ, including the Sichuan Basin and the middle and lower reaches of the YRB, were the strongest positive temperature-negative precipitation anomalies in the 21st century (figure 2(a)). High temperatures significantly impact plant physiological processes and have the potential to induce heat stress responses in vegetation, potentially leading to reduced growth rates, flowering, and fruit ripening in this region.

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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 m³ 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.

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.

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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.

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 (R²) ranges from 0.60 to 0.82.

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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.

In 2022, the YRB experienced a nearly four-month-long 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.

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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 in 2012 (Wolf et al 2016), the compound event in the YRB in 2022 had a longer duration and a larger elevation span, and thus greater spatial heterogeneity in the GPP response. Following an extreme climatic event, vegetation may adapt to environmental stress by adjusting parameters like photosynthesis rate, stomatal conductance, and water use efficiency, which can cause a time lag in GPP response.

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.

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 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.