Response of Vegetation Productivity to Greening and Drought in the Loess Plateau Based on VIs and SIF

Gross primary productivity (GPP) is the amount of organic carbon fixed by plants via photosynthesis per unit time [1], and is the largest and most uncertain component of the global carbon cycle [2]. As the basis of human production and life, changes in GPP are related to human welfare. Understanding how photosynthesis responds to global environmental change is particularly important because small perturbations in terrestrial productivity have implications for global biodiversity, agriculture, and climate change [3,4].

Long-term satellite data show a significant greening trend in global vegetation area since the 1980s, driven by human land use management (e.g., revegetation in China), climate change, and CO₂ fertilization [5,6]. Continued greening has led to an increase in vegetation productivity. According to recent studies, the global terrestrial carbon sink increased from (−0.2 ± 0.9) Pg C yr⁻¹ (1 Pg = 10¹⁵ g) in the 1960s to (1.9 ± 1.1) Pg C yr⁻¹ in the 21st decade [7]. Changes in evapotranspiration (ET) due to vegetation greening can also have an impact on local/regional climate. In general, an increase in ET due to the greening of vegetation enhances the local water vapor cycle, leading to an increase in precipitation in downwind areas and a weak but significant downward trend in near-surface air temperatures [8]. There is increasing evidence that the greater atmospheric water demand with rising temperatures (i.e., radiative effects of CO₂) may lead to an increased intensity and frequency of drought, which could notably affect vegetation growth and crop yields [9,10,11]. Drought is a persistent and abnormal shortage of rainfall. The World Meteorological Organization (WMO) classified drought according to the affected domain as meteorological, agricultural, hydrological, and socioeconomic [12]. Drought is one of the most prevalent natural disasters in the world and has the most severe and widespread impacts on terrestrial ecosystem GPP. Droughts have tremendous impacts on ecosystem composition, structure, and functioning, largely affecting GPP accumulation and weakening the carbon sink function of terrestrial ecosystems [13,14,15]. In addition to this, drought events are often associated with complex emergencies involving multiple and compound hazards (e.g., food shortages, economic crises, or human/livestock/crop diseases). The interaction between a number of such factors can affect exposure, vulnerability, and capacity to cope with the crisis. Therefore, it is important to study the response of GPP to drought under the greening trend for future agricultural security and ecological protection.

Changes in gross primary productivity (GPP) profoundly affect food security and the stability of the carbon cycle. Among the methods of ground-based observation of GPP, the inventory method estimates changes in carbon stocks in terrestrial ecosystems based on comparisons of inventory data from different periods. The lack of long-term continuous inventory data and the uncertainty of the conversion process from sample points to regional scales have led to a large bias in the results at the regional scales [16]. The eddy covariance (EC) technique is considered to be the most stable and accurate method for estimating GPP at the ecosystem scale [17]. It is based on the principle of micrometeorology and directly measures the net CO₂ exchange between land ecosystems and the atmosphere over a fixed coverage area. Nevertheless, the number of EC stations is sparse and unevenly distributed, making it challenging to study aspects of GPP at the regional scale [18]. The remote sensing technique is more effective than the ground-based observation method. Remote sensing techniques are well suited to monitor changes in ecosystem GPP at broad spatial scales compared to ground-based observation methods. Over the past 40 years, a series of time-continuous and spatially consistent GPP products have been generated based on satellite remote sensing, providing directly observed data for ecosystem research and management [19]. Thus, in this paper, Moderate-resolution Imaging Spectroradiometer (MODIS) GPP products were used to analyze the study area.

Vegetation indices (VIs) based on remotely sensed reflectance, which have been widely used to monitor changes in vegetation growth, can help us to better understand the changes in GPP in areas during periods of drought [20]. Previous studies have shown that the normalized difference vegetation index (NDVI) can indicate the greenness of vegetation and can be used to monitor vegetation growth [21]. Two recently developed indices, near-infrared reflectance of vegetation (NIR_V) and kernel NDVI (kNDVI), both of which solve the background contamination problem well, are stronger than NDVI in linking with GPP [22,23]. NIR_V only needs to input two parameters when estimating GPP, which drastically reduces the complexity of GPP estimation. It has been shown that NIR_V can replace SIF to study the photosynthesis of vegetation [24]. The coupling between the components of canopy structure that influence NIR reflectance and stress-constrained canopy photosynthetic capacity remains strong at drought stress events, making NIR_V and GPP maintain a strong coupling during drought events [24]. kNDVI computes all higher-order relationships for NDVI and resolves the nonlinear relationship with GPP. kNDVI is more highly correlated with GPP and less problematic in terms of noise and instability than products such as NDVI and NIR_V [23].

Solar-induced chlorophyll fluorescence (SIF) is a promising index for satellite monitoring of vegetative photosynthesis. It is fundamentally different from VIs. SIF is a product of vegetative photosynthesis, which can reflect the intensity of vegetative photosynthesis and has a very close physiological and metabolic connection with GPP [25,26,27,28,29,30]. It is well known that an arid environment will weaken the photosynthesis ability and metabolic function of plants, which in turn will weaken the signal release of SIF. VIs are only sensitive to changes in the canopy structure and chlorophyll concentration of vegetation and are not directly related to the photosynthesis of plants. Hence, when drought occurs, the greenness of vegetation does not decrease immediately, resulting in a certain lag effect of vegetation indexes on drought [31,32,33]. Compared with VIs, SIF has a more sensitive response to drought and is more suitable for monitoring changes in GPP during drought.

China has one of the widest distribution of ecologically fragile areas in the world, accounting for 22% of the national territory, with diverse types of fragility and severe vulnerability [34]. The Loess Plateau in China has experienced severe vegetation loss, soil erosion, and land degradation [35]. In view of this, China has carried out a series of ecological restoration measures, the most successful of which was the Grain for Green Project in 1999 [36]. This project aims to prevent soil erosion, alleviate flooding, and store carbon by increasing forest and grassland cover on previously cropped hillslopes, as well as converting cropland, barren hills, and wasteland into forested areas [37]. Using the ecological restoration measures, the Loess Plateau has shown a clear trend of “greening” [38,39,40]. However, the region is still vulnerable to disturbance and damage, and ecological restoration is difficult, with low carrying capacity, which is an obstacle to carbon reduction and sequestration. Drought will make the fragile ecosystem of the Loess Plateau more unstable and cause irreversible impacts on the ecosystem. The rising greening trend and unstable climate change in the Loess Plateau bring great challenges to GPP’s accounting. Therefore, it is important to study the response of GPP to drought under the greening trend of the Loess Plateau.