Assessing the impact of land conversion and management measures on the net primary productivity in the Bailong River Basin, in China
Introduction
As an important parameter of ecosystem function and carbon cycle, net primary productivity (NPP) can be used to quantify the impact of LUCC on earth systems and corresponding global change (Xu et al., 2007). In recent years, China has suffered severe environmental consequences owing to overgrazing and excessive afforestation, especially in areas with fragile ecology and complex environments (Ding, 2003, Wang et al., 2013). To reduce deforestation, promote forest profits, and reduce human pressure on the environment, China has implemented a series of vegetation-related programs. The most famous one is the Grain for Green Project (GGP) launched in the late 1990s (Song et al., 2015). The goal of the project is to increase vegetation coverage and reduce soil erosion by planting trees and converting bare land on farmland or steep slopes into forests and grasslands (Wang et al., 2012, Lu et al., 2013). In 2003, another large-scale ecological restoration program, Grazing Withdrawal Program (GWP), was launched to supplement the impact of GGP. The GWP places special emphasis on reducing grazing pressure and restoring degraded grasslands by means of grazing exclusion, employing cultivated pastures, or rotational grazings (Zhang et al., 2016, Mu et al., 2013). Some researchers have also concluded that ecological construction projects have a significant impact on vegetation changes (Cao et al., 2006, Zhuo et al., 2007, Wang et al., 2009, Li et al., 2011, Huang et al., 2012). Zhu et al. (2016) pointed out that land use changes (caused by afforestation) are the main reasons for greening in Southeast China. Compared with before and after China launched the GGP, Li et al. (2017a) assessed that the GGP strongly promoted the growth of Normalized Difference Vegetation Index in the Loess Plateau, with slope greater than 4.75 × 10−3 a−1. Zhao et al. (2017) reported that the correlation coefficient (R2) between cumulative afforestation and vegetation greening trends in the Loess Plateau of China from 2000 to 2014 was 0.96 (p < 0.01). Wu et al. (2013) also proposed that ecological restoration plans may be one of the main driving forces for increased vegetation activities in arid and semi-arid environments. The Bailong River Basin (BRB) in southern Gansu Province is located in a typical transition zone between the subtropical and northern temperate climate zones, with obvious ecosystem fragility and sensitivity. In the last few decades of the 20th century, unreasonable developmental activities, such as deforestation, have reduced the vegetation coverage in the basin and increased barren areas (Li et al., 2019). The vegetation in the BRB area has undergone major changes and has attracted widespread attention. Fortunately, several national ecological restoration projects had been implemented (Gong et al., 2021). After the implementation of the Natural Forest Protection Project in 1998, the natural forest resources in BRB have been effectively protected, and significant results had been achieved in forest resource management and biodiversity protection. To reduce the risk of floods and soil erosion disasters, and to improve regional vegetation conditions and ecological quality in the BRB, the GGP was implemented in 1999, which stipulates that the reclamation of slopes above 25°was prohibited. After more than 10 years since project implementation, vegetation coverage in the study area has increased significantly (Gong et al., 2019).
There have been numerous previous studies on the relative contribution of climate change and human activities to NPP changes (Yang et al., 2021, Zhou et al., 2015, Xu et al., 2016), and from 2000 to 2015, the role of human activities in grassland degradation in northern China was greater than that of climate change (Yan et al., 2019). Mu et al. (2013) showed that the total amount of NPP in the grasslands of Inner Mongolia increased from 2001 to 2009, and the contribution of human activities was as high as 80%. According to a report (Gang et al., 2018), human activities contributed to a 78.45% increase in the NPP of grasslands in the Loess Plateau from 2000 to 2015. However, most previous studies were limited to assessing vegetation changes and their responses to climate change (Mu et al., 2013, Gang et al., 2018, Zhang et al., 2020) and did not consider the different vegetation types (de Beurs et al., 2015, Evans and Geerken, 2004, Lioubimtseva, 2014). In the ecological project implementation regions, how land conversion and land management measures affect the spatial pattern and NPP of vegetation and to what extent climate change promotes these changes remains an unresolved issue. Indeed, understanding the spatiotemporal changes of NPP and the impact of land conversion and management measures is essential for ecological monitoring and ecosystem management (Dirnböck et al., 2003, Scurlock et al., 2002). Therefore, we studied the impact of land conversion on NPP changes after the implementation of the government's ecological plan. In contrast, NPP is used as a metric to distinguish between human activities (land conversion and management alternatives) and climate change leading to LUCC, which is essential for maintaining optimal ecosystem functions and predicting future global carbon cycle trends.
Our main aims were to: (i) analyse the spatial pattern of BRB land use and NPP changes from 2000 to 2019; (ii) quantify the changes in NPP induced by land conversion during this period; and (iii) distinguish between climate change and human activities (land conversion and management measures) based on the relative contribution to NPP. The results of this study will not only provide basic data for the comprehensive evaluation of the efficiency of ecological restoration plans but also help land managers and policymakers understand the status of BRB vegetation and formulate future ecological policies related to it.
Section snippets
Study area
The BRB is a first-level tributary on the right bank of the Jialing River in China. It originates in Langmuir, a town at the junction of Sichuan and Gansu Provinces (Fig. 1). It then flows from the northwest to southeast through Diebu County, Tanchang County, Zhouqu County, Wudu District, and Wen County, and finally flows into the Jialing River in Sichuan Province. We selected a part of the BRB in Gansu Province as the study site (32°36′–34°24′N, 103°00′–105°30′E), covering a total area of
Temporal changes in ANPP
Mean annual ANPP was in the range of 0.45–0.65 kg C m−2 a-1, with an average of 0.54 kg C m−2 a-1, a lowest value of 0.456 kg C m−2 a-1 in 2012 and a highest of 0.645 kg C m−2 a-1 in 2016 (Fig. 2(a)). ANPP increased by 0.0063 kg C m−2 a-1. The results of the MK test for NPP of the BRB from 2000 to 2019 are shown in Fig. 2(b). Overall, the value of the UF(k) curve from 2000 to 2019 was positive, showing an upward trend. This result indicates that ANPP showed a clear upward trend. The UF(k) and
Effects of land conversion on vegetation dynamics
The ecological restoration plan has played an active role in vegetation and ecosystem restoration in China (Chen et al., 2019, Zhou et al., 2021b). An analysis of the main human activities that affect the vegetation change of BRB showed that the vegetation greening trend of BRB is mainly attributed to human-driven factors, and the positive contribution of human activities (land conversion and management measures) to vegetation change is far greater than that of climate-driven factors. Some
Conclusions
We analysed the temporal and spatial changes in vegetation coverage of BRB from 2000 to 2019 and the effects of climate change and human activities (mainly land conversion and management measures) on vegetation dynamics. These man-made vegetation changes are closely related to ecological restoration projects. Based on the above analysis, the following conclusions can be drawn: the ecological restoration project greatly changed the land use type and the spatial pattern of NPP in the basin from
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgments
This work was supported by the National Key Research and Development Plan (Grant No. 2017YFC1501005), the National Natural Science Foundation of China (Grant Nos. 42077230), the Science and Technology Major Project of Gansu Province (Grant No. 19ZD2FA002), and the Science and Technology Planning Project of Gansu Province (Grant No. 18YF1WA114).
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