Spatiotemporal variation in vegetation coverage and its driving factors in the Guanzhong Basin, NW China
Introduction
Vegetation is an essential element of the terrestrial ecosystem. It connects the water, soil, air and other environmental components (Foley et al., 2000). Vegetation coverage is one of the important indexes used to evaluate ecosystem conditions (Ollinger, 2010; Zheng et al., 2018). In the past few decades, the development of remote sensing technology has made satellite–based images (such as Moderate Resolution Imaging Spectroradiometer (MODIS) and Landsat) one of the most important sources of data for large-scale monitoring of vegetation dynamics (Pettorelli et al., 2005). The Normalized Difference Vegetation Index (NDVI), one of the most important remote sensing products, is an important indicator of vegetation coverage and is highly efficient in monitoring the vegetation dynamic at both the regional and global scales (Band et al., 1993; Lawley et al., 2016). Attribution vegetation changes can provide basic information for environmental resource management, especially in areas with fragile ecosystems (Li et al., 2017). Vegetation cover variations are strongly influenced by multiple natural and anthropogenic factors (Jiang et al., 2017; Liu et al., 2015). Climate-induced near-surface atmospheric warming and the corresponding rise in the surface temperature lead to significant changes in the water and energy balances in terrestrial ecosystems (Hilbich et al., 2008; Nemani et al., 2003). Precipitation and temperature were considered to be the two most important climatic factors affecting vegetation dynamics (He et al., 2015). Previous studies have confirmed that the response of vegetation to climatic factors exhibits significant lag time effects (Qi et al., 2017). This can be explained by different natural factors. For example, in the case of precipitation, this can be caused by the following processes: (1) the delay between the absorption of water by the plant and the subsequent increase in photosynthesis; and (2) the soil acts as an accumulator and spreads the release of the collected water resources over a certain period of time (Muradyana et al., 2019). Human activities cause additional disturbances, including growing populations, economic development, overgrazing, and numerous engineering projects, which are becoming the main factors driving vegetation changes in most parts of the world (Liu et al., 2018a). Studying the impacts of human activities on environmental changes is essential to ensuring a sustainable relationship between humans and the environment (Li et al., 2015). Generally, separately or together, the vegetation changes triggered by human activities and climatic factors, influence the vegetation greenness on the global and regional scales (King et al., 2015; Wang et al., 2015). Thus, understanding vegetation dynamics and its response to human activities and climate changes is important.
Previous studies have used NDVI time series and climate datasets to analyze the responses of vegetation to climatic factors in the Guanzhong Basin. These studies determined that the vegetation cover in the Guanzhong Basin has significantly increased, and the vegetation cover changes respond well to temperature and precipitation (Bai et al., 2013; Chen et al., 2014). However, the relative contributions of climate change and human activities to the vegetation changes in the Guanzhong Basin remain obscure and cannot be precisely calculated. Neglecting the effects of human activities on vegetation may lead to errors in land condition assessments. Therefore, understanding the changes and dynamics of the vegetation cover in relation to climate change and anthropogenic activities is important to developing adaptation strategies to address the challenges caused by climate change and human activities in vegetation ecosystems in arid and semi-arid regions. This study aims to explore the spatial and temporal vegetation dynamics and its response to climate change and human activities in the Guanzhong Basin, Northwestern China from 2000 to 2020. The objectives of this study are to (1) quantify the long-term trends of the NDVI at the regional scale; (2) to analyze the relationships between the NDVI and the climatic factors and human activities; and (3) to explore the relative contributions of climate change and human activities to vegetation changes in arid and semi-arid regions.
Section snippets
Study area
The Guanzhong Basin is located in the central part of Shaanxi Province in China and covers an area of 20,440 km2 (107°30′–110°37′E, 33°39′–35°50′N). The region is bordered by Tongguan to the east, Baoji to the west, the Qinling Mountains to the south, and the Weibei Mountains to the north (Fig. 1). The Guanzhong Basin has a developed economy and plays an important role in Northwestern China, which includes Xi'an, Baoji, Xianyang, Weinan, Tongchuan and Yangling. Xi'an is one of the most
Temporal and spatial distributions of the NDVI
Based on the annual change characteristics, the vegetation coverage in the Guanzhong Basin was good and exhibited a fluctuating increase (Fig. 2a), with an annual growth rate of 0.003 a−1. The annual average NDVI was 0.68 from 2000 to 2020, indicating a high vegetation coverage, and the variation range was 0.64–0.73. The overall level of vegetation coverage was relatively high due to the highly developed agricultural civilization in the Guanzhong Basin (Wen and Yao, 2018). The highest NDVI
Discussion
During 2000–2020, the long-term average value of the annual maximum synthesis NDVI was 0.68. The growth rate of the vegetation coverage in the Guanzhong Basin from 2000 to 2020 was 0.003 a−1, which is higher than the growth rate of 0.002 a−1 on the China's Loess Plateau during 1982–2015 (Li et al., 2017) and is much lower than the growth rate of 0.006 a−1 in the Shaanxi Province during 2001–2018. Moreover, the rate of change was spatially variable. From 2000 to 2020, the NDVI decreased in the
Conclusions
In this study, the spatial-temporal variations in the NDVI, precipitation and temperature during 2000–2020 were analyzed, the relationships and lag time effects between the NDVI, precipitation, and temperature were studied, and the contributions of climate change and human activities to the vegetation cover changes in the Guanzhong Basin, northwestern China, were determined. The results show that the NDVI, precipitation, and temperature increased, with annual growth rates of 0.003 a−1,
Declaration of Competing Interest
No potential conflict of interest was reported by the authors.
Acknowledgements
This work was supported by the National Natural Science Foundation of China (grant number 41601029), Shaanxi Provincial Department of Education Scientific Research Project (grant number 16JS032), Young Talent fund of University Association for Science and Technology in Shaanxi, China (grant number 20180705), and Natural Science Talent Project of Weinan Normal University (grant number 18ZRRC04). The authors are grateful to the reviewers for their helpful comments and suggestions to improving the
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