**1. Introduction**

The impact of climate change on terrestrial vegetation growth and its mechanism are important issues in the field of global change research. A large number of studies have proven that global climate change is an important driving factor for changes in terrestrial vegetation in recent decades [1–3]. For the Tibetan Plateau, it is generally believed that simultaneous increases in temperature and precipitation will be beneficial to alpine vegetation growth [4–6]. However, the sensitivity of vegetation productivity to temperature changes at the beginning of this century has declined, and climate warming during nongrowing seasons is considered as one of the main reasons [7]. Climate warming in the nongrowing seasons is not conducive to the preservation of snow in winter, and to a certain extent, it affects vegetation growth indirectly by affecting the water supply in the early growing season [8]. Another study found that temperature increases are likely to affect the plant growth of alpine grassland by changing the forms of precipitation at the beginning of the growing season [9]. In the context of climate change, compared

**Citation:** Zhao, W.; Luo, T.; Wei, H.; Alamu; Zhang, L. Relative Impact of Climate Change and Grazing on NDVI Changes in Grassland in the Mt. Qomolangma Nature Reserve and Adjacent Regions during 2000–2018. *Diversity* **2022**, *14*, 171. https://doi.org/10.3390/d14030171

Academic Editor: Michael Wink

Received: 13 February 2022 Accepted: 25 February 2022 Published: 27 February 2022

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with temperature, although the changes in precipitation are not necessarily the main limiting factor for the variations in the normalized difference vegetation index (NDVI) on the Tibetan Plateau [10], changes in moisture caused by temperature (such as increased evaporation) are likely to affect vegetation growth. In addition, the effects of temperature and precipitation on the NDVI of the Tibetan Plateau vary significantly depending on the seasons and regions [11,12]. Therefore, to explore the impact of climate change on alpine grasslands, we need to fully consider the temporal and spatial changes in temperature and precipitation.

The Tibetan Plateau is one of the main pastoral areas in China, and long-term grazing activities have a dual impact on the grassland of the plateau. On the one hand, grazing affects vegetation biomass and soil compactness through gnawing and trampling [13]. On the other hand, grazing may stimulate plant growth [14] and improve soil fertility through manure [15]. Therefore, it seems that the grazing intensity determines the effect on alpine grassland to a large extent, and the influence of grazing activities on the vegetation in pastoral areas may vary with the grazing density and the length of the grazing period. Exploring a reasonable grazing intensity is a key issue for vegetation protection and sustainable use of forage; however, the related research is still controversial. For example, Zhang et al. (2015) and Lehnert et al. (2016) pointed out that climate is the main controlling factor for changes in alpine grassland ecosystems on the Tibetan Plateau [16,17]. However, Pan et al. (2017) believed that the nonclimatic factors from 1982 to 2013 were responsible for the changes in the alpine grassland on the plateau [18].

As an important national barrier for ecological security and the Asia Tower [19], the Tibetan Plateau plays a unique role in protecting the ecological security of China and even Southeast Asia. However, under the background of global climate change and increasing human activities, the function of the national barrier for ecological security on the Tibetan Plateau is facing serious threats and challenges. The Mt. Qomolangma National Nature Reserve and the seven surrounding counties of Zhongba, Saga, Gyirong, Nyalam, Tingri, Dinggye, and Ngamring, belonging to the Tingri Ecological Observation Station of the Ecology and Environment Bureau in Tibet, are an important ecosystem type on the Tibetan Plateau. The protection of grassland resources is an important part of the construction of ecological security barrier in Tibet [20]. According to the 1:1,000,000 Vegetation Atlas of China [21], the forest, shrub, and grassland in the Mt. Qomolangma National Nature Reserve and adjacent regions accounted for 0.66%, 3.80%, and 68.15%, respectively. According to statistical data, the available grassland area in this region is approximately 82,800 km2, of which degraded grassland accounts for approximately 42% [22]. Additionally, grassland degradation poses important challenges to the protection and construction of regional ecological security barriers. In this study, based on MODIS (Moderate Resolution Imaging Spectroradiometer) and SPOT (Systeme Probatoire d'Observation de la Terre) remote sensing data, the temperature and precipitation data, and the data of the number of livestock during 2000–2018 in Mt. Qomolangma National Nature Reserve and adjacent regions, we aimed to (1) illustrate the temporal and spatial patterns of grassland NDVI and (2) disclose the relative effects of climate change and grazing on NDVI changes. Here, temperature and precipitation were regarded as the main indicators of climate change and the number of livestock at the end of the year as the key indicators of grazing.

### **2. Materials and Methods**

### *2.1. Study Area*

The Mt. Qomolangma National Nature Reserve and adjacent regions (27.70◦ N–31.90◦ N; 82.10◦ E–88.40◦ E) are located in the south Tibetan Plateau, including the seven counties of Zhongba, Saga, Gyirong, Nyalam, Tingri, Dinggye, and Ngamring (Figure 1). This area is located between the middle section of the Himalayas and the Gangdise Mountains. The terrain is high in the north and south, with the Yarlung Zangbo River passing between. The middle part is, therefore, characterized by wide valleys and alluvial plains. The landform types are complex and diverse, and the climate exhibits cold and dry conditions. Based

on the long-term meteorological observations (1967–2019) at Dingri station (a.s.l., 4300 m), the mean annual, January, and July air temperature and annual rainfall were 3.08, −5.84, 11.69 ◦C, and 286 mm, respectively [23]. Alpine grassland (mainly comprising alpine meadow and alpine steppe) is the main land-cover type (Figure 1). The common species in alpine steppe contain *Stipa purpurea*, *Lasiocaryum densiflorum*, *Artemisia wellbyi*, *Incarvillea compacta*, and *Androsace tapete*, while those in alpine meadow include *Kobresia pygmaea*, *Potentilla bifurca*, *Lancea tibetica*, *Arenaria bryophylla*, *Leontopodium pusillum*, and *Gentian* sp. The Mt. Qomolangma National Nature Reserve and adjacent regions have an area of approximately 87,600 km2, where usable grassland area covers 82,800 km2. The areas of lightly degraded, moderately degraded, and severely degraded grassland of the study area are 23,400; 10,400; and 33,300 km2, respectively, accounting for 26.71%, 11.93%, and 3.78% of the total grassland area, respectively [22].

**Figure 1.** The location and vegetation types of the study area. The map was created using ESRI ArcMap 10.7 software; the topographic base of the map was created with National Catalogue Service For Geographic Information data (https://www.webmap.cn, figure number: GS(2016)2556); the vegetation map source from the CASEarth Data Sharing and Service Portal (http://data.casearth.cn, accessed on 19 December 2018).
