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Journal of Geographical Sciences >

2021 , Vol. 31 >Issue 9: 1299 - 1327

DOI: https://doi.org/10.1007/s11442-021-1899-8

Research Articles

Coupled effect of climate change and human activities on the restoration/degradation of the Qinghai-Tibet Plateau grassland

  • YUAN Qin , 1, 2 ,
  • YUAN Quanzhi , 1, 2, * ,
  • REN Ping 1, 3
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  • 1. Institute of Geography and Resources Science, Sichuan Normal University, Chengdu 610101, China
  • 2. Sustainable Development Research Center of Resources and Environment of Western Sichuan, Sichuan Normal University, Chengdu 610066, China
  • 3. Key Laboratory of Land Resources Evaluation and Monitoring in Southwest, Ministry of Education, Sichuan Normal University, Chengdu 610066, China
* Yuan Quanzhi (1985-), PhD, specialized in physical geography. E-mail:

Yuan Qin (1989-), Master Candidate, specialized in physical geography. E-mail:

Received date: 2020-11-30

  Accepted date: 2021-03-09

  Online published: 2021-11-25

Supported by

National Natural Science Foundation of China(41930651)

National Natural Science Foundation of China(41701100)

Application Foundation Project of Sichuan Science and Technology Department(2017JY0155)

Copyright

Copyright reserved © 2021. Office of Journal of Geographical Sciences All articles published represent the opinions of the authors, and do not reflect the official policy of the Chinese Medical Association or the Editorial Board, unless this is clearly specified.
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Abstract

Climate change (CC) and human activities (HA) are the main reasons for the restoration/degradation of the Qinghai-Tibet Plateau (QTP) grassland. Many related studies have been conducted thus far, but the impact mechanism of CC coupled with HA on QTP remains unclear. We summarized the two main coupling factors in recent years (specifically, in the past five years) and obtained the following conclusions. (1) CC and HA have positive and negative effects on the QTP grassland ecosystem. CC primarily affects grassland ecology through temperature, humidification, and extreme climate, and HA mainly affects ecosystems through primary, secondary, and tertiary industries and restoration measures. (2) CC coupled with HA affects soil, plants, animals, and fungi/microbes. CC makes the snow line rise by increasing the temperature, which expands the zone for HA. CC also restricts HA through hydrological changes, extreme climate, and outbreak of pikas and pests. Simultaneously, measures are implemented through HA to control and adapt to CC. Hence, the grassland ecosystem is comprehensively influenced by CC and HA. (3) The grassland ecosystem dynamically adapts to the disturbance caused by CC and HA by changing its physiological characteristics, distribution range, diet structure, community structure, and physical state. Simultaneously, it responds to environmental changes through desertification, poisonous weeds, rodent outbreak, release of harmful gases, and other means. This work can be used as a reference for the sustainable development of the QTP grassland.

Key words： Qinghai-Tibet Plateau; geochemistry; soil organic carbon; net primary productivity; pikas; pests; Cordyceps sinensis

Cite this article

YUAN Qin , YUAN Quanzhi , REN Ping . Coupled effect of climate change and human activities on the restoration/degradation of the Qinghai-Tibet Plateau grassland[J]. Journal of Geographical Sciences, 2021 , 31(9) : 1299 -1327 . DOI: 10.1007/s11442-021-1899-8

1 Introduction

The Qinghai-Tibet Plateau (QTP) is an ecological security barrier for China, East Asia, and the world (Huang et al., 2017). It is located in Southwest China and has an average elevation of over 4,000 m and an area of approximately 2.5724×106 km2; it is the highest and most extensive plateau in the world. QTP has a huge impact on the environment of China and even East Asia due to its extremely thermal and dynamic effects (Zhao et al., 1995). With the exception of the north and south poles, QTP is the largest and the most complete natural ecology that is rarely disturbed by humans (Qiu, 2008). QTP is not only an important area for global carbon sequestration and biodiversity conservation (Mu et al., 2017), but also an indicator of world climate and ecological changes (Chen and zhu, 2015; Zhang et al., 2015b).
Grassland is the main body of QTP, and it accounts for 60% of QTP, which is about 1.5×106 km2 (Mo, 2020). The vast grassland ecosystem has many ecological functions, such as nitrogen and oxygen sequestration, water conservation, climate regulation, prevention of desertification, provision of living places for grassland organisms, provision of food and medicinal materials to humans, and husbandry basis (Fayiah et al., 2020). The alpine grassland ecosystem has important ecological and economic values. Research on the QTP grassland ecosystem is thus essential.
Grassland degradation has become a worldwide ecological problem in recent years. The QTP grassland ecosystem is considerably degraded under the combined effects of climate change (CC) and human activities (HA) (Harris, 2010; Fassnacht et al., 2015; Zhang, 2015d). Degradation causes pest and rodent outbreaks (Cao et al., 2016); decreases in biomass, biodiversity, soil nutrition, vegetation cover, and service function (Dong et al., 2013; Hao et al., 2020); and increases in greenhouse gases (Hu et al., 2017), soil erosion, and sandstorms (Hopping et al., 2018a). Therefore, grassland restoration has recently become an important topic of research on the sustainable development of QTP. Since the 1990s, the Chinese government has been focusing on restoring and protecting the QTP grassland ecosystem via agricultural technology (Dong et al., 2020), cultural education, economic methods, and policy measures (Han et al., 2016). Although these measures have exerted positive impacts on protecting the grassland ecosystem (Cai et al., 2015), several negative effects cannot be ignored; these include fencing (Dong et al, 2020), grazing and hunting exclusion (Lu et al., 2017a), economic compensation, establishment of nature reserves (Jiang et al., 2019), elimination of pests and plateau pikas (Tian et al., 2019), and privatization of grassland (Yeh et al., 2017). The negative effects of restoration measures have gradually attracted the attention of scientists. The reasons for QTP grassland restoration must be clarified, existing measures should be re-evaluated, and scientific and effective grassland restoration strategies based on different degradation reasons must be established.
The QTP grassland ecological restoration/degradation is driven by complex factors of CC and HA. Apart from CC (Lu et al., 2017b; Zheng et al., 2020) and HA (Zhang et al., 2017; Li et al., 2019d), which have been extensively studied, other factors, such as biological invasions (Wen et al., 2020), rodents (Yu et al., 2017), urbanization (Walt et al., 2015), land use change (Hopping et al., 2018a), railway construction (Luo et al., 2020), mining activities (Liu et al., 2020), tourism (Ding et al., 2018), digging and hunting (Xu et al., 2015), population migration (Wang et al., 2016c), and human restoration activities (Dong et al., 2020), play vital roles in the QTP grassland ecological restoration/degradation.
Previous researchers have focused on the impact of CC or HA on the QTP grassland restoration/degradation, but studies on the coupling relationship of CC and HA are insufficient. With this in mind, we discuss the following scientific issues through a review of previous studies: (1) the positive and negative effects of CC and HA on the restoration/degradation of the QTP grasslands; (2) the mechanisms and impacts of CC coupled with HA on grassland geochemical properties, soil organic carbon (SOC), net primary productivity (NPP), pikas, pests, and Cordyceps sinensis; and (3) how the grassland ecosystem dynamically adapts to the disturbance caused by CC and HA.
The data used for this review were from secondary sources, such as peer-reviewed articles on QTP and other grasslands. In total, 148 articles and other sources of materials were reviewed. Among them, 88% were from 2015 to 2020, 10% were from 2010 to 2014, and 3 were published before 2010.

2 Role of climate change in Qinghai-Tibet Plateau grassland restoration/ degradation

Scientists generally believe that CC is an important driving factor for the restoration/degradation of the QTP grasslands, and many observations and experiments have been conducted on this matter. Numerous studies performed in the past five years (Table 1) have shown that CC is mainly affected by temperature and humidification. Scientists have tested the sensitivity of the QTP grassland ecosystems to CC through experiments, and the results showed that CC is a primary driving factor of the QTP grassland degradation (Wang et al., 2016c; Che et al., 2018). However, the driving factors of climate in alpine grasslands are spatially heterogeneous. Alpine grasslands in the south are sensitive to temperature. Alpine grasslands in the northeast show a strong response to precipitation, and those in the middle are mainly affected by the combined effects of radiation and temperature. Climate sensitivity increases considerably with altitude (Li et al., 2019b). Han et al. (2019) used the normalized differential vegetation index (NDVI) of the mesoscale resolution imaging spectrum from 2000 to 2018 to combine the changes in temperature and precipitation and systematically study the characteristics of the QTP vegetation coverage and its driving factors. In general, the restored area is larger than the degraded area, and NDVI is significantly related to temperature and precipitation, of which temperature is dominant. The QTP grassland has spatial heterogeneity, and the increase in precipitation and temperature cannot explain the vegetation change characteristics in the entire QTP alpine grassland (Chen, 2020). However, given the warm and humid background, the degree of greening of the QTP grassland vegetation has increased significantly, and the overall habitat quality has become stable (Jiang and Zhang, 2016).
Table 1 Impact of climate change on Qinghai-Tibet Plateau restoration/degradation
Effect Factors Mechanism Study area Research period
(year)		 Reference
Grassland restoration Warming Warming increases
microbial residues, and this increase is conducive to SOC formation		 Beiluhe Observation and Research Station of Chinese Academy of Sciences (34°51′N, 92°56′E) on QTP 2008-2010 Ding et al., 2019b
Vegetation photosynthesis is strengthened, and the green area is expanded R1 & R2: The Three Rivers Headwater Region R1: 2000-2012;
R2: 1982-2013		 R1: Jiang and Zhang, 2016;
R2: Gao et al., 2019
Warming increases
vegetation height		 R2: Nagqu County (31°26′N, 92°11′E) and Bange County (31°23′N, 92°11′E) 2010-2013 Zhang et al., 2015d
Wetting Humid conditions help alleviate sandstorms, wind erosion, and desertification 165 random grass plots on the QTP 2000-2015 Huang et al., 2017
In the short term, the amount of water is
increasing, which is
conducive to plant growth		 Three Rivers Headwater Region 2000-2012 Jiang and Zhang, 2016
Warm and wet conditions Vegetation growth is
promoted in warm and
wet conditions		 QTP (26°30′-39°30′N, 78°18′-103°60′E) 2001-2013 Wang et al., 2016c
Vegetation activity is enhanced in warm and
wet conditions		 QTP (25°60′-39°12′N, 76°42′-105°30′E) 1982-2011 Cong et al., 2017
NPP is enhanced in warm and wet conditions QTP (26°30′-39°30′N, 78°54′-103°60′E) 2001-2015 Zheng et al., 2020
Microbial activity
increases as the
temperature rises under warm and wet conditions		 Nam Co Monitoring and Research Station for Multisphere Interactions (Nam-MI), which is located on central QTP (30°47′N, 90°58′E) 2012-2014 Chen et al., 2020a
Microorganisms multiply and SOC increases under warm and wet conditions R1: Nam Co Monitoring and Research Station for Multisphere Interactions (Nam-MI), which is located on central QTP (30°47′N, 90°58′E);
R2: fenced alpine meadow located at Damxung Grassland Station, QTP (30°51′N, 91°05′E, 4333 m above sea level)		 R1: 2012-2014
R2: 2015-2044		 R1: Chen et al., 2020a; R2: Guan et al., 2018
Grassland degradation Warming The grassland ecological changes from carbon sinks to sources through the collapse of permafrost and increment in soil
respiration		 Eboling Mountain Region (38°00′N, 100°54′E) 2014-2016 Mu et al., 2017
The oxidation capacities of carbon, nitrogen, and methane are affected by warming QTP 2010-2019 Chen et al., 2013
Large amounts of greenhouse gases are emitted into the atmosphere by warming Boundary region between permafrost areas and seasonal frozen ground areas on southern QTP September 2014 Wu et al., 2018
Soil nutrients are lost because of SOC decomposition in warming condition QTP 2010-2017 Liu et al., 2018
Warming significantly increases the soil water- soluble organic carbon and affects the chemical
composition of SOC		 Qinghai Province (31°39′-39°19′N, 89°35′-103°04′E) 1985-2044 Zhang et al., 2017
Effect Factors Mechanism Study area Research period
(year)		 Reference
Grassland degradation Warming Soil dryness is increased by warming Administrative village in Namtso Township in the Tibet Autonomous Region of China 2006-2013 Hopping et al., 2018a
Soil warming in early spring partially offsets productivity gains from the growing season Haibei Grassland Ecological Monitoring Station (36°57′N, 100°51′E) 1997-2011 Guo et al., 2018a
Plant diversity is reduced by warming Nagqu County (31°26′N, 92°11′E) and Bange County (31°23′N, 92°17′E) July to August 2013 Zhang et al., 2015d
The adaptation of weeds and pests is increased by warming Haibei Alpine Meadow Ecosystem Research Station (37°37′N, 101°12′E) 2010-2011 Cao et al., 2015
Soil microbial diversity is significantly reduced by warming in cultivated grasslands Nagqu County (31°26′N, 92°11′E) and Bange County (31°23′N, 92°16′E) 2013-2016 Zhang et al., 2016a
Soil microorganisms convert into poor nutrient content and low active state in warming
condition		 Haibei Alpine Meadow Ecosystem Research Station (37°37′N, 101°12′E) 2006-2012 Che et al., 2018
In the long run, the amount of water will decrease because of
glacier retreat and
permafrost thawing		 Tanggula Mountains over central QTP 1976-2013 Ke et al., 2017
Wetting The soil’s capacity for water conservation is decreasing because of the increase in rain erosion Tibet 2000-2019 Chen et al., 2020b
Excessive humidity is not conducive to vegetation growth in humid areas Nam Co Monitoring and Research Station for Multisphere Interactions (30°47′N, 90°58′E) Every April to September from 2012 to 2014 Chen et al., 2020a
Soil microbial diversity is greatly reduced by concentrated heavy rain in natural alpine grasslands Nagqu County (31°26′N, 92°11′E) and Bange County (31°23′N, 92°17′E) 2013-2016 Zhang et al., 2016a
Extreme climate Alpine animals and plants are covered and frozen to death, and the soil is eroded by extreme
climate disasters, such as snowstorm, rainstorm, and thunderstorm		 R1: Three Rivers Headwater Region (31°39′-36°12′N, 89°45′-102°23′E)
R2: About 2946 km long from east to west, west from the Pamirs and east to the Hengduan Mountains; about 1532 km wide from south to north, south from the Himalayan Mountains and north to the Kunlun-Qilian Mountains
R3: QTP		 R1: 1960-2010
R2: 1949-2015
R3: 1961-2008		 R1: Wang et al., 2014;
R2: Wang et al., 2019b;
R3: He et al., 2016
Relevant studies conducted in recent years have revealed the following spatial and temporal features. With regard to spatial features, many micro-scale studies were conducted in central and eastern QTP where monitoring and research stations, such as Nagqu, Bange, Beiluhe, Nam-MI, and Haibei, can be found. Meanwhile, many macro-scale studies were conducted in the Three Rivers Headwater Region, Qinghai province and Lhasa city, Himalayan Mountains, Tanggula Mountains, and entire QTP by using remote sensing technology, meteorological data, and literature reviews. With regard to temporal features, the time span was large in many studies, especially those on the effect of CC on the grassland ecosystem from 2000 to 2019. In addition, the growing season (April to September) is a hot period for many studies, and only a few scientists have paid attention to the ecological resilience in winter.

2.1 Role of climate change in Qinghai-Tibet Plateau grassland restoration

2.1.1 “Warm and wet” trend
The “warm and wet” trend of CC has been recognized by most scientists (Schuur et al., 2015; Hoham and Remias, 2020). Therefore, most studies on QTP grassland ecosystems are based on the background of “warm and wet.” However, a few scientists have proven through experiments that the warm and humid trend is conditional. Wen’s eight-year (2011-2018) warming experiment showed that short-term warming (three years) brings about a “dry and hot” trend, and long-term warming (>three years) brings about a “warm and humid” trend (Wen et al., 2020). Therefore, warming and wetting are conducive to the restoration of the QTP grasslands within a certain range (Chen et al., 2015).
2.1.2 Role of warming in Qinghai-Tibet Plateau grassland restoration
Solar radiation and temperature are the dominant factors in alpine meadows for the restoration of grassland (Zheng et al., 2020). NPP increases in warm conditions, and warming has increased the photosynthesis of vegetation, extended plant growth seasons, expanded grassland areas, and increased grassland NPP (Zhang et al., 2014; Mao et al., 2015). Glacier retreat and permafrost thawing are caused by increasing temperature (Wu et al., 2012; Ke et al., 2017). Hence, the altitude that can be covered by vegetation increases, and grassland expands. Warming has significantly increased the overall NPP of grassland, but due to the regional imbalance of driving factors, the NPP of several areas has weakened. In detail, the western region of QTP has become relatively dry, and its NPP has decreased. The eastern region has become warmer and wetter than before, and its NPP has increased gradually from the east to the west (Yuan et al., 2017). Moreover, a certain correlation exists among temperature increase, permafrost thawing, and vegetation cover. Yuan’s experiments showed that a slight increase in soil temperature and vegetation cover caused by a temperature increase can offset the impact of CC on permafrost (Yuan et al., 2020).
2.1.3 Role of wetting in Qinghai-Tibet Plateau grassland restoration
Microorganisms multiply in humid conditions, which is beneficial to plant growth (Wang et al., 2018a). Ding reported that humid conditions contribute to microbial residues (Ding et al., 2019b). Chen showed that microbial activity increases in moderately wet conditions (Chen, 2020). In addition, wind speed is subject to humidity, and the frequency of wind erosion and sandstorms is reduced in wet conditions (Jiang and Zhang, 2016), which is conducive to the restoration of QTP grasslands. Altogether, humid conditions are conducive to the restoration of QTP grasslands within a certain range.

2.2 Role of climate change in Qinghai-Tibet Plateau grassland degradation

CC leads to QTP grassland degradation directly and indirectly. Warming, wetting, drought, and extreme weather affect the grassland ecosystem directly, and ecological participants, such as soil, vegetation, animals, and atmospheric composition, lead to indirect degradation of the QTP grasslands.
2.2.1 Grassland degradation caused by climate change directly
First, many scientists believe that the drought brought about by CC exacerbates the degradation of the QTP grasslands because warming increases evaporation, which is not conducive to the growth of the grassland vegetation. A short-term temperature increase (three years) makes air and soil dry and hot (Wen et al., 2020), thereby limiting the growth of grassland vegetation (Fu et al., 2013; Hopping et al., 2018a).
Second, grassland is degraded in over-wet areas. Glacier retreat and permafrost melting, which are caused by CC, lead to an over-humid condition in the atmosphere and soil, and NDVI is negatively correlated with precipitation in humid areas (Han et al., 2019). Precipitation increases plant productivity, leading to an increase in nutrient consumption and a decrease in the content of SOC. SOC is lost under excessive humid conditions in alpine meadows (Yang et al., 2017). McCarthy reported that plants allocate a relatively small proportion of their body out of surface to conduct photosynthesis, thereby reducing the carbon input from vegetation to roots under a condition of relatively sufficient moisture (Mccarthy and Enquist, 2007).
Third, as the amount of water increases, rain erosion intensifies, and the risk of being eroded by wind and water in grasslands increases (Chen et al., 2020a).
Fourth, warming in the non-growing season is not conducive to grasslands. Guo found that warming before the growing season (March to April) in winter and spring reduces soil water use efficiency and decreases grassland productivity (Guo et al., 2018a).
Fifth, extreme weather also causes grassland degradation (Chi et al., 2016). He analyzed the spatial pattern and trend of extreme weather in QTP alpine meadows by using meteorological data from 1961 to 2008. His results showed that climate experiences many warm extremes and few cold extremes (He et al., 2016). In addition, extreme weather affects the physiological and biochemical processes of plants (Han et al., 2019). Heavy rains, hail, snowstorms, mudslides, ice avalanches, and thunderstorms are the main extreme weather disasters in QTP (Wang et al., 2014; Bai et al., 2019). The monitoring of the QTP snow disasters indicates that 238 large-scale snow disasters have occurred in the past 50 years. A high-risk zone of snow disasters from northeast to southwest was identified in space (Wang et al., 2019b). The grassland here was covered by severe ice avalanches and blizzards, and the animals and plants were covered or froze to death. Meanwhile, heavy rainfall and hail aggravate soil erosion (He and Richards, 2017). Frequent extreme weather is another reason for the degradation of the QTP grasslands.
2.2.2 Grassland degradation caused by climate change indirectly
Grassland degradation can be caused by soil changes under CC. An increase in temperature causes permafrost thawing; the water seeps, and the soil surface becomes dry, which is not conducive to vegetation growth (You et al., 2017). Moreover, warming increases evaporation and significantly reduces the average soil moisture in the growing season (Fu et al., 2013). Although warming has increased the amount of water via glacier retreat and permafrost thawing, the water volume is expected to decrease in the long run, eventually leading to soil water shortage and grassland degradation (Wang et al., 2016b; Ke et al., 2017). Moreover, warming accelerates the decomposition of organic matter and strengthens soil respiration, thereby changing the soil carbon cycle (Ding et al., 2019b). Carbon flux measurements have shown that QTP grassland is an important carbon sink (Doetterl et al., 2015). In warm and wet conditions, the carbon sink function of the QTP grassland ecology is significantly strengthened (Mu et al., 2017), and the increasing temperature causes soil and grassland degradation under CC.
Furthermore, grassland degradation can be caused by changes in species relationships in grassland ecosystems under CC. The eight-year warming experiment of Wen showed that warming has increased the drought on the surface soil layer, and this increment is conducive to the growth of pikas and poisonous weeds. The adaptation to CC of weeds, pests, and pikas is increasing, thereby reducing grassland biodiversity (Wen et al., 2020). Dorji conducted a five-year (2009-2015) study on the relationship among warming, grazing, spring melting, and grassland organisms in the central part of QTP and discovered that warming reduces species richness and promotes the proportion of shrubs in total vegetation (Dorji et al., 2018). At the same time, the living environments of vegetation and animals are changed by the increasing soil temperature, and the grassland ecosystem is disturbed (Guan et al., 2018). Chen et al.’s (2020a) six-year warming experiment showed that under warming conditions, the DNA replication, transcription, translation, and signal transduction of soil microorganisms are significantly inhibited, and the soil protozoan community shifts to a less nutritious and less active state. Overall, the increase in pests, decline in biomass and biodiversity, and suppression of microorganisms lead to grassland degradation under CC.

3 Role of human activities in Qinghai-Tibet Plateau grassland restoration/ degradation

HA plays an important role in the restoration/degradation of the QTP grasslands. In addition to husbandry and agriculture, which have been widely studied, changes in lifestyle brought about by urbanization, changes in economic growth patterns, and changes in land use brought about by tourism and industries are new factors that influence the QTP grasslands. Furthermore, the disturbance of the biological chain caused by mining and hunting and the ecological restoration measures implemented by humans affect the ecological restoration/degradation of the QTP grasslands. As a result, people are looking for new restoration measures. For example, several scientists believe that artificial grassland is a good replacement for degraded grassland (Li et al., 2017; Li et al., 2018a). Others think that we do not need to carry out any restoration. Li et al.’s (2019a) research on aeolian desertification in the northeast of QTP for nearly a thousand years predicted that as long as moderate HA is maintained, the grassland ecology of QTP will recover naturally even if no restoration is carried out.
Recent relevant studies have revealed the following spatial and temporal features. With regard to spatial features, many studies have adopted typical counties, such as Nagqu, Damzhung, Xihai, Gonghe, Tiebujia, Hongyuan, Maqu, Damxun, and Lhasa, as samples to examine HA effects on grassland. Other studies have focused on the region along the Qinghai-Tibet Railway. In this region, the ecologically fragile areas, such as permafrost and the agro-pastoral zone, are popular research subjects. With regard to temporal features, several management and restoration measures have been implemented since 1984, and these include the privatization of pastures, fencing, and setting of reserve areas. In sum, relevant studies were mainly from the 1990s to the 21st century, and most of them have been conducted for more than 10 years.

3.1 Role of human activities in Qinghai-Tibet Plateau grassland restoration

3.1.1 Moderate grazing is beneficial to grassland restoration
The positive effects of moderate grazing have been widely recognized (Li et al., 2018b; Dong et al., 2020). Experiments have shown that the above-ground biomass, vegetation coverage, SOC, and total soil nitrogen of grassland ecosystems have a slight linear growth relationship with changes in grazing intensity. Furthermore, light and moderate grazing activities exert a positive impact on vegetation diversity and soil respiration. Mu evaluated nectar (fodder) and plant diversity in areas with non-, light, moderate, and heavy grazing. The results showed that the nectar and plant diversity of light and moderate grazing grasslands is higher than that of severe and non-grazing areas (Mu et al., 2016). Cui compared the grazing situation in warm and cold seasons. They found that moderate grazing significantly increases the production of alpine grassland plants in both seasons (Cui et al., 2014). Miehe reported that traditional migratory grazing is the best strategy for protecting QTP grasslands because people have been living with this lifestyle for thousands of years, proving that grasslands are sustainable in this moderate grazing condition (Miehe et al., 2019). Altogether, moderate grazing is beneficial to the ecological health of QTP grasslands.
3.1.2 Urbanization is conducive to grassland restoration to a certain extent
Urbanization is conducive to popularizing education and changing the diet structure in QTP. With the introduction of rice and flour, the demand for beef and lamb declines (Wang et al., 2016a). Several scientists believe that population growth does not necessarily lead to a growth in husbandry. In addition, certain herders move to urban locations to improve their life and educational resources; afterward, they rent out or abandon their ranches (Cao et al., 2018a). Therefore, the problem of overgrazing is alleviated by urbanization, which is beneficial to QTP grasslands.
3.1.3 Tourism is conducive to grassland restoration to a certain extent
Scientists have found that practitioners in the tourism industry are inclined to protect the environment (Li et al., 2019c). Tourism brings income to the local government so that they will have funds for grassland restoration and construction. Furthermore, tourism is a new means to make a living for several residents (Yang et al., 2019); hence, the problem of overgrazing is alleviated to a certain extent. The direction of the future development of QTP is how to manage the tourism in QTP so that it can improve the economic conditions of local residents and local governments and contribute to the protection of grassland ecosystems.
3.1.4 Restoration measures are effective
Beginning in the 1990s, a series of protection and restoration measures were implemented by humans in the QTP grassland ecosystem, and these included agriculture, economy, culture, and policy (Table 2). Han et al. (2016) conducted a sensitivity analysis of the response of ecological service value to land use change. The results showed that the average sensitivity index of ecological service in Qinghai Province was 0.693 and 1.137, respectively from 1988 to 2000 and from 2000 to 2008, which means that for every 1% increase in land use change, the value of ecological services fluctuates by 0.693% and 1.137%, respectively. Monitoring of the ecological status of the QTP grassland from 2000 to 2008 and 2008 to 2015 shows that the grassland degradation trend has weakened significantly, and the restoration trend has strengthened. In particular, the proportions of degraded grassland have decreased by 19.9%, and restored grassland has increased by 33%. Vegetation coverage, especially alpine meadows, has also increased by an average of 1.9%. Furthermore, the pressure of grazing on grassland has decreased, the supply of forage has improved, and the contradiction between forage and livestock has eased (Huang et al., 2017). Evidently, the restoration measures are effective (Shao et al., 2017).
Table 2 Protection and restoration measures being implemented in Qinghai-Tibet Plateau since the 1990s
Methods Reference
Agricultural Reduce livestock
Seasonal grazing
Collective grazing
Fencing
Fertilization
Increase seeding rate
Replanting
Improve grassland quality
Control rodents and pests
Reduce rain erosion by technology		 Dong et al., 2013;
Yeh et al., 2017;
Hopping et al., 2018b;
Wang et al., 2018b;
Miehe et al., 2019;
Dong et al., 2020;
Cao et al., 2011;
Cao et al., 2018c;
Chen et al., 2016a;
Wu et al., 2019
Economic Financial compensation
Ecological protection reward
Compensation for animal accidents		 Yang et al., 2019;
Chen and Zhu, 2015
Cultural Law education
Environmental education
Popularize knowledge/skills of ranch management		 Han et al., 2016
Policy Seasonal fallow
Grazing exclusion
Establish administrator management system
Return ranch to grass
Return construction to grass
Nature reserves
Biodiversity conservation		 Cai et al., 2015;
Lu et al., 2017a;
Lu et al., 2017b;
Jiang and Zhang, 2016;
Fu et al., 2019;
Zhang et al., 2016b

3.2 Role of human activities in Qinghai-Tibet Plateau grassland degradation

Several scientists believe that HA is the dominant factor affecting the QTP grassland degradation. Pan estimated that non-climatic drivers account for 66.1% of the QTP grassland changes (Pan et al., 2017). Among them, farming, grazing, hunting, mining, tourism, urban construction, and railway construction contribute to the degradation of the QTP grassland to varying degrees.
3.2.1 Overgrazing aggravates grassland degradation
Overgrazing is one of the main causes of QTP grassland degradation. In the past 30-50 years, overgrazing in QTP has led to severe degradation in the vegetation and soil of alpine grassland. SOC has decreased by 42% relative to that in non-degraded grassland (Liu et al., 2018). Zhang’s warming and cattle grazing experiments showed that overgrazing reduces the height and quantity of plants, and it is the main driving force for vegetation changes in alpine grasslands (Zhang et al., 2015d). Li et al. (2019d) selected five sections along 1100 km from east to west of the Three Rivers Headwater Region to analyze the soil erosion data in the past 50 and 100 years. The results showed that in the past 100 years, almost no soil erosion occurred, but soil accumulation did occur. However, soil erosion has intensified in the past 50 years, and soil erosion is closely related to the number of livestock and intensity of grazing. In addition, many experiments have attempted to quantify the difference between grazing and non-grazing grasslands. Lu analyzed and summarized the average data of 61 related studies (Lu et al., 2017b). The SOC, soil total nitrogen, NH4+-N, microbial carbon, above-ground biomass, underground biomass, vegetation coverage, and vegetation height of grazing grassland decreased by 1185.19 g/m2, 97.4 g/m2, 2.56 mg/kg, 66.70 g/m2, 88.92 g/m2, 197.30 g/m2, 16.92 g/m2, and 5.53 cm, respectively, compared with the values for non-grazing grassland. Notably, if no appropriate and effective restoration methods are implemented, some grassland in QTP may disappear within a few decades due to the rapid loss of SOC and the slow recovery (Zhang et al., 2015c).
3.2.2 Over-digging and over-hunting aggravate grassland degradation
QTP has many well-known medicine resources, such as Cordyceps sinensis, saffron, and snow lotuses (Sun et al., 2020b). People on QTP have a thousand-year tradition of digging for natural medicines, and this approach has been proven to be environment friendly. However, the demand for these medicines has increased in recent years (Shrestha and Bawa, 2014). Excessive digging disrupts the alpine grassland ecosystem. Take cordyceps, a traditional Chinese herbal medicine, as an example. Given that Cordyceps sinensis only emerges 2-3 cm above the ground, digging activities require crawling or creeping on the grass to explore them inch by inch. Furthermore, the time of digging for natural medicines coincides with the season of plant growth, which is from May to June; hence, digging and trampling activities cause serious disturbance to soil, plants, and organisms in QTP. Xu’s quantitative research showed that digging and trampling activities have reduced the biodiversity in grassland ecosystems by 10%-13%, vegetation coverage by 11%-19%, and above-ground biomass by 21%-40% (Xu et al., 2015).
In addition, QTP has many well-known wild animal resources, such as Tibetan antelopes, wild donkeys, and wild yaks. Over-hunting has reduced the number of wild animals greatly and has even made certain species extinct. The carnivores in the food chain are hunted, which destroys the biodiversity and causes herbivores to multiply; this situation intensifies the damage on grassland vegetation. Although the Chinese government has excluded several areas from hunting and digging activities, poaching is still common. One reason is that local residents have low awareness of legal and environmental protection. Another reason is that the prohibition of digging and hunting affects their livelihood (Cui et al., 2017). In summary, over-digging and over-hunting cause grassland degradation and disturb the living habitat of other organisms. Thus, they deserve our attention.
3.2.3 Secondary and tertiary industries accelerate grassland degradation
City construction damages the grassland ecosystem. The speed of urbanization in the QTP region accelerates with the increase in population. As a result, cities invade grasslands, impervious surfaces change the reflectance and groundwater flow in cities, and artificial structures break the original grassland landscape into fragments; consequently, the native grassland habitat is lost (Gong et al., 2017).
Moreover, the Qinghai-Tibet Railway is causing interference to the environment. It crosses vast grasslands and permafrost areas, causing serious interference to the environment along the line. Researchers have found that the impact of the Qinghai-Tibet Railway includes a 30-km range along the line (Luo et al., 2020). Specifically, in addition to the destruction caused by railway construction and operation (Shang et al., 2015), the interception of the railway has disrupted the migration paths of animals, leading to species extinction and destruction of the grassland biological chain, which are unfavorable for the grassland ecosystem.
Furthermore, mining on QTP has attracted widespread attention in recent years (Liu et al., 2020). The development of mining has brought about changes in land use and damaged the ecology of the QTP grassland. Qian conducted research on six open-pit coal mines on the south side of Qilian Mountain (Qian et al., 2018). The results showed that the capacity for soil water conservation and the value of ecological services are decreasing in these places.
Lastly, the influence of tourism cannot be ignored. QTP is a famous destination for its unique natural scenery and mysterious Tibetan Buddhist culture. In 2008, the tourist population of Lhasa (the capital city of Tibet) was 1.5 times larger than the permanent population. By 2014, the tourist population was 14.1 times larger than the permanent population and produced 174.8 tons of garbage daily in Lhasa. Estimations indicate that Lhasa will have 9,260,000 tourists in 2025 and will produce 960 tons of garbage per day (Ding and Wang, 2018). Apart from garbage, Le believes that tourism can also lead to soil and vegetation degradation by reducing plant height, species diversity, and above-ground biomass (Le et al., 2014). Furthermore, tourism-related services, such as catering, accommodation, transportation, shopping, and entertainment, have developed accordingly. Horse riding, bull riding, grass sliding, hunting, vehicle rolling, and other play activities have also further aggravated the degradation of the QTP grassland.
3.2.4 Negative effects of improper restoration measures
First, “grazing exclusion” involves removing the important participant in the original stable grassland ecosystem quickly; then, the regular pattern is broken, which is not conducive to the health of the grassland ecosystem. In addition, the “grazing exclusion” policy has been rejected by several residents for the reason that they must give up their basic income, which is from grazing (Cui et al., 2017).
Second, “fencing” has protected the grassland in several severely degraded areas for a certain period. As the grassland recovers, “fencing” will have negative effects. For example, the “no-fencing” zone will become a seriously degraded area. Moreover, “fencing” disturbs animal migration and will destroy the grassland ecosystem ultimately (Dong et al., 2020).
Third, the privatization strategy of pastures implemented by China has had little positive effect because herders camp in fixed areas and the grassland around the camps is trampled on seriously (Yeh et al., 2017; Cao et al., 2018a).
Fourth, although the establishment of the Nature Reserve Zone has a certain protective effect on ecological restoration (Jiang et al., 2019), the conflict of interest between the protected area and local residents is prominent. For example, residents are not satisfied with the compensation because it is too small to make up for the loss of grazing and hunting in the Nature Reserve Zone. In addition, the Nature Reserve Zone was set up by the government suddenly and directly, which means the local residents must change their lifestyle, which they have had for thousands of years, quickly. This change is understandably difficult (Shrestha and Bawa, 2014). Furthermore, several local residents have low environmental and legal awareness; they still enter the reserve for grazing and hunting, thinking that doing so is not a serious matter. Lastly, certain restoration measures are short-lived and discontinuous. Overall, the negative effects of improper restoration methods have been widely identified gradually (Lu et al., 2017a).

4 Coupled effect of climate change and human activities on Qinghai-Tibet Plateau grassland restoration/degradation

We summarized the effects of CC and HA on the restoration/degradation of the QTP grasslands in the past five years (Tables 1 and 3) in Sections 2 and 3, respectively, but the effects of CC and HA are not independent, and many coupling relationships exist (Xiong et al., 2019). CC expands the zone for humans, plants, and animals. It restricts HA by increasing the adaptability of plateau pikas and pests and the occurrence frequency of extreme weather conditions, such as snow storms, thunderstorms, and extremely cold or hot weather. Meanwhile, HA directly affects the grassland ecosystem through grazing, digging, mining, traveling, trampling, and fertilization and indirectly aggravates CC by changing the surface reflectance and atmospheric composition (Figure 1).
Table 3 Impact of human activities on Qinghai-Tibet Plateau grassland restoration/degradation
Effect Impact Mechanism Study area Research period Reference
Grassland
restoration		 Urbanization Changing from grazing to purchasing food, which is conducive to grazing management and restoration measures Nagqu County in northern Tibet (30°31′-31°55′N, 91°12′-93°02′E) 1991-2011 Wang et al., 2016a
Change the food structure with rice and noodles, and the demand of grazing will decline Maqu County on eastern QTP (33°99′N, 102°07′E) 1996-2016 Cao et al., 2018a
Improving and popularizing the knowledge of grazing management and environment protection Damzhung County in Tibet Autonomous Region of China 1990-2012 Hopping et al., 2016
Grazing Plant diversity can be increased by grazing Nagqu County (30°31′-31°55′N, 91°12′-93°02′E) and Bange County (31°23′N, 90°17′E) July-August of 2013 Zhang et al., 2015d
Plant photosynthesis can be enhanced by grazing directly and indirectly Xihai Town of Haiyan County (36°56′N, 100°57′E) and Tiebujia Town of Gonghe County (99°35′N, 37°02′E) 2012-2017 Shen et al., 2019
Grazing can increase the nitrogen content in soil and promote nitrogen uptake by plants Xihai Town of Haiyan County (36°56′N, 100°57′E) and Tiebujia Town of Gonghe County (99°35′N, 37°02′E) 2012-2017
Plant production in alpine grassland is significantly increased by moderate grazing QTP 2010-2020 Dong et al., 2020
Plant diversity and nectar (forage) sustainable production are promoted by light and moderate grazing Hongyuan County (32°48′-32°52′N, 102°01′-102°33′E) 2004-2014 Mu et al., 2016
Traditional migratory pastures are conducive to sustainable pasture development Kobresia pastures in eastern Tibetan highlands 1984-2016 Miehe et al., 2019
Multi-family grazing management can maintain better soil fertility and support grassland sustainable use Magqu County on
eastern QTP		 1996-2016 Cao et al., 2018b
Nitrogen adding (fertilization) can significantly increase the coverage of grassland Damxung, northern Tibet 2012-2014 Zong et al., 2016
Restoration Restoration methods, such as nature reserve, return farmland to grass, and the Three North Shelter Belt Project, are effective R1: QTP;
R2: Qinghai Province ( 31°09′-39°19′N, 89°35′-103°04′E);
R3: QTP;
R4: China;
R5: Water Tower region of China		 R1: 2000-2100, 2030-2050, and 2080-2100;
R2: 1988-2008;
R3: 2000-2015;
R4: 2001-2010;
R5: 2000-2010		 Zhang et al., 2015c;
Han et al., 2016;
Huang et al., 2017;
Lu et al., 2018;
Wang et al., 2015
Effect Impact Mechanism Study area Research period Reference
Grassland
degradation		 Urbanization Population growth has a negative impact on grassland QTP 2002-2019 Fayiah et al., 2020
Secondary industry The quality of soil along the railway is declining Continuous permafrost regions along the
Qinghai-Tibet Highway in the north of Kunlun Mountains		 2013 Shang et al., 2015;
Sun et al., 2020b
Grassland ecology is destroyed by mining Southern slope of
Qilian Mountains		 1975-2016 Qian et al., 2018
Tertiary
industry		 Tourism has a negative impact on plant height, vegetation coverage, species diversity, and above-ground biomass in grassland Lhasa 2015-2025 Le et al., 2014;
Ding and Wang, 2018
Grazing SOC is reduced by grazing QTP The past 100 years Li et al., 2019d
The height, coverage, and living conditions of vegetation are reduced by grazing R1: Nagqu County (30°31′-31°55′N, 91°12′-93°02′E);
R2: Haibei Alpine Meadow Ecosystem Research Station (37°37′N, 101°12′E)		 R1: 2010-2013;
R2: 2010-2011		 R1: Zhang et al., 2015d;
R2: Cao et al., 2015
Biomass is reduced by grazing R1: Qinghai Province (31°39′-39°19′N, 89°35′-103°04′E)
R2: Northern Tibetan Plateau		 R1: 1985-2044;
R2: 2010-2013		 R1: Zhang et al., 2017;
R2: Zeng et al., 2015
The uptake of nitrogen by plants and microorganisms is reduced strongly by grazing Nagqu County on the Tibetan Plateau (30°31′-31°55′N, 91°12′-93°02′E) Growing season Jiang et al., 2017b
The size and reproduction of pests are significantly increased by grazing Haibei Alpine Meadow Ecosystem Research Station (37°37′N, 101°12′E) 2010-2011 Cao et al., 2015
Soil erosion is accelerated by livestock R1: Qinghai Province;
R2: across 1100-km East-West transect in the Three-River Headwaters Region		 R1: 2009-2012;
R2: 1919-2019		 R1: Harris et al., 2016;
R2: Li et al., 2019d
Restoration Several improper restoration methods, such as grazing exclusion and fencing, are harmful to the health of grasslands to a certain degree R1: Damzhung County in Tibet Autonomous Region of China
R2: QTP		 R1: 1990-2012;
R2: 2010-2020		 R1: Hopping et al., 2016;
R2: Dong et al., 2020
Figure 1 Coupled effect of climate change and human activities on Qinghai-Tibet Plateau grassland restoration/degradation

4.1 Coupled effect on soil organic carbon

Researchers have attempted to determine whether HA or CC dominates SOC. In terms of leading factors, Wang’s warming-grazing experiment evaluated the coupling relationship between CC and grazing in SOC change. The experiment showed that CC dominated by temperature rise directly reduces the SOC content, and the factor of grazing only results in a small variation in SOC concentration (Zhang et al., 2017). In terms of propelling factors, soil and plants are trampled by grazing, thereby reducing soil carbon uptake from vegetation to roots, which has a negative impact on SOC. However, grazing can also have a positive impact on SOC through trampling, falling leaves, and manure return. In summary, no significant correlation exists between grazing and SOC (Wang et al., 2019a).
Moreover, complex relationships exist among CC, HA and grassland. HA further aggravates CC, such as increasing the amount of greenhouse gas, which affects the change in SOC. Grazing exclusion experiments have shown that grazing exclusion reduces the soil respiration of QTP meadows and grasslands but increases their sensitivity to temperature; in addition, grazing areas have a higher potential to release carbon than non-grazing areas under future warming conditions (Zhang et al., 2017; Guo et al., 2018b). Notably, although CC is generally recognized as the main controlling factor of SOC, Doetterl discovered that SOC is controlled by the interaction between geochemistry and climate. After the relationship between geochemical factors is removed, the relationship between CC and carbon flux is significantly reduced (Doetterl et al., 2015) (Figure 2).
Figure 2 Coupled effect on soil organic carbon

4.2 Coupled effect on the production of the grassland ecosystem

NPP is affected by CC and HA. Although the green area of alpine vegetation has been proven to increase in warm and wet climate conditions (Jiang and Zhang, 2016), in areas with high human pressure, especially in grazing and traveling areas around Lhasa, the decline of alpine vegetation is strong (Luo et al., 2020), and grassland degradation is obvious.
Furthermore, glacier retreat forms new pastures at high altitudes, and the impact of HA expands as a result. Digging, grazing, and tourism activities have expanded to high elevations with the expansion of grasslands. The grassland degradation caused by HA in high-altitude zones aggravates soil erosion by wind and water and brings about soil nutrient loss (He and Richards, 2017). In turn, soil nutrient loss causes the degradation of grassland again at high elevations (Liu et al., 2018).
With regard to the total NPP, Wang calculated the contribution of the interaction between CC and HA to the total NPP of QTP vegetation through a quantitative assessment using the CASA model and the results showed that CC plays a leading role in the total NPP(Wang et al., 2016c). Zhang obtained similar results, but he clarified that air temperature is the main cause of grassland changes in biomass and total NPP, and precipitation and grazing are the secondary causes (Zhang et al., 2017). However, several scientists hold the opposite view. They believe that overgrazing instead of CC is the main cause of vegetation changes in QTP. Grazing offsets the increase in vegetation height and surface biomass caused by warming (Zhang et al., 2015d). Zhang’s scenario simulation results showed that in the past 30 years, biomass in grassland and grazing intensity has been negatively correlated, and biomass has declined at all grazing degrees against the background of warming (Zhang et al., 2017) (Figure 3).
Figure 3 Coupled effect on the production of the grassland ecosystem

4.3 Coupled effect on pikas and pests

Primarily, pikas on QTP grassland cannot be ignored (Yu et al., 2017). Pikas are important participants in the grassland ecosystem, and the burrowing and gnawing activities of pikas reduce the soil moisture, hardness, organic carbon, and nitrogen, thereby changing the vegetation cover, soil nutrient content, microbial community, physical structure, and porosity (Ding et al., 2019a). Pikas also increase the top soil gravel, which is not conducive to vegetation restoration and excavate large pores to form many bald spots, mounds, and pores, which change the soil structure in such a way that air and water flow only in the shallow pores created by them. Thus, the connectivity of soil pores is destroyed and soil erosion is aggravated (Hu et al., 2020). Furthermore, the adaptability of pikas to CC is increasing, and the prediction results of the food chain model show that the number of pikas on the QTP is between 70 and 288 per hectare without external interference, which is a relatively high number (Sun et al., 2015). To improve the pasture economy, humans have carried out various interventions to limit or eliminate pikas on QTP. Examples include the use of pesticides to eliminate pikas or the adoption of eagles to hunt them; both measures have been in vain. Experiments have shown that in 80% of the areas subjected to drug elimination, the number of pikas can be restored to 70% of the original number within four years, and the number of pikas is much greater than the demand for food for eagles (Tian et al., 2019). Meanwhile, the methods of grass improvement and replanting have produced better results. Experiments have shown that the height of plants is negatively correlated with the density of pikas. Therefore, ecologists have proposed a biological control model of “increasing the proportion of grasses in the plant community to control the hazards of pikas”. Notably, pikas are important participants in the grassland ecosystem of QTP, and they can provide homes for plateau lizards and birds, form habitats for creatures, and become the staple food of eagles and other animals (Zhao et al., 2020). Appropriate human interference can help improve the habitat of pikas and promote the quality of alpine grassland, but transitional intervention would lead to reverse succession of the ecosystem. Hence, the pika exclusion strategy is inappropriate, and ecological measures should consider the comprehensive influence of CC and HA.
Additionally, grassland caterpillars are important participants in the QTP grassland ecosystem and essential indicators of the extreme ecological environment of alpine hypoxia grassland. An example is the grassland caterpillar Gynaephora menyuanensis, which is one of the main pests in the alpine meadow in the northeast of QTP. An outbreak of G. menyuanensis causes a shortage in livestock production by causing sickness and death among livestock, but it also causes pest interaction in forage (Zhang and Yuan, 2013). Thus, the community development of caterpillars is crucial for the ecology of pastures and grasslands. Previous warming-grazing experiments have shown that the growth rate of grassland caterpillars increases by 9%, and the weight of females increases by 26% in warming-grazing treatment (Cao et al., 2016). Specifically, CC and HA significantly increase the body size, potential reproductive capacity, and density of caterpillars, which is unfavorable for grasslands (Cao et al., 2015). In addition, the distribution of caterpillars is mainly restricted by the degree of grazing. Comparative tests conducted on non-, light, moderate, and heavy grazing have revealed that heavy grazing increases the density of caterpillars in grasslands, and moderate grazing inhibits the increase in species density (Chen et al., 2016b). Furthermore, the environmental adaptability of caterpillars increases under conditions of grazing and warming, and caterpillars exacerbate grassland degradation by reducing the biomass of sedge and plants (Cao et al., 2015). Notably, spring melting and moderate grazing can offset several of the impacts of warming on plant communities (Dorji et al., 2018) (Figure 4).
Figure 4 Coupled effect on pikas and pests

4.4 Coupled effect on Cordyceps sinensis

The coupled effect of CC and HA exerts a significant influence on the fungi/microbes in QTP grassland. An example is Cordyceps sinensis, a valuable medicine with a good reputation worldwide (Xu et al., 2019). Glacier retreat makes the snow line move up (Ke et al., 2017; Cao et al., 2017), and the C. sinensis produced near the snow line moves along the snow line (Li et al., 2014). To obtain high-quality C. sinensis, diggers dig at high altitudes, increasing the damage to the fragile environment. During the two-month digging period, diggers need to build fires for heating, cooking, and setting up tents. Service industries, such as supply stations, movie theaters, and hotels, are also available near the camps. Furthermore, new tools have resulted in increased vehicle crushing and oil burning waste and litter, which increase the temperature and CO2 content and further aggravate the glacier retreat at high-altitude zones. Consequently, the snow line moves up further, and diggers need to constantly move their camps to even higher altitudes in the two months of digging. A vicious circle is therefore formed (Yan et al., 2019).
Furthermore, the excavation of C. sinensis has increased the amount of bare ground and the number of poisonous weeds on the surface, thus providing an enlarged habitat for the settlement of pikas and voles. The local people of QTP do not kill creatures because of their religious belief (Sangji, 2015), and this situation creates suitable conditions for the outbreak of rodents; rodent outbreaks destroy the grassland seriously (Danjiu, 2019).
Furthermore, the income obtained from digging for C. sinensis for two months is equivalent to the income from farming for one year. Therefore, excavators are unwilling to engage in agriculture and husbandry. The phenomenon of “abandoning agriculture and husbandry” has spread in these zones, thus affecting CC and HA (Fan, 2016; Danjiu, 2019) because excavation activities and husbandry are important to the changes in the alpine grassland ecosystem (Figure 5).
Figure 5 Coupled effect on Cordyceps sinensis

4.5 Dynamic adaptation to climate change and human activities

CC and HA cause grassland degradation and reduce the ecological service functions of the QTP grasslands; for example, they can change pasture growth, carbon sequestration, biodiversity, above-ground/underground biomass, vegetation coverage, soil physical/chemical properties, and seed banks (Hopping et al., 2018b; Li et al., 2020). Simultaneously, the participants in the QTP grasslands adapt to these changes dynamically by changing the community structure, diet structure, morphology, and distribution range. The grassland ecological environment also responds to CC and HA disturbance through soil nutrient loss, desertification, sandstorms, drought, downstream floods, poisonous weeds, rodents, and harmful gases, such as N2O and CO2 (Harris, 2010; Fayiah et al., 2020; Guo et al., 2020b). They are summarized in Table 4.
Table 4 Dynamic adaptation of Qinghai-Tibet Plateau’s grassland ecology to climate change and human activities
Participants Dynamic adaptation References
Animals Change the distribution range (e.g., C. sinensis distribution changes with the snow line) Li et al., 2014
Change the diet structure (e.g., the diet structure of caterpillars changes with altitude) Zhang et al., 2016a
Change the physical characteristics Cao et al., 2016
Plants Change the state of existence Mccarthy and Enquist, 2007;
Yuan et al., 2017
Change the community structure (e.g., increase the number of poisonous weeds) Li and Liu., 2017;
Hao et al., 2020;
Chu et al., 2019;
Wen et al., 2020
Phenology changes Guo et al., 2020a
Change the distribution range Wang et al., 2016a
Soil Change the physical state (e.g., water conservation) Miehe et al., 2019;
Guo et al., 2020b;
Li et al., 2020
Change the chemical properties (e.g., acidity and alkalinity) Yang et al., 2017;
Li et al., 2020
Change the respiration rate Chen et al., 2017;
Qiao et al., 2015
Water Change the physical state (e.g., glacier melting) Jiang and Zhang, 2016;
Jiang et al., 2017a;
Hoham and Remias, 2020;
Lei et al., 2012
Atmosphere Changes in composition (e.g., CO2 and N2O) Hu et al., 2017;
Leng et al., 2020;
Sun et al., 2020a;
Zhao et al., 2017
The invasion of woody plants has become a global problem in grasslands. Given that grassland degradation has become severe, resulting in reduced groundwater height and soil water retention, grasslands have changed to shrubs. Scientists predict that alpine meadows will gradually be replaced by shrubs, and the distribution area of alpine grasslands will expand and occupy the west and north areas in the warmer and wetter future (Adam et al., 2020). Although the presence of woody plants in alpine hypoxia places is unsuitable at present (Zhang et al., 2015a), the invasion of woody plants in QTP is still worthy of further research.

5 Conclusion

5.1 Effect of climate change

CC is one of the main reasons for grassland restoration/degradation. Warm and wet climate conditions are the primary influencing factors. QTP has high spatial heterogeneity, and warm and wet conditions have different effects on different regions and times. The positive effects of warm and wet conditions are as follows: (1) enhanced photosynthesis of vegetation; (2) increased NPP; (3) expansion of the greening area; (4) increase in microbial activity; and (5) decrease in wind erosion and sandstorms. The negative effects of warm and wet conditions are as follows: (1) soil erosion is aggravated in over-humid areas; (2) evaporation intensifies in arid areas, resulting in soil water shortage; (3) the proportion of shrubs is increasing; (4) the interaction between pests is intensified; (5) the microorganisms in grasslands are inhibited because of the increasing adaptability of pests and pikas; (6) the warming in winter and spring causes a decrease in soil water use efficiency; and (7) extreme climate affects the physiological process of plants and creatures in the alpine grassland ecosystem.

5.2 Effect of human activities

HA is one of the main reasons for grassland restoration/degradation in QTP. The positive effects of HA are as follows: (1) moderate grazing is beneficial to grassland ecological health; (2) urbanization is conducive to population education and change in residents’ diet structure and lifestyle; (3) restoration measures are effective; and (4) tourism is a new means to make a living, and it provides funds for local environmental protection. The negative effects of HA are as follows: (1) over-grazing, over-hunting, and over-digging are widespread; (2) urban construction, railway construction, mining, and tourism activities destroy the grassland ecosystem; and (3) grazing exclusion, fencing, grassland privatization, and nature reserves have certain drawbacks. In sum, apart from husbandry and agriculture, which have been widely studied, changes in lifestyle, economic growth patterns, and land use brought about by tourism and industries are new factors that deserve our attention.

5.3 Coupled effects of climate change and human activities

CC, HA, and grassland in QTP exert many coupled effects. First, SOC is significantly affected by the coupled effect of these factors. CC increases plant respiration, but HA decreases it. HA is affected and restricted by CC, and HA intensifies the change in CC and then changes SOC. Second, NPP is significantly affected by the coupled effects of CC and HA. CC enlarges the grassland area and increases NPP, and HA can offset the effect of CC on vegetation; in turn, changes in vegetation affect CC and HA again. Third, the population structure and environmental adaptability of creatures are changed under the coupled effect of CC and HA. Then changes in creatures affect HA and CC again. For example, C. sinensis growing along the snow line moves to a high altitude because of CC. Then, HA moves too, which increases CC again.

5.4 Dynamic adaptation to climate change and human activities

The grassland ecosystem mainly adapts to the coupled effects of CC and HA through the following: (1) changing the population structure; (2) changing the food structure; (3) changing the physical state; (4) changing the physical and chemical properties; (5) changing the distribution range; (6) causing outbreak of pests and poisonous weeds; (6) causing outbreaks of pikas; (7) desertification; (8) invasion of woody plants; and (9) changing the reproductive status.

6 Outlook

6.1 Comprehensive understanding of grassland degradation

Although the restoration/degradation of QTP grassland ecosystems has been extensively studied, no standard for degradation exists at present. Several scientists regard the reduction in soil nutrients and NPP in grasslands as the standard (Su et al., 2015; Wang et al., 2016c; Gu et al., 2019). Meanwhile, other scientists view dominant populations, biomass, grass mass, caterpillars, animal caves, number of herders’ camps, distance between camps, and time of fencing as degradation standards. Scientists obviously have different assessments of QTP grassland degradation. Several of them believe that the degradation of QTP grassland ecology has reached about 90%, 50%, and 30% (Dong and Sherman, 2015; Fayiah et al., 2020). Thus, scientists suggest that apart from focusing on vegetation cover, biomass, and biodiversity, attention should also be paid to the characteristics of plant communities and ecosystem resilience (Cao et al., 2019). Presently, comprehensive evaluation of the degradation of QTP grassland is lacking. Thus, decision makers cannot formulate suitable strategies for the sustainable development of QTP grassland. Related studies should incorporate HA into the grassland ecosystem and conduct a comprehensive evaluation from the perspective of ecology in the future. Attention must be devoted to the ecological service function and the adaptation and feedback of soil, plants, animals, and humans in QTP grassland.

6.2 Comprehensive understanding of climate change

At present, studies on the impact of CC mainly focus on warming, glacier retreat, permafrost thawing, and hydration changes. Specifically, warming methods are used to obtain research results in many experiments. However, apart from overall warming, the definition of global warming also includes local cooling and increased frequency of extreme weather conditions. In recent years, a few scientists have used models to predict the impact of cooling on the QTP grassland ecosystem. Although the warm and wet phenomenon is already recognized as a CC trend, studies under cold conditions are also forward-looking, especially in the QTP area because QTP is characterized by widespread glaciers and permafrost, and warming makes glaciers melt. Then, the icy-cold water flows to low-altitude areas, bringing low-temperature water to QTP grassland and cooling the air and soil. Will it bring about cool zones in the background of increasing temperature?
Another feature of global warming is the exacerbation of climate extremes (He et al., 2016), such as snowstorms, thunderstorms, extremely cold and hot weather, extreme droughts, and floods. Relevant literature on the impact of extreme climate is still lacking, which inspires us to focus on the overall perspective of global change in the future.

6.3 Coupled relationship among climate change, human activities, and grassland ecology needs further discussion

Although we focused on the coupled relationship among CC, HA, and grassland ecology, our work is still at the phenomenon description stage and lacks a quantitative analysis. Therefore, a comprehensive evaluation is yet to be systematically summarized. We recommend using a certain species as a clue to explore the coupled impact of CC and HA and then expanding the results to populations and communities to establish the coupling relationship between them gradually.
Basing on the summary provided by this review, we advise future studies in this field to focus on the following aspects: (1) tracking the status of the QTP grassland restoration/degradation; (2) studying the impact of glacier retreat and permafrost thawing the QTP grassland under the coupled influence of CC and HA; (3) focusing on the adaptation and impact of pikas and pests on grassland ecology under the influence of CC and HA; (4) following the changes in the biological chain of the QTP grassland under the influence of CC and HA; (5) investigating the impact of extreme climate of the QTP grassland ecology; (6) exploring the development of ecotourism of the QTP; (7) solving the livelihood issues of herders in QTP; (8) evaluating the restoration measures implemented in QTP; and (9) investigating the grassland restoration technology in alpine regions.

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