Humans have grazed on the Qinghai–Tibetan Plateau (QTP) for many thousands of years. In recent decades, the intensity of grazing has increased and several new management strategies have been put into place to address the resulting changes in rangeland condition. Effective management of grazing activities in this region requires understanding the impact of livestock grazing across the diverse array of alpine grassland ecosystems present in the QTP, but recent studies have identified a number of critical uncertainties in the ecological science that underlies these management principles. To address these uncertainties, we carried out a synthesis analysis of the effect of livestock grazing on 26 indicators of ecosystem structure and function based on 61 studies from 88 independent research sites within the QTP. Our synthesis results indicate that livestock grazing exerts complex controls on ecosystem structure and function, which vary according to local landscape characteristics. We found that grazing contributes to greater plant species diversity (Shannon–Wiener index, Simpson dominance index, and Pielou evenness index significantly increased 0.18, 0.05, and 0.03, respectively, due to grazing), but decreased aboveground biomass (47.15%), soil organic carbon (12.41%), soil total nitrogen (12.75%), and microbial biomass carbon (9.42%). Further, ecosystem function is controlled by interactions between grazing and other landscape characteristics such as elevation and mean annual temperature. The management regime currently in place in the QTP, which involves complete exclusion of grazing in some areas, can have variable effects on grassland health. Therefore, the complexity of these responses is an indication that livestock and grassland management may benefit from a more nuanced management regime than is currently utilized in the QTP.

Introduction Grasslands cover 41% of Earth's land surface and provide livelihoods for nearly 800 million people, as well as forage for livestock, wildlife habitat, valuable ecosystem services, and locations for recreation and tourism (Zhang 2006, Strömberg et al. 2013). Grassland degradation, the decline of productivity and ecological function of grassland ecosystems due to human activity or natural processes, is recognized as ecological and environmental problem worldwide and can have far‐reaching implications including changes to local hydrology, dust storms, commodity scarcity, and the societal consequences of displaced populations (Feng et al. 2009, Harris 2010, Dlamini et al. 2014). Overgrazing is one of the primary contributors to grassland degradation around the world, through reduction in vegetation cover, degradation of topsoil, causing soil compaction as a result of trampling, reduction in soil infiltration rates, and enhancement of the susceptibility of soils to erosion (Su et al. 2005, Hilker et al. 2014). Grazing management is one of the most effective strategies to prevent grassland degradation and to support sustainable grazing (Zhang 2006, Wiesmeier et al. 2012). However, response of grassland ecosystems to livestock management is mediated by local environmental factors such as climate, water accessibility, elevation, and slope (Gao et al. 2010, Bütof et al. 2012). Therefore, it is important to take local environmental characteristics into account when making recommendations for grazing management. The Qinghai–Tibetan Plateau (QTP), which encompasses Tibet, Qinghai, and parts of other provinces in southwestern China, contains one of the largest areas of alpine grassland in the world (Lu et al. 2012, Zhang et al. 2014). With an area of about 2.5 million km2, and an average elevation of more than 4000 m above sea level (a.s.l.), the QTP is one of the most important pastoral areas in China and has served as the dominant pastures for Tibetan communities for thousands of years (Klein et al. 2007, Zhang et al. 2014). In addition, the QTP is located upstream and upwind of more than 40% of the world's human population, making it an important region for maintaining water and air quality resources (Harris 2010, Chen et al. 2014, He and Richards 2015). Currently, the alpine grasslands resources of the QTP are under pressure to sustain increasing livestock production and meet the demands of an ever‐growing milk and meat market (Hafner et al. 2012, Wu et al. 2012). In the Tibet Autonomous Region, located within the QTP, human populations have increased from 1.1 to 3.2 million in the past 60 yr. Livestock numbers have increased by 0.3 million/yr from 1951 to 2003, but are now decreasing due to a grassland protection policy introduced in 2004 (Fig. 1). The implementation of new grazing policies that exclude grazing in specific regions has resulted in high concentrations of grazing outside of the enclosures and excessive grazing has led to regions of grassland degradation (Arthur et al. 2008, Ma et al. 2010). Currently, about one half million km2 of alpine grasslands in the QTP is degraded, and grassland degradation has reduced the productivity of alpine grasslands by approximately 30% over the last 20 yr in the QTP (Cui and Garf 2009, Yu et al. 2010, Dong et al. 2012). Figure 1 Open in figure viewer PowerPoint The human population (a) and livestock inventories (b) changes from 1951 to 2014 in Tibet Autonomous Region. In response to the problem of grassland degradation in the QTP, China's state and local authorities initiated a program in 2004 called “retire livestock and restore grassland” (RLRG). This campaign has focused mostly on grazing exclusion by fencing to restore degraded alpine grasslands (Fig. 2). In the last decade (from 2004 to 2013), more than 2.5 billion Chinese Yuan (US $0.4 billion) has been invested in management of grazing activities in more than 6 million ha of alpine grasslands in Tibet Autonomous Region (Sun 2014). Figure 2 Open in figure viewer PowerPoint Free grazing alpine grassland (a) and grazing exclusion alpine grassland (b, area left of the fence) after implementing “retire livestock and restore grassland,” which is an alpine grassland protection policy introduced by China's state and Qinghai–Tibetan Plateau local authorities since 2004. Following the large investments in grazing policies in the QTP, a large number of studies exploring the effect of livestock grazing in the QTP have been published in recent years (Hirota et al. 2005, Xie et al. 2014). Grazing disturbance in the QTP is generally recognized to have extensive and profound impacts on alpine grassland ecosystem structure and function, including plant species composition and diversity (Miehe et al. 2011, Yang et al. 2013), vegetation productivity (Niu et al. 2009, Wu et al. 2014a), and soil carbon and nitrogen cycle (Hirota et al. 2005, Unteregelsbacher et al. 2012, Wang et al. 2012). However, the majority of these studies focus on a single alpine grassland ecosystem type (Chen et al. 2008, Miehe et al. 2011) and single ecosystem effects (Hafner et al. 2012, Dorji et al. 2014) within a limited regional scale (Ma et al. 2014, Wu et al. 2014b). At present, there have been no attempts, to our knowledge, to formulate a synthesis of the effects of livestock grazing on alpine grasslands from the perspectives of both ecosystem functional and community structural perspectives, even though a comprehensive assessment of the ecological benefits and risks of the RLRG program across the wide diversity of ecosystems within the QTP is an important reference for policy makers. The purpose of this paper was to present a synthesis of the effects of livestock grazing on ecosystem structure and function within alpine grasslands of the QTP based on data sets from existing publications. To complete this review, we collected all available studies with information on the effects of livestock grazing on ecosystems of the QTP. For all studies that investigate the same response variables, we used each study result as a single data point in paired comparisons of grazed vs. ungrazed sites. The objectives of this study were to: Determine how livestock grazing impacts alpine grassland ecosystem structure and function in the QTP. Explore how the effect of livestock grazing on ecosystem structure and function varies according to grazing intensity and local environmental conditions such as temperature, precipitation, and altitude. Evaluate whether the RLRG policy could restore degraded grassland by improving ecosystem structure and function compared with free grazing grasslands in the QTP.

Materials and Methods To collect data for this study, we performed a systematic search of the scientific literature by using the ISI Web of Knowledge (https://www.webofknowledge.com) database with the keywords: grazing, livestock grazing, grazing intensity, fencing, grazing exclusion, alpine grassland, alpine steppe, alpine meadow, Tibet, Tibetan Plateau, QTP. To find additional papers, we checked all references of papers revealed in the database search and searched other databases including Elsevier and Springer to find articles in press. Only studies that compared grazed sample plots with nearby ungrazed control plots were included. This condition eliminated all studies that compared only different intensities of grazing without an ungrazed control, and studies that made temporal comparisons of the same sites before and after grazing or grazing exclusion. Sixty‐one studies met the criteria for our investigation (Appendix S1). Several papers included appropriate data for more than a single indicator, such as articles with data on both vegetation indicators and soil indicators in grazed vs. ungrazed sites. Some papers also contributed two or more observations from different sample sites to a given analysis. In addition, in some papers, ungrazed sites were compared with two or more sites that were grazed at different intensities. In such cases, the lower‐intensity grazing data, “lightly grazed” rather than “heavily grazed,” were used to represent grazing effects for comparison with an ungrazed control when two levels of grazing were used. The intermediate‐intensity grazing data, “moderately grazed” rather than “lightly grazed” or “heavily grazed,” were used for comparison with an ungrazed control when three levels of grazing were used. Based on these studies, only five indicators, Shannon–Wiener index, aboveground biomass, vegetation cover, soil organic carbon, and soil total nitrogen, were used to analyze grazing intensity separately from grazing/ungrazing comparisons because the data pools of other indicators were too small for analyzing the grazing intensity influences. Grazing intensities were determined based on the stocking rates according to the calculation of the proper carrying capacity of rangelands. The average stocking rates of the alpine grasslands with light grazing (LG), moderate grazing (MG), and heavy grazing (HG) were 1.75, 3.59, and 5.85 sheep units/ha, respectively. All observations of the effect of livestock grazing on alpine grassland ecosystem structure and function were derived from 88 research sites in the QTP based on these 61 studies. In general, these 88 sites covered the main alpine grassland types, including alpine meadow, alpine steppe, alpine shrub meadow. And they are mainly concentrated in the east and west regions of the QTP (Fig. 3; Appendix S2: Table S1). In the east part of the QTP, the research sites were distributed around three alpine grasslands ecological stations: Haibei Alpine Meadow Ecosystem Research Station (37°37′ N, 101°12′ E, 3200 m a.s.l., run by the Northwest Institute of Plateau Biology, Chinese Academy of Sciences), Tianzhu Alpine Grassland Experimental Station (37°11′ N, 102°47′ E, 2960 m a.s.l., run by Gansu Agricultural University), and Maqu Alpine Meadow and Wetland Ecosystem Research Station (33°39′ N, 101°53′ E, 3650 m a.s.l., run by Lanzhou University). In the western portion of the QTP, the research sites were also distributed around three alpine grasslands ecological stations: Naqu Alpine Grassland Ecosystem Research Station (31°39′ N, 92°01′ E, 4600 m a.s.l., run by the Institute of Geographic Sciences and Natural Resources Research, Chinese Academy of Sciences), Xainza Alpine Steppe and Wetland Ecosystem Observation and Experiment Station (30°57′ N, 88°42′ E, 4675 m a.s.l., run by the Institute of Mountain Hazards and Environment, Chinese Academy of Sciences), and Nam Co Station for Multi‐sphere Observation and Research (30°47′ N, 90°59′ E, 4730 m a.s.l., run by the Institute of Tibetan Plateau Research, Chinese Academy of Sciences). Figure 3 Open in figure viewer PowerPoint Location of 88 sites around six alpine grassland ecological stations where grazing studies were conducted in the Qinghai–Tibetan Plateau. We used 26 indicators of ecosystem structure and function in seven different categories to assess the effect of livestock grazing on alpine grassland in the QTP (Table 1). The seven categories consisted of one category to assess ecosystem structure: plant species diversity, and six categories to assess ecosystem function: vegetation growth, seed bank, carbon stocks, soil nitrogen and phosphorus, dissolved organic matter, and microbial biomass. Each category is made up of several indicators; for example, plant species diversity included four indices: species richness, Shannon–Wiener index, Simpson dominance index, and Pielou evenness index. The other six categories for assessing ecosystem function included aboveground biomass, belowground biomass, vegetation cover, vegetation height, soil organic carbon, and others (22 indicators in total; Table 1). Table 1. Summary of trends for the 26 ecosystem structural and functional indicators with grazed and ungrazed mean values in response to livestock grazing in Qinghai–Tibetan Plateau Indicators Type No. observations Mean P value Trend Total Increasing (%) Declining (%) M g M ug M g − M ug Plant species diversity Species richness S 20 12 (60) 8 (40) 28.50 26.85 1.65 0.207 No change Shannon–Wiener index S 16 13 (81) 3 (19) 2.23 2.05 0.18 0.015 Increase Simpson dominance index S 8 7 (87) 1 (13) 0.78 0.73 0.05 0.025 Increase Pielou evenness index S 11 8 (73) 3 (27) 0.76 0.73 0.03 0.018 Increase Vegetation growth Aboveground biomass (g/m2) F 47 1 (1) 46 (99) 107.34 196.26 −88.92 <0.001 Decrease Belowground biomass (g/m2) F 24 6 (25) 18 (75) 1001.77 1199.07 −197.30 0.006 Decrease Vegetation cover (%) F 34 2 (6) 32 (94) 51.54 68.46 −16.92 <0.001 Decrease Vegetation height (cm) F 19 0 (0) 19 (100) 7.78 13.31 −5.53 0.002 Decrease Seed bank Seeds density (per m2) F 5 4 (80) 1 (20) 2803.27 2324.59 478.68 0.196 No change Seeds species numbers F 3 1 (33) 2 (67) 33.40 33.77 −0.37 0.616 No change Seeds species richness F 3 1 (33) 2 (67) 25.88 24.70 1.18 0.773 No change Carbon stocks Aboveground biomass carbon (g/m2) F 6 1 (17) 5 (83) 109.56 228.85 −119.29 0.201 No change Belowground biomass carbon (g/m2) F 5 1 (20) 4 (80) 406.83 586.07 −179.24 0.263 No change Soil organic carbon (g/m2) F 17 6 (35) 11 (65) 8362.63 9547.82 −1185.19 0.038 Decrease Soil respiration (g·m−2·h−1) F 7 4 (57) 3 (43) 0.50 0.45 0.05 0.276 No change Q 10 of soil respiration F 6 6 (100) 0 (0) 3.67 2.17 1.50 0.004 Increase Soil nitrogen and phosphorus Soil total nitrogen (g/m2) F 17 3 (18) 14 (82) 666.59 763.99 −97.40 0.049 Decrease Soil total phosphorus (g/kg) F 12 6 (50) 6 (50) 0.57 0.64 −0.07 0.170 No change Soil available nitrogen (mg/kg) F 6 1 (17) 3 (50) 66.16 130.56 −64.40 0.158 No change Soil available phosphorus (mg/kg) F 6 2 (33) 4 (67) 2.23 3.15 −0.92 0.237 No change NH 4 +–N (mg/kg) F 7 0 (0) 7 (100) 13.23 15.79 −2.56 0.022 Decrease NO 3 −–N (mg/kg) F 5 3 (60) 2 (40) 7.94 6.15 1.79 0.277 No change Dissolved organic matter Dissolved organic carbon (mg/g) F 5 2 (40) 3 (60) 0.30 0.36 −0.06 0.291 No change Dissolved organic nitrogen (mg/g) F 6 1 (17) 5 (83) 0.06 0.07 −0.01 0.279 No change Microbial biomass Microbial biomass carbon (mg/kg) F 12 2 (17) 10 (83) 797.28 863.98 −66.70 0.044 Decrease Microbial biomass nitrogen (mg/kg) F 9 0 (0) 9 (100) 161.49 210.73 −49.24 0.122 No change Synthesis analysis for the 26 indicators in these seven categories was performed using results from the papers identified in the literature search. We grouped studies with the same response variable in order to assess the effects of grazing on these response indicators. Each analysis included between 3 and 47 data points of paired comparisons of grazed vs. ungrazed areas. We extracted data directly from tables or text in literatures or indirectly from figures using GetData Graph Digitizer 2.25 (http://getdata-graph-digitizer.com). The units of some indicators were converted where necessary; other variables such as latitude, longitude, altitude, mean annual temperature, and mean annual precipitation were also collected for further analysis. t tests to determine the effect of livestock grazing on each indicator of ecosystem structure or function by comparing grouped data of grazed vs. ungrazed sites using IBM SPSS Statistics software 19 (IBM, Armonk, New York, USA). Next, we calculated the effect size of grazing using the natural log response ratio (Mcsherry and Ritchie 2013 P < 0.05), we selected the function that maximized the R2 value. Regression analyses were also conducted with IBM SPSS Statistics software 19 (IBM). We used paired differencetests to determine the effect of livestock grazing on each indicator of ecosystem structure or function by comparing grouped data of grazed vs. ungrazed sites using IBM SPSS Statistics software 19 (IBM, Armonk, New York, USA). Next, we calculated the effect size of grazing using the natural log response ratio (Mcsherry and Ritchie):where Grazed and Ungrazed are the values of each indicator in the plot with and without grazing, respectively. Effect size was used as the dependent variable in our meta‐analyses. We evaluated the relationships between effect size and environmental factors, such as elevation, mean annual temperature, and mean annual precipitation, using linear, quadratic, and non‐linear (logarithmic, power, exponential, etc.) functions. When two or more functions were significant (< 0.05), we selected the function that maximized thevalue. Regression analyses were also conducted with IBM SPSS Statistics software 19 (IBM).

Results We report results from all 26 indicators of ecosystem structure and function in Table 1, even though some of the indicators, such as seeds density, seeds species numbers, and seeds species richness, were only derived from three or five observations. In general, 12 of the 26 indicators increased or decreased in the presence of livestock grazing (P < 0.05), while the remaining 14 indicators were not statistically significant different in grazed and ungrazed areas (P > 0.05; Table 1). For the remainder of the results, we focus on indicators with greater than nine observations. Ecosystem structural indicators generally increased in regions open to grazing (Fig. 4). Specifically, the Shannon–Wiener index, Simpson dominance index, and Pielou evenness index were 0.18, 0.05, and 0.03 greater in regions grazed by livestock than in regions that were not grazed (P < 0.05; Table 1). According to the mean effect sizes, livestock grazing had a negative effect on vegetation growth (Fig. 4). Aboveground biomass, belowground biomass, vegetation cover, and vegetation height—vegetation growth indicators—decreased by 88.92 g/m2, 197.30 g/m2, 16.92%, and 5.53 cm, respectively, in the presence of grazing (P < 0.01; Table 1). Figure 4 Open in figure viewer PowerPoint Mean effect sizes of livestock grazing on ecosystem structural indicators (a) and functional indicators (b) in Qinghai–Tibetan Plateau. The positive effect sizes indicate that the indicator values in grazed area are larger than those in ungrazed areas, and the negative effect sizes indicate the opposite; the numbers in brackets represent the total observations of each indicator; error bars represent standard errors; the dashed line was drawn at mean effect size = 0. Livestock grazing also had a negative effect on most soil functional indicators, including carbon stocks, soil nitrogen and phosphorus, dissolved organic matter, and microbial biomass, and resulted in significant decreases in some soil indicators (Fig. 4, Table 1). Soil organic carbon, soil total nitrogen, NH 4 +–N, and microbial biomass carbon declined by 1185.19 g/m2, 97.40 g/m2, 2.56 mg/kg, and 66.70 mg/kg, respectively, in the presence of grazing (P < 0.05; Table 1). Nevertheless, the paired difference t tests indicated that the Q 10 , which was used to assess the temperature sensitivity of soil respiration, increased due to grazing (P < 0.01, Table 1). The effect sizes of livestock grazing on grassland diversity were positive at all grazing intensities (Fig. 5). Furthermore, there was a slight trend of greater diversity in MG than that in light and HG intensities: The effect size of Shannon–Wiener index was 0.14 at MG intensity and the effect sizes of Shannon–Wiener index were 0.09 and 0.06 at light and HG intensities, respectively. However, the effect sizes of livestock grazing on Shannon–Wiener index were not statistically significantly different among three grazing intensities (P > 0.05). For some ecosystem functional indicators, including aboveground biomass, vegetation cover, soil organic carbon, and soil total nitrogen, there was a linear decreasing trend with increasing grazing intensities (Fig. 5). Figure 5 Open in figure viewer PowerPoint Mean (SE) effect sizes of livestock grazing on vegetation cover (VC), soil organic carbon (SOC), soil total nitrogen (STN), aboveground biomass (AB), and Shannon–Wiener index (SWI) in three different grazed intensities (LG, lightly grazed; MG, moderately grazed; HG, heavily grazed). The positive effect sizes indicate that the indicator values in grazed area are larger than those in ungrazed areas, and the negative effect sizes indicate the opposite. The effect sizes of grazing on vegetation growth indicators, including aboveground biomass and vegetation cover, were positively correlated with altitude (Appendix S2: Table S2; Fig. 6). The effect sizes of grazing on vegetation cover had significant quadratic relationships with MAT (Fig. 6); though for vegetation cover, the curvilinear relationship found was driven by a single datum point. Indeed, when this point was removed, a negative linear relationship between this variable and MAT was found (r = 0.30, P = 0.094). The effect size of grazing on the carbon contents of aboveground biomass was positively correlated with MAT and MAP. The effect size of grazing on soil respiration was significantly linearly related to MAP (Appendix S2: Table S2). Nevertheless, the effect of grazing on most community structure and function did not vary according to local environmental gradients of temperature and precipitation (Appendix S2: Table S2). Figure 6 Open in figure viewer PowerPoint Regression of altitude (a) and mean annual temperature (b) with the effect sizes of vegetation cover. Details of models fitted are given within each panel.

Discussion This study uses data from 61 published studies to analyze the effects of livestock grazing on ecosystem structure and function of alpine grasslands in the QTP and provide landscape‐scale insights into the effects of grazing across the QTP. Our analysis reveals that grazing exerts complex controls on ecosystem structure and function indicators, and the ecosystem response to grazing varies with grazing intensity and local environmental conditions. These results highlight the importance of considering the context of grazing in alpine grassland management and degraded alpine grassland restoration. Ecosystem structure Grazing has an important role in the plant species diversity and vegetation structure of grasslands (Cingolani et al. 2003, Pucheta et al. 2004, Metzger et al. 2005). Grazed plots have higher plant species diversity values compared with ungrazed plots (Table 1, Fig. 4); therefore, grazing appears to play an important role in affecting the community structure and plant species diversity of alpine grassland ecosystems in the QTP. The effect of grazing on plant species diversity may be a result of individual plant‐specific responses to grazing because various species differ in their resistance and tolerance to native herbivores. For instance, in the alpine steppe ecotone of central Tibet, grazing tolerance species were the prevalent functional types. However, species with no specific protection against grazing, such as Kobresia schoenoides and Poa albertii, constituted no more than 15% of the total plant cover (Miehe et al. 2011). Grazing may promote higher plant species diversity by increasing spatial heterogeneity and enhancing niche breadth and overlap in grassland communities, thus facilitating species coexistence (Woldu and Saleem 2000, McGranahan et al. 2012). Grazing resulted in the removal of palatable species, which reduced the abundance of some dominant species (Niu et al. 2010). This reduction benefited some unpalatable and grazing‐resistant species through reduced competition and increased nutrient availability and/or light availability (Zhu et al. 2008, Li et al. 2013a). Ecosystem function Plant biomass and productivity are important metrics of ecosystem function in alpine grasslands. We find that all four measures of vegetation growth—aboveground biomass, belowground biomass, vegetation cover, and vegetation height—were all lower in grazed grasslands than those in ungrazed grasslands (P < 0.01). Almost all published papers (>90%) that explored the effect of grazing on vegetation growth found that livestock grazing reduced the aboveground vegetation growth indicators in alpine grasslands (Table 1). However, for the belowground biomass, the research results were not consistent in terms of the effect of livestock grazing among different study sites and different alpine grassland types: 75% of published papers reported that the belowground biomass decreased due to grazing (Fan et al. 2013, Xiong et al. 2014), while the remaining 25% of published papers reported the opposite results (Niu et al. 2009, Fu et al. 2014, Table 1). The contrasting effects of grazing on aboveground and belowground biomass may be a result of how herbivores graze and how plants respond (Fu et al. 2014, Xiong et al. 2014, Wu et al. 2015). Grazing decreases aboveground biomass because livestock use the vegetation for feed, especially graminoids and sedge species (Wu et al. 2004, Shi et al. 2013). Grazing may also result in lower plant biomass as a result of deteriorated soil conditions in grazed regions, such as soil erosion and nutrient loss (Yates et al. 2000, Zhou et al. 2006). In contrast to aboveground biomass, belowground biomass can decrease or increase due to livestock grazing both in the QTP (Fu et al. 2014, Xiong et al. 2014) and in grasslands in other parts of the world (Semmartin et al. 2007, Koerner and Collins 2014, López‐Mársico et al. 2015). Grazing may increase belowground biomass by causing plants to reduce allocation to aboveground parts and increase allocation of biomass to belowground parts in order to germinate and resist grazing pressures (Sun et al. 2014, López‐Mársico et al. 2015) or decrease belowground biomass if continuous grazing reduces the source size of carbon‐assimilating organs and intensifies the re‐translocation of root carbohydrates to shoot meristems (Koerner and Collins 2014, Xiong et al. 2014). Grazing reduced the carbon storage of alpine grassland ecosystem in the QTP, especially belowground carbon stocks including soil organic carbon and carbon in belowground biomass (Table 1, Fig. 4). Livestock grazing reduced soil organic carbon storage in the QTP through three possible mechanisms: first by reducing the quantity of resources returned to the soil, particularly by removing the palatable grasses and sedges that produce higher‐quality litter for decomposers than unpalatable species (Sun et al. 2011); second by reducing belowground net primary productivity and biomass allocation (Fan et al. 2013); and third through higher loss of carbon to the atmosphere as a result of higher soil temperature and increased soil biota and root activities after grazing (Wang et al. 2008, Zou et al. 2014). Additionally, over long time periods, grazing can result in soil compaction, alter soil infiltration rates, increase soil bulk density, decrease soil porosity, and increase soil erosion by wind, which also lead to alterations in the soil organic carbon (Wang et al. 2012, Sun et al. 2014). Soil total nitrogen was significantly lower in grazed alpine grasslands compared with ungrazed areas (Table 1, Fig. 4). The decrease in soil total nitrogen was probably due to the combined effects of the loss of organic matter following animal trampling, lower aboveground and belowground organic matter input as a consequence of grazing, and increased wind erosion leading to a preferential loss of fine and nitrogen‐rich particles (Neff et al. 2005, Bisigato et al. 2008, Wang et al. 2012). Our results indicated that livestock grazing did not significantly affect soil total phosphorous and available phosphorous contents of alpine grassland (Table 1). In general, soil phosphorous content was lower in the QTP compared with the Chinese average and global average value due to the extreme environmental stress, such as considerable cold, limited precipitation, short growing season, and the intense erosion effects of wind (Hong et al. 2014, Sun et al. 2014). Thus, any change in the phosphorus content as a result of grazing was not obvious. Our results showed that livestock grazing significantly reduced soil microbial biomass carbon of alpine grassland ecosystems in the QTP (Table 1). The decrease in soil microbial biomass could be explained by the enhanced mineralization due to plant regrowth (Hamilton and Frank 2001, Rui et al. 2011). Alternatively, grazing may reduce the availability of organic substrates for microorganisms and microbial biomass through indirect effects, including alteration to run‐off and erosion processes after grazing (Holt 1997, Li et al. 2013b). Decreases in soil microbial biomass following grazing have also been reported in experimental research in an alpine meadow in central Tibet (Fu et al. 2012, 2014), and studies have also found that grazing reduces landscape‐level measures of soil microbial biomass in dry tropical grassland ecosystems of Australia (Holt 1997, Northup et al. 1999). Grazing intensities Plant communities and ecosystems can tolerate some degree of grazing disturbance, but typically if the disturbance surpasses existing thresholds, they can become unstable (Westoby 1989, Zhu et al. 2008, Villnäs et al. 2013). Based on our results, we believe this is the case for the plant species diversity of grassland ecosystems under varying degrees of grazing. We find that livestock grazing had the positive effect on grassland diversity at all grazing intensities. Nevertheless, there is a slight trend of greater diversity in medium grazing than that in LG intensities and HG intensities in the QTP (Fig. 5). This result is generally consistent with the intermediate disturbance hypothesis, which suggests that moderate levels of disturbance can play an important role in promoting community succession and maintaining community structure and species diversity (Grime 1973, Duru et al. 2012, Kiełtyk and Mirek 2015). Livestock grazing decreases functional indicators of alpine grasslands and the effects are enhanced at greater grazing intensities (Fig. 5), likely because grazing reduces plant density, plant basal area, and the presence of organic residues that act as an important soil carbon and nitrogen source to grasslands (Wang et al. 2012). For instance, in the Stipa purpurea alpine grassland of northern Tibet, aboveground biomass decreased by 20.43%, 32.11%, and 41.54% and total coverage decreased by 5.76%, 14.01%, and 14.44%, under lightly grazed, moderately grazed, and heavily grazed conditions, respectively (Duan et al. 2011). In an alpine meadow of southeastern Qinghai, the soil organic carbon decreased by 56.80%, 64.17%, and 70.40% and soil total nitrogen decreased by 38.20%, 49.44%, and 57.30% under light, moderate, and HG conditions, respectively, compared with ungrazed grasslands (Wu et al. 2015). Therefore, we expect that continuous HG would result in the deterioration of vegetation and soil conditions in the QTP and could lead to significant changes in the composition and structure of the plant community, including decreases in the regenerative ability of the grasslands, decreases in biomass, decreases in the amount of nutrients returned to the soil as litter, and eventual grassland degradation (Zhou et al. 2005, Shang and Long 2007). Interactions between grazing and environmental factors The structure and function of grazed grassland ecosystems depend on both grazing management and environmental conditions such as temperature and precipitation (Milchunas and Lauenroth 1993, Turner 1999, Yan et al. 2013). The effect of livestock grazing on aboveground biomass and vegetation cover indicates that the negative effects of grazing weakened with the increasing altitude in the QTP (Appendix S2: Table S2; Fig. 6). The effect sizes of grazing on vegetation cover had significant quadratic relationships with MAT (Fig. 6), although it must be noted that the relationship between vegetation cover and MAT was dependent on a single datum point, which was not in the main part of the QTP and is relatively low attitude with 2500 m a.s.l. (Fig. 3), and a negative linear relationship was found when it was removed (r = 0.30, P = 0.094). The effect of grazing on aboveground vegetation growth was consistent between altitude and temperature, likely because temperature decreases with elevation in the QTP (Körner 2007, Liu et al. 2009). In the QTP, temperature appears to be a key limiting factor for plant growth due to high altitude and low mean temperature (Kong et al. 2012, Ma et al. 2012). Therefore, these local environmental conditions such as altitude and MAT should be considered when making recommendations for management of grazing intensity. However, most other structural and functional indicators did not exhibit a relation with the altitude, MAT, or MAP (Appendix S2: Table S2); therefore, more work is needed to understand whether these patterns are indicative of real differences in grassland responses with elevation or due to some other factors such as changing grazing intensity at different elevation grasslands. “Retire livestock and restore grassland” The RLRG program was initiated in 2004 as a cost‐effective method for restoring degraded grasslands on the QTP (Wu et al. 2010, Yan and Lu 2015). Some ecological benefits of this program have been reported (Wei et al. 2012, Wu et al. 2013), but our analyses suggest that complete exclusion of grazing in grassland ecosystems can have variable effects on grassland health with evidence for positive (Wu et al. 2010, Yan and Lu 2015), negative (Hafner et al. 2012, Shi et al. 2013), and neutral effects (Dong et al. 2012, Lu et al. 2015a, b). In other ecosystems, there is growing evidence that complete exclusion from grazing for a prolonged period of time may not lead to an improvement in vegetation and soil conditions (Chaneton and Lavado 1996, Hart and Scientist 2001, Firincioğlu et al. 2007, Thompson and Willms 2014). From our metadata analysis, low or moderate grazing could be an appropriate alpine grassland management strategy. Low, moderate, and high grazing intensities provide some benefits in plant diversity (compared to no grazing), but the metadata analysis results suggest increasing risk of degradation in most indicators with increasing grazing intensity (Fig. 5). Many other studies also suggest that a moderate level of grazing offers a reasonable compromise when balancing forage production, plant species diversity, soil organic carbon, and plant mortality during drought (McNaughton 1979, Franzluebbers 2010, Kimoto et al. 2012, Luo et al. 2012). Therefore, we suggest that implementing a low or moderate grazing intensity management strategy may be effective at enhancing grassland production and sustaining ecosystem health and sustainability of alpine grassland on the QTP.

Conclusions Livestock grazing is one of the most important factors influencing community structure and ecosystem function in natural grasslands. In this work, we produce a synthesis of the effects of livestock grazing on alpine grassland ecosystem structure and function in the QTP based on a comprehensive review of the current literatures. We find that livestock grazing has an important role in community structure, and specifically, species richness and biodiversity were higher in grazed alpine grasslands than in ungrazed grasslands. In contrast, grazing significantly decreased vegetation biomass, vegetation cover, and some soil functional indicators, such as soil organic carbon, soil total nitrogen, soil available nitrogen, soil microbial biomass carbon and nitrogen. With increasing grazing intensity, there is greater risk of loss of some ecosystem function and associated degradation, while benefits from increased diversity remain relatively constant. In addition, this analysis also shows that the effect of livestock grazing on grassland ecosystems varies according to local environmental conditions. Some ecological benefits have been gained from the RLRG program, but complete exclusion of grazing in grassland ecosystems can have variable effects on grassland health. These observations combined with evidence from studies of grazing systems elsewhere (Firincioğlu et al. 2007, Kimoto et al. 2012, Thompson and Willms 2014) suggest that a more nuanced management regime, such as moderate grazing strategy, may offer the best compromise between balances of ecological protection of grasslands and forage production for livestock in the QTP region.

Acknowledgments This study was supported by the National Natural Science Foundation of China (41371267 and 41671262) and by the Strategic Leading Science and Technology Projects of Chinese Academy of Sciences (XDB03030505).

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