Animal husbandry and crop farming are specialized for development through a long history in the northern plateau and southern valleys, respectively, on the Tibetan Plateau. Such a pattern of isolation has led to current concerns over rangeland degradation and forage shortages with the increasing pressures of livestock growth and declining soil fertility due to intensive farming (Duan et al., 2019). Rangeland degradation is a widespread concern, especially in the highly vulnerable and sensitive alpine environment. On the Tibetan Plateau, reports indicate that more than half of the rangeland has been degraded (Dong et al., 2010), which directly threatens ecosystem goods and services. Moreover, as the major terrestrial ecosystem, rangeland degradation will also impair its functioning nationally as an ecological security shelter. Cropland degradation is also a common phenomenon, showing a decline in soil organic matter, soil compaction and poor soil structure, and thereby resulting in decreasing food production in southern Tibet. Intensive farming, low input, monoculture and soil erosion are the main causes of agricultural land degradation and decreasing food production (Cai, 2003).

Plant photosynthesis is the fundamental driver of all the biospheric functions. Alpine meadow on the Tibetan Plateau is sensitive to rapid climate change, and thus can be considered an indicator for the response of terrestrial ecosystems to climate change. However, seasonal variations in photosynthetic parameters, including the fraction of photosynthetically active radiation by canopy (FPAR), the light extinction coefficient (k) through canopy, and the leaf area index (LAI) of plant communities, are not known for alpine meadows on the Tibetan Plateau. In this study, we used field measurements of radiation components and canopy structure from 2009 to 2011 at a typical alpine meadow on the northern Tibetan Plateau to calculate these three photosynthetic parameters. We developed a satellite-based (NDVI and EVI) method derived from the Beer-Lambert law to estimate the seasonal dynamics of FPAR, k ,and LAI, and we compared these estimates with the Moderate Resolution Imaging Spectroradiometer (MODIS) FPAR (FPAR_MOD) and LAI product (LAI_MOD). The results showed that the average daily FPAR was 0.33, 0.37 and 0.35, respectively, from 2009 to 2011, and that the temporal variations could be explained by all four satellite-based FPAR estimations, including FPAR_MOD, an FPAR estimation derived from the Beer-Lambert law with a constant k (FPAR_LAI), and two FPAR estimations from the nonlinear functions between the ground measurements of FPAR (FAPRg) and NDVI/EVI (FPAR_NDVI and FPAR_EVI). We found that FPAR_MOD seriously undervalued FPARg by over 40%. Tower-based FPAR_LAI also significantly underestimated FPARg by approximately 20% due to the constant k (0.5) throughout the whole growing seasons. This indicated that using FPAR_LAI to validate the FPAR_MOD was not an appropriate method in this alpine meadow because the seasonal variation of k ranged from 0.19 to 2.95 in this alpine meadow. Thus, if the seasonal variation of k was taken into consideration, both FPAR_NDVI and FPAR_EVI provided better descriptions, with negligible overestimates of less than 5% of FAPRg (RMSE=0.05), in FPARg estimations than FPAR_MOD and FPAR_LAI. Combining the satellite-based (NDVI and EVI) estimations of seasonal FPAR and k, LAI_NDVI and LAI_EVI derived from the Beer-Lambert law also provided better LAIg estimations than LAI_MOD (less than 30% of LAIg). Therefore, this study concluded that satellite-based models derived from the Beer-Lambert law were a simple and efficient method for estimating the seasonal dynamics of FPAR, k and LAI in this alpine meadow.

Ecosystems can simultaneously provide multiple functions and services. Knowledge on the combinations of such multi-dimensional functions is critical for accurately assessing the carrying capacity and implementing sustainable management. However, accurately quantify the multifunctionality of ecosystems remains challenging due to the dependence and close association among individual functions. Here, we quantified spatial patterns in the multifunctionality of alpine grassland on the Tibetan Plateau by integrating four important individual functions based on data collected from a field survey and remote sensing NDVI. After mapping the spatial pattern of multifunctionality, we extracted multifunctionality values across four types of grassland along the northern Tibet Plateau transect. Effects of climate and grazing intensity on the multifunctionality were differentiated. Our results showed that the highest values of multifunctionality occurred in the alpine meadow. Low values of multifunctionality were comparable in different types of grassland. Annual precipitation explained the large variation of multifunctionality across the different types of grassland in the transect, which showed a significantly positive effect on the multifunctionality. Grazing intensity further explained the rest of the variation in the multifunctionality (residuals), which showed a shift from neutral or positive to negative effects on multifunctionality across the different types of grassland. The consistently rapid declines of belowground biomass, SOC, and species richness resulted in the collapse of the multifunctionality as bare ground cover amounted to 75%, which corresponded to a multifunctionality value of 0.233. Our results are the first to show the spatial pattern of grassland multifunctionality. The rapid decline of the multifunctionality suggests that a collapse in the multifunctionality can occur after the vegetation cover decreases to 25%, which is also accompanied by rapid losses of species and other individual functions. Our results are expected to provide evidence and direction for the sustainable development of alpine grassland and restoration management.

Livestock grazing is one of primary way to use grasslands throughout the world, and the forage-livestock balance of grasslands is a core issue determining animal husbandry sustainability. However, there are few methods for assessing the forage-livestock balance and none of those consider the dynamics of external abiotic factors that influence forage yields. In this study, we combine long-term field observations with remote sensing data and meteorological records of temperature and precipitation to quantify the impacts of climate change and human activities on the forage-livestock balance of alpine grasslands on the northern Tibetan Plateau for the years 2000 to 2016. We developed two methods: one is statical method based on equilibrium theory and the other is dynamic method based on non-equilibrium theory. We also examined the uncertainties and shortcomings of using these two methods as a basis for formulating policies for sustainable grassland management. Our results from the statical method showed severe overgrazing in the grasslands of all counties observed except Nyima (including Shuanghu) for the entire period from 2000 to 2016. In contrast, the results from the dynamic method showed overgrazing in only eight years of the study period 2000-2016, while in the other nine years alpine grasslands throughout the northern Tibetan Plateau were less grazed and had forage surpluses. Additionally, the dynamic method found that the alpine grasslands of counties in the northeastern and southwestern areas of the northern Tibetan Plateau were overgrazed, and that alpine grasslands in the central area of the plateau were less grazed with forage surpluses. The latter finding is consistent with field surveys. Therefore, we suggest that the dynamic method is more appropriate for assessment of forage-livestock management efforts in alpine grasslands on the northern Tibetan Plateau. However, the statical method is still recommended for assessments of alpine grasslands profoundly disturbed by irrational human activities.

Many recent studies have focused on vegetation feedback to climate systems in sensitive areas like the Qinghai-Tibetan Plateau. Providing allowances and awards to households engaged in animal husbandry that practice grazing exclusion to restore degraded grassland is an important eco-compensation policy effort in China. Grazing exclusion influences grassland variations (Cai et al., 2015). Numerous observational and modelling studies have confirmed that land sur-face conditions play a crucial role in climate change (Pielke et al., 2002; Kalnay and Cai, 2003; Feddema et al., 2005; Pitman and Narisma, 2005; Seneviratne et al., 2006; Pielke et al., 2007). Land surfaces impact the atmosphere through the exchange of energy, momentum, water, carbon dioxide and other gases with the atmospheric boundary layer (Cox et al., 2000; Bounoua et al., 2002).

Enclosure is one of the most widely used management tools for degraded alpine grassland on the northern Tibetan Plateau, but the responses of different types of grassland to enclosure may vary, and research on these responses can provide a scientific basis for improving ecological conservation. This study took one site for each of three grassland types (alpine meadow, alpine steppe and alpine desert) on the northern Tibetan Plateau as examples, and explored the effects of enclosure on plant and soil nutrients by comparing differences in plant community biomass, leaf-soil nutrient content and their stoichiometry between samples from inside and outside the fence. The results showed that enclosure can significantly increase all aboveground biomass in these three grassland types, but it only increased the 10-20 cm underground biomass in the alpine desert. Enclosure also significantly increased the leaf nutrient content of the dominant plants and contents of total nitrogen (N), total potassium (K), and organic carbon (C) in 10-20 cm soil in alpine desert, thus changing the stoichiometry between C, N and P (phosphorus). However, enclosure significantly increased only the N content of dominant plant leaves in alpine steppe, while other nutrients and stoichiometries of both plant leaves and soil did not show significant differences in alpine meadow and alpine steppe. These results suggested that enclosure has differential effects on these three types of alpine grasslands on the northern Tibetan Plateau, and the alpine desert showed the most active ecological conservation in the responses of its soil and plant nutrients.

In order to investigate the general tendency of soil microbial community responses to fertilizers, a meta-analysis approach was used to synthesise observations on the effects of inorganic and organic fertilizer addition (N: nitrogen; P: phosphorus; NP: nitrogen and phosphorus; PK: phosphorus and potassium; NPK: nitrogen, phosphorus and potassium; OF: organic fertilizer; OF+NPK: organic fertilizer plus NPK) on soil microbial communities. Among the various studies, PK, NPK, OF and OF+NPK addition increased total phospholipid fatty acid (PLFA) by 52.0%, 19.5%, 334.3% and 58.3%, respectively; while NP, OF and OF+NPK addition increased fungi by 5.6%, 21.0% and 8.2%, respectively. NP, NPK and OF addition increased bacteria by 6.4%, 9.8% and 13.3%, respectively; while NP and NPK addition increased actinomycetes by 7.0% and 14.8%, respectively. Addition of ammonium nitrate rather than urea decreased gram-negative bacteria (G -). N addition increased total PLFA、bacteria and actinomycetes in croplands, but decreased fungi and bacteria in forests, and the F/B ratio in grasslands. NPK addition increased total PLFA in forests but not in croplands. The N addition rate was positively correlated with the effects of N addition on gram-positive bacteria (G +) and G -. Therefore, different fertilizers appear to have different effects on the soil microbial community. Organic fertilizers can have a greater positive effect on the soil microbial community than inorganic fertilizers. The effects of fertilizers on the soil microbial community varied with ecosystem types. The effect of N addition on the soil microbial community was related to both the forms of nitrogen that were added and the nitrogen addition rate.

Drought is an important climate event and is linked to water availability, temperature, sunshine, and wind speed (Dai, 2011; Trenberth et al., 2014). Generally, water availability plays an important role in terrestrial ecosystems, and drought has an adverse effect on vegetation growth (Mu et al., 2013; Wang et al., 2013). Several studies have focused on how drought changes under climate change. However, neither modeling nor observations provide a widely accepted answer to the question of how climate change impacts drought (Sheffield et al., 2012; Dai, 2013; Trenberth et al., 2014). Understanding the relationship between drought and changes in climate factors is crucial for predicting future changes in aridity and coping with drought-related issues.

Low temperature is an important limiting factor for alpine ecosystems on the Tibetan Plateau. This study is based on data from on-site experimental warming platforms (open top chambers, OTC) at three elevations (4300 m, 4500 m, 4700 m) on the Qinghai-Tibet Plateau. The carbon and nitrogen stoichiometry characteristics of plant communities, both above-ground and below-ground, were observed in three alpine meadow ecosystems in August and September of 2011 and August of 2012. Experimental warming significantly increased above-ground nitrogen content by 21.4% in September 2011 at 4500 m, and reduced above-ground carbon content by 3.9% in August 2012 at 4300 m. Experimental warming significantly increased below-ground carbon content by 5.5% in August 2011 at 4500 m, and the below-ground ratio of carbon to nitrogen by 28.0% in September 2011 at 4300 m, but reduced below-ground nitrogen content by 15.7% in September 2011 at 4700 m, below-ground carbon content by 34.3% in August 2012 at 4700 m, and the below-ground ratio of carbon to nitrogen by 37.9% in August 2012 at 4700 m. Experimental warming had no significant effect on the characteristics of community carbon and nitrogen stoichiometry under other conditions. Therefore, experimental warming had inconsistent effects on the carbon and nitrogen stoichiometry of plant communities at different elevations and during different months. Soil ammonium nitrogen and nitrate nitrogen content were the main factors affecting plant community carbon and nitrogen stoichiometry.

Nearly half of the land in Tibet is arid or semi-arid. Due to its special topographical, geomorphic and climatic conditions, the natural conditions are harsh, the ecosystem is fragile, and the carrying capacity is very limited (Sun et al., 2012; Zhang et al., 2015). Global climate change and the impact of human activities have further aggravated the situation in recent years, and grassland degradation has become one of the most serious ecological problems facing Tibet (Liu et al., 2012; Fu et al., 2018). In order to relieve the pressure of natural grassland and protect the ecological environment in Tibet, the vigorous promotion of human-made grassland has become a key measure (Gao et al., 2014; Duan et al., 2019). Pennisetum centrasiaticum Tzvel is a perennial forage of Pennisetum in Gramineae. P.centrasiaticum is widely distributed in arid and semi-arid areas of Tibet. Its rhizome system is developed and has strong resistance to adversity. It has genes to resist disease, insects, cold and drought that wheat crops lack. The biomass and nutritional quality of P.centrasiaticum are high at jointing stage (Li, 1983; Gao, 2008). PEG-6000 can simulate drought stress by regulating the osmotic pressure of solutions to limit water entering seeds, so the use of PEG-6000 offers a fast and reliable method to simulate a drought environment for the purpose of screening drought-resistant varieties (Hegarty, 1977; Van den et al, 2006). At present, there are few studies of P.centrasiaticum in Tibet. There is no data about the use of PEG-6000 to simulate a drought environment to screen out the P.centrasiaticum from different areas. The meteorological data from 1971 to 2014 showed that drought conditions in Tibet have worsened over the years, and this may cause devastating, long-term damage to the agriculture, economy and ecosystem of Tibet (Li et al., 2019). Seedling stage is the key stage of forage growth, and is sensitive to water response. Therefore, identifying varieties with strong drought resistance at seedling stage can be of significant value as a guide for production. Drought resistant forage types are key to the development of animal husbandry in Tibet (Cui et al., 2015).