Chengqun Yu

Ecological and Environmental Issues Faced by a Developing Tibet

The Tibetan plateau, covering an area of 2.6 million km 2 with an average elevation of over 4000 m, often called “the third pole of the world”, has fundamental significance to the environment of China, Asia, and the world. The Tibetan plateau is called a “water tower” due to its downstream influence on approximately 40% of the world’s population. It is a region with rich species diversity and a high-altitude plateau biodiversity conservation base site, where some ancient species were preserved and new species evolved under the unique geology development process. In recent years, a series of ecological and environmental issues have emerged due to enhanced anthropogenic disturbances and climatic change. These issues are gradually eroding the capacity of the Tibet plateau to act as an “ecological security barrier” of atmosphere circulation and water sources for China and southern Asia. This study critically reviews several imminent ecological and environmental issues faced by Tibet and has the goal of drawing the attention of governments and international societies. The effects of global warming are more obvious in Tibet than in other areas at similar latitude. The temperature in Tibet has been increasing at a faster rate than other inland areas of China in the past decades. 1 Precipitation exhibited no obvious trend, but has occurred in a more concentrated way during each year. The permafrost soil of Tibet historically covered an area of 1,401,000 km 2 , accounting for 54.3% of Tibetan plateau. 2 In the past 30 years, the lower altitude limit of permafrost in Tibet has moved up on average 50 m. The thickness of the active soil

Response of Soil Respiration to Grazing in an Alpine Meadow at Three Elevations in Tibet

Alpine meadows are one major type of pastureland on the Tibetan Plateau. However, few studies have evaluated the response of soil respiration (R s) to grazing along an elevation gradient in an alpine meadow on the Tibetan Plateau. Here three fenced enclosures were established in an alpine meadow at three elevations (i.e., 4313 m, 4513 m, and 4693 m) in July 2008. We measured R s inside and outside the three fenced enclosures in July–September, 2010-2011. Topsoil (0–20 cm) samples were gathered in July, August, and September, 2011. There were no significant differences for R s, dissolved organic C (DOC), and belowground root biomass (BGB) between the grazed and ungrazed soils. Soil respiration was positively correlated with soil organic C (SOC), microbial biomass (MBC), DOC, and BGB. In addition, both R s and BGB increased with total N (TN), the ratio of SOC to TN, ammonium N (NH4 +-N), and the ratio of NH4 +-N to nitrate N. Our findings suggested that the negligible response of R s to grazing could be directly attributed to that of respiration substrate and that soil N may indirectly affect R s by its effect on BGB.

Relationship between the Growing Season Maximum Enhanced Vegetation Index and Climatic Factors on the Tibetan Plateau

Temperature and water conditions affect vegetation growth dynamics and associated spectral measures. We examined the response of the growing season maximum enhanced vegetation index (MEVI) to the growing season temperature, precipitation, vapor pressure and relative humidity on the Tibetan Plateau. The responses of the MEVI to climatic factors changed with the vegetation type, which may be attributed to the finding that the background values and climatic factor changes varied with the type of vegetation. The spatially averaged MEVI over the entire plateau exhibited a non-significant decreasing trend. Approximately 5% and 12% of the vegetation area exhibited significant MEVI decreasing and increasing trends, respectively. Both vapor pressure and relative humidity significantly affected the MEVI, whereas the temperature and precipitation did not significantly correlate with the MEVI over the entire plateau. Specifically, the environmental humidity dominated the MEVI variation over the entire plateau.

Response of ecosystem respiration to experimental warming and clipping at daily time scale in an alpine meadow of tibet

The alpine meadow, as one of the typical vegetation types on the Tibetan Plateau, is one of the most sensitive terrestrial ecosystems to climate warming. However, how climate warming affects the carbon cycling of the alpine meadow on the Tibetan Plateau is not very clear. A field experiment under controlled experimental warming and clipping conditions was conducted in an alpine meadow on the Northern Tibetan Plateau since July 2008. Open top chambers (OTCs) were used to simulate climate warming. The main objective of this study was to examine the responses of ecosystem respiration (Reco) and its temperature sensitivity to experimental warming and clipping at daily time scale. Therefore, we measured Reco once or twice a month from July to September in 2010, from June to September in 2011 and from August to September in 2012. Air temperature dominated daily variation of Reco whether or not experimental warming and clipping were present. Air temperature was exponentially correlated with Reco and it could significantly explain 58∼96% variation of Reco at daily time scale. Experimental warming and clipping decreased daily mean Reco by 5.8∼37.7% and −11.9∼23.0%, respectively, although not all these changes were significant. Experimental warming tended to decrease the temperature sensitivity of Reco, whereas clipping tended to increase the temperature sensitivity of Reco at daily time scale. Our findings suggest that Reco was mainly controlled by air temperature and may acclimate to climate warming due to its lower temperature sensitivity under experimental warming at daily time scale.

Response of Soil C and N, Dissolved Organic C and N, and Inorganic N to Short-Term Experimental Warming in an Alpine Meadow on the Tibetan Plateau

Although alpine meadows of Tibet are expected to be strongly affected by climatic warming, it remains unclear how soil organic C (SOC), total N (TN), ammonium N (NH4 +-N) , nitrate N (NO3 +-N), and dissolved organic C (DOC) and N (DON) respond to warming. This study aims to investigate the responses of these C and N pools to short-term experimental warming in an alpine meadow of Tibet. A warming experiment using open top chambers was conducted in an alpine meadow at three elevations (i.e., a low (4313 m), mid-(4513 m), and high (4693 m) elevation) in May 2010. Topsoil (0–20 cm depth) samples were collected in July–September 2011. Experimental warming increased soil temperature by ~1–1.4°C but decreased soil moisture by ~0.04 m3 m−3. Experimental warming had little effects on SOC, TN, DOC, and DON, which may be related to lower warming magnitude, the short period of warming treatment, and experimental warming-induced soil drying by decreasing soil microbial activity. Experimental warming decreased significantly inorganic N at the two lower elevations,but had negligible effect at the high elevation. Our findings suggested that the effects of short-term experimental warming on SOC, TN and dissolved organic matter were insignificant, only affecting inorganic forms.

Clipping Alters the Response of Biomass Production to Experimental Warming: A Case Study in an Alpine Meadow on the Tibetan Plateau, China

Predicting how human activity will influence the response of alpine grasslands to future warming has many uncertainties. In this study, a field experiment with controlled warming and clipping was conducted in an alpine meadow at three elevations (4313 m, 4513 m and 4693 m) in Northern Tibet to test the hypothesis that clipping would alter warming effect on biomass production. Open top chambers (OTCs) were used to increase temperature since July, 2008 and the OTCs increased air temperature by approximately 0.9°C ∼ 1.8°C during the growing in 2012. Clipping was conducted three times one year during growing season and the aboveground parts of all live plants were clipped to approximately 0.01 m in height using scissors since 2009. Gross primary production (GPP) was calculated from the Moderate-Resolution Imaging Spectroradiometer GPP algorithm and aboveground plant production was estimated using the surface-measured normalized difference vegetation index in 2012. Warming decreased the GPP, aboveground biomass (AGB) and aboveground net primary production (ANPP) at all three elevations when clipping was not applied. In contrast, warming increased AGB at all three elevations, GPP at the two lower elevations and ANPP at the two higher elevations when clipping was applied. These findings show that clipping reduced the negative effect of warming on GPP, AGB and ANPP, suggesting that clipping may reduce the effect of climate warming on GPP, AGB and ANPP in alpine meadows on the Tibetan Plateau, and therefore, may be a viable strategy for mitigating the effects of climate change on grazing and animal husbandry on the Tibetan Plateau.

PARTITIONING SOURCES OF ECOSYSTEM AND SOIL RESPIRATION IN AN ALPINE MEADOW OF TIBET PLATEAU USING REGRESSION METHOD

ABSTRACT: Partitioning sourćes of ecosystem and soil respiration (R eco and Rs) is important for understanding how climate change affects carbon cycling. Plant and microbial biomass analyses and daytime measurements of Reco and Rs were performed for 25 plots in an alpine meadow at elevation 4313 m on the Tibetan Plateau. Plant and microbial biomass were determined by harvesting method and the chloroform fumigation-extraction method, respectively. Respiration fluxes were measured by an automated CO2 flux system (LI-8100, LI-COR Biosciences, Lincoln, NE, USA). Soil respiration can be estimated by a linear or exponential relationship between Reco and aboveground plant biomass (AGB). Microbial respiration (Rm) can be estimated by a linear or exponential relationship between Rs and belowground plant biomass (BGB) or by a multiple relationship between Reco and AGB and BGB. Soil respiration (or Rm) is respiration flux when AGB (or BGB) is extrapolated to zero for the linear and exponential regression methods. Similarly, Rm is respiration flux when both AGB and BGB are zero for the multiple regression method. Our findings suggest that the exponential regression method to partition sources of Reco and R s may be more appropriate compared to other methods for this alpine meadow of Tibet.

Hydrochemical characteristics and element contents of natural waters in Tibet, China

Sixty water samples (35 groundwater samples, 22 surface water samples and three hot-spring water samples) were collected at 36 points from villages and towns in Lhasa city, Nagchu (Nagqu) prefecture, Ali (Ngari) prefecture and Shigatse (Xigaze) prefecture (Tibet) in 2013 to study the hydrochemical characteristics and element contents of natural waters. The concentrations of elements were determined in the water samples and compared with the concentrations in water samples from other regions, such as southeast Qinghai, south Xinjiang, east Sichuan and west Tibet. The hydrochemical species in different areas were also studied. Water in most parts of Tibet reaches the requirements of the Chinese national standard and the World Health Organization international standard. The pH values of the water samples ranged from 6.75 to 8.21 and the value for the mean total dissolved solids was 225.54 mg/L. The concentration of arsenic in water from Ali prefecture exceeded the limit of both the Chinese national standard and the international standard and the concentration of fluoride in water from Shuanghu exceeded the limit of both the Chinese national standard and the international standard. The main hydrochemical species in water of Tibet is Ca (HCO3)2. From south to north, the main cation in water changes from Ca2+ to Na+, whereas the main anions in water change from HCO3− to Cl− and SO42−. The chemistry of river water and melt water from ice and snow is dominated by the rocks present at their source, whereas the chemistry of groundwater is affected by many factors. Tectonic divisions determine the concentrations of the main elements in water and also affect the hydrochemical species present.

The Soil Drying Along the Increase of Warming Masks the Relation between Temperature and Soil Respiration in an Alpine Meadow of Northern Tibet

ABSTRACT A warming experiment with two magnitudes was performed in an alpine meadow of Northern Tibet since late June, 2013. Open top chambers (OTCs) with two top diameters (0.60 m and 1.00 m) were used to increase soil temperature. Soil respiration (Rs) was measured during the growing season in 2013–2014. The OTCs with top diameters of 1.00 m and 0.60 m increased soil temperature by 1.30 and 3.10oC, respectively, during the whole study period, but decreased soil moisture by 0.02 and 0.05 m3 m-3, respectively. However, the two patters of OTCs did not affect Rs . These results implied that a higher warming did not result in a higher Rs but a greater soil drying. Therefore, a higher warming may not cause a higher soil respiration, which was most likely due to the fact that a higher warming may result in a greater soil drying.

Patterns and dynamics of the human appropriation of net primary production and its components in Tibet

Anthropogenic activities have induced profound changes across the globe. Human appropriation of net primary production (HANPP) is a useful indicator for quantifying anthropogenic influences on natural ecosystems. We applied a detailed HANPP framework to the Tibet Autonomous Region of China for the period 1989-2015 and performed clustering analysis to explore county-level dynamics of HANPP components. The results indicated a continuous increase in HANPP per unit area from 10.3 g C/m2/yr in 1989 to 18.5 g C/m2/yr in 2008, with some fluctuation and a decline to 16.8 C/m2/yr in 2015. As a percentage of potential net primary production (NPPpot), HANPP increased from 6.9% to 13.5%. This rise was mainly driven by the commercialization of animal husbandry and by ecological conservation policies. Animal stocks dominated HANPP in Tibet in 1989, and by 2015 beef or crop production had become predominant in 30 of 73 counties. However, HANPP did not change uniformly across all locations. Changes were mainly concentrated in the south-central river valley area because of the growth in beef and crop production there. While in almost half of the 73 counties located in the northwestern regions, HANPP was dominated by sheep stocks and changed only slightly over the study period. These findings indicate that a comprehensive spatiotemporal analysis of HANPP components in Tibet provides deeper insights into changes in production and livelihood strategies of local residents, aligned with ecological conservation policies and economic development. Moreover, it unravels the complex impacts of human activities on alpine ecosystems, and indicates the need to optimize local ecosystem management and conservation policies.