Shi Peili

Causes and Restoration of Degraded Alpine Grassland in Northern Tibet

Abstract: Grassland in northern Tibet plays an important role in the eco—security of the Qinghai-Tibet Plateau and the restoration of deserted and degraded grassland is now a focus for governments. We used remote sensing, simulations and field surveys to analyze the current status, trends and causes of grassland degradation across northern Tibet. We develop several recovery models for degraded grassland based on field experiments in the region. We found that slightly degraded grassland covers 62% and that moderate to severely degraded grassland occupied 15.1% in the Chang Tang Plateau. The amount of degraded alpine steppe increased from 1991, and the amount of area classified as severely degraded increased sharply from 2000. The cause of degraded steppe in northwestern Tibet may be the result of warming and an arid climate; the cause of severe degradation in mid and eastern regions was mainly from overgrazing. Three restoration models are proposed for different levels of degradation: “enclosures” for slightly degraded areas, “enclosures with fertilization” for moderately degraded areas, and “enclosure with oversowing and fertilization” for severely degraded areas.

Satellite-Based Estimation of Gross Primary Production in an Alpine Swamp Meadow on the Tibetan Plateau: A Multi-Model Comparison

Abstract: Alpine swamp meadows on the Tibetan Plateau, with the highest soil organic carbon content across the globe, are extremely vulnerable to climate change. To accurately and continually quantify the gross primary production (GPP) is critical for understanding the dynamics of carbon cycles from site-scale to global scale. Eddy covariance technique (EC) provides the best approach to measure the site-specific carbon flux, while satellite-based models can estimate GPP from local, small scale sites to regional and global scales. However, the suitability of most satellite-based models for alpine swamp meadow is unknown. Here we tested the performance of four widely-used models, the MOD17 algorithm (MOD), the vegetation photosynthesis model (VPM), the photosynthetic capacity model (PCM), and the alpine vegetation model (AVM), in providing GPP estimations for a typical alpine swamp meadow as compared to the GPP estimations provided by EC-derived GPP. Our results indicated that all these models provided good descriptions of the intra-annual GPP patterns (R2>0.89, P<0.0001), but hardly agreed with the inter-annual GPP trends. VPM strongly underestimated the GPP of alpine swamp meadow, only accounting for 54.0% of GPP_EC. However, the other three satellite-based GPP models could serve as alternative tools for tower-based GPP observation. GPP estimated from AVM captured 94.5% of daily GPP_EC with the lowest average RMSE of 1.47 g C m-2. PCM slightly overestimated GPP by 12.0% while MODR slightly underestimated by 8.1% GPP compared to the daily GPP_EC. Our results suggested that GPP estimations for this alpine swamp meadow using AVM were superior to GPP estimations using the other relatively complex models.

Response of canopy quantum yield of alpine meadow to temperature under low atmospheric pressure on Tibetan Plateau

An open-path eddy covariance system was set up in Damxung rangeland station to measure the carbon flux from July to October, 2003. The canopy quantum yield (α) of alpine meadow was calculated by the linear function between the net ecosystem carbon dioxide exchange (NEE) and the photosynthetic active radiation (PAR) under low light, and how it was influenced by the temperature was also discussed. Results showed that the canopy α decreased almost linearly with temperature, with the decrease in every 1 °C increase of temperature similar to those measured on leaf level of C3 plant. At the beginning, the decrease of canopy α with temperature was 0.0005 μmol CO2·μmol−1 PAR; while it increased to 0.0008 μmol CO2·μmol−1 PAR in September, showing a rising trend with plant growing stages. Compared with the canopy α calculated with rectangular hyperbola function, the value in the paper was lower. However, the method advanced here has the advantages in examining the relationship between α and the key environmental factors, such as temperature.

Characteristics of net ecosystem carbon dioxide exchange (NEE) from August to October of Alpine meadow on the Tibetan Plateau, China

The Alpine meadow is one of the vegetation types widely distributed on the Tibetan Plateau in China with an area of about 1.2 million square kilometers. The Damxung rangeland station, located in the hinterland of the Tibetan Plateau, is covered with an typical vegetation. The continuous carbon flux data (from August to middle October, 2003) measured with the open-path eddy covariance system was used to analyze the diurnal variation pattern of net ecosystem carbon dioxide exchange (NEE) and its relationship with the environmental factors, such as photosynthetically active radiation (PAR), precipitation, and temperature. Results showed that NEE presented obvious diurnal variation pattern with single-peak of diurnal maximum carbon assimilation at 11: 00–12: 00 (local time) with an average of −0.268 mg CO2·m−2·s−1, i.e., −6.08 μmol CO2·m−2·s−1. During the daytime, NEE fitted fairly well with PAR in a rectangular hyperbola function with the apparent quantum yield (0.020 3 μmol CO2 μmol−1 PAR) and maximum ecosystem assimilation (9.741 1 μmol CO2·m−2·s−1). During the night-time, NEE showed a good exponential relation with the soil temperature at 5 cm depth.

Livestock Dynamic Responses to Climate Change in Alpine Grasslands on the Northern Tibetan Plateau: Forage Consumption and Time-Lag Effects

Abstract: Climate change and forage-intake are important components of livestock population systems, but our knowledge about the effects of changes in these properties on livestock is limited, particularly on the Northern Tibetan Plateau. Based on corresponding independent models (CASA and TEM), a human-induced NPP (NPPH) value and forage-intake threshold were obtained to determine their influences on livestock population fluctuation and regrowth on the plateau. The intake threshold value provided compatible results with livestock population performance. If the forage-intake was greater than the critical value of 1.9 (kg DM d-1 sheep-1), the livestock population increased; otherwise, the livestock population decreased. It takes four years to transfer a disturbance in primary productivity to the next trophic level. The relationships between livestock population and NPPH value determined population dynamics via the forage-intake value threshold. Improved knowledge on lag effects will advance our understanding of drivers of climatic changes on livestock population dynamics.

Biophysical Regulation of Carbon Flux in Different Rainfall Regime in a Northern Tibetan Alpine Meadow

Abstract: Inter-annual variability in total precipitation can lead to significant changes in carbon flux. In this study, we used the eddy covariance (EC) technique to measure the net CO2 ecosystem exchange (NEE) of an alpine meadow in the northern Tibetan Plateau. In 2005 the meadow had precipitation of 489.9 mm and in 2006 precipitation of 241.1 mm, which, respectively, represent normal and dry years as compared to the mean annual precipitation of 476 mm. The EC measured NEE was 87.70 g C m-2 yr-1 in 2006 and –2.35 g C m-2 yr-1 in 2005. Therefore, the grassland was carbon neutral to the atmosphere in the normal year, while it was a carbon source in the dry year, indicating this ecosystem will become a CO2 source if climate warming results in more drought conditions. The drought conditions in the dry year limited gross ecosystem CO2 exchange (GEE), leaf area index (LAI) and the duration of ecosystem carbon uptake. During the peak of growing season the maximum daily rate of NEE and Pmax and α were approximately 30%–50% of those of the normal year. GEE and NEE were strongly related to photosynthetically active radiation (PAR) on half-hourly scale, but this relationship was confounded by air temperature (Ta), soil water content (SWC) and vapor pressure deficit (VPD). The absolute values of NEE declined with higher Ta, higher VPD and lower SWC conditions. Beyond the appropriate range of PAR, high solar radiation exacerbated soil water conditions and thus reduced daytime NEE. Optimal Ta and VPD for maximum daytime NEE were 12.7°C and 0.42 KPa respectively, and the absolute values of NEE increased with SWC. Variation in LAI explained around 77% of the change in GEE and NEE. Variations in Re were mainly controlled by soil temperature (Ts), whereas soil water content regulated the responses of Re to Ts.

Impact of Water Vapor on Elevation-Dependent Climate Change

Abstract: Elevation dependency amongst climate change signals has been found in major mountain ranges around the world, but the main factors causing this dependency have not been clarified. In this study, four different datasets of observation and reanalysis for China were used to examine the elevation dependency of climate change. A lack of consistency was found in dependency between warming magnitude and elevation across the Tibetan Plateau and China. However, a dependency of climate change on water vapor was detected whereby the temperature trend initially increased at low specific humidity, and then decreased as specific humidity increased. At ground level the maximum trend in temperature appeared in the specific humidity range 2.0–3.0 g kg-1. This suggests that water vapor is a mediator of climate change and may be responsible for elevation-dependent climate change.

Declining Precipitation Enhances the Effect of Warming on Phenological Variation in a Semiarid Tibetan Meadow Steppe

Abstract: Vegetation phenology is a sensitive indicator of global warming, especially on the Tibetan Plateau. However, whether climate warming has enhanced the advance of grassland phenology since 2000 remains debated and little is known about the warming effect on semiarid grassland phenology and interactions with early growing season precipitation. In this study, we extracted phenological changes from average NDVI in the growing season (GNDVI) to analyze the relationship between changes in NDVI, phenology and climate in the Northern Tibetan Damxung grassland from 2000 to 2014. The GNDVI of the grassland declined. Interannual variation of GNDVI was mainly affected by mean temperature from late May to July and precipitation from April to August. The length of the growing season was significantly shortened due to a delay in the beginning of the growing season and no advancement of the end of the growing season, largely caused by climate warming and enhanced by decreasing precipitation in spring. Water availability was the major determinant of grass growth in the study area. Warming increased demand for water when the growth limitation of temperature to grass was exceeded in the growing season. Decreased precipitation likely further exacerbated the effect of warming on vegetation phenology in recent decades due to increasing evapotranspiration and water limitations. The comprehensive effects of global warming and decreasing precipitation may delay the phenological responses of semiarid alpine grasslands.

The Rangeland Livestock Carrying Capacity and Stocking Rate in the Kailash Sacred Landscape in China

Abstract: Maintaining the health and productivity of rangelands by controlling the livestock stocking rate to remain within carrying capacity is of significance to ensure sustainable management of rangeland ecosystems. But we know little about the safe carrying capacity in particular rangeland landscapes. This has hampered efforts to use rangelands in a risk-averse manner in fluctuating rainfall environments, and especially in arid and semiarid areas. To address this lack of information, we took Kailash Sacred Landscape in China (KSL-China) as our study site and used remote sensing data, meteorological data and statistical data from 2000 to 2015 to analyze rangeland carrying capacity, stocking rate, and major influencing factors. Rangeland carrying capacity presented an increasing trend, while stocking rate was gradually decreasing, resulting in an increase of carrying rate in the study area. The increased carrying capacity was closely related to increased rainfall. Stocking rate declined owing to government regulations, particularly implementation in 2004 of the national policy of Returning Grazing Land to Grassland. There was a sharp reduction of livestock number below 200 000 standard sheep units (SU) after 2005. The decrease of stocking rate had a stronger effect on rangeland carrying rate than did the increase of carrying capacity. Ecosystem restoration programs have provided subsidies to pastoralists to encourage them to reduce livestock numbers. Our findings suggest that a safe rangeland carrying capacity is ca. 170 000 SU in KSL-China. There is a carrying capacity surplus of ca. 50 000 SU for safe animal husbandry development in the study area. More importantly, future climate warming and increases in grazing may jointly play a key role in affecting rangeland carrying capacity.

Impact of Drought Stress on Net CO2 Exchange above an Alpine Grassland Ecosystem in the Central Tibetan Plateau

Abstract: Drought may impact the net ecosystem exchange of CO2 (NEE) between grassland ecosystems and the atmosphere during growth seasons. Here, carbon dioxide exchange and controlling factors in alpine grassland under drought stress in the hinterland of Tibetan Plateau (Damxung, Tibet, China) were investigated. Data were obtained using the covariance eddy technique in 2009. Severe drought stress appeared in the early growing season (May to early July) and September. Drought conditions during the early growing season limited grass production and the green leaf area index (GLAI) increased slowly, with an obvious decline in June. When encountering severe water stress, diurnal patterns of NEE in the growth season altered with a peak carbon release around 16:00 h or a second carbon uptake period before sunset. NEE variations in daytime related most closely with &thgr; other than PAR when daily averaged &thgr;<0.1 m3 m-3 . Seasonal patterns of gross primary production (GPP) and NEE were also influenced by drought: the maximum and minimum of daily-integrated NEE were 0.9 g C m-2 d-1 on 3 July 2009, and -1.3 g C m-2 d-1 on 12 August 2009 with a GPP peak (-2.3 g C m-2 d-1) on the same day, respectively. Monthly NEE from May to July remained as carbon release and increased gradually; peak values of monthly NEE and GPP both appeared in August, but that of ecosystem respiration (Reco) was reached in July. Annual NEE, GPP and Reco of the alpine grassland ecosystem were 52.4, -158.1 and 210.5 g C m-2, respectively. Therefore, the grassland was a moderate source of CO2 to the atmosphere in this dry year. Interannual variation in NEE was likely related to different water conditions in the growing season. The three greatest contributors to seasonal variation in NEE, GPP and Reco respectively were GLAI>Ta>&thgr;, GLAI>&thgr;>PPT, and Ta>GLAI>PAR. Seasonality of GLAI explained 60.7% and 76.1% of seasonal variation in NEE and GPP, respectively. GPP or NEE was more sensitive than Reco to variation in GLAI, and ecosystem water conditions.