He Yongtao

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.

Study on the characteristics of precipitation resources and the afforestation suitability in the Loess Plateau.

Located in the arid and semi-arid regions,the Loess Plateau is a fragile region with low precipitation and intensive evaporation.Therefore,water is the most important limiting factor,and how to restore suitable vegetation in different regions based on the precipitation is a foundation to the ecological conservation and regeneration in the plateau.Based on the 40-year(1961-2000) climate data observed by 68 meteorological stations in the Loess Plateau,the precipitation characteristics in the plateau are analyzed.The results show that precipitation is distinctly different from the different areas,the average annual precipitation decreases from 600 mm in the south to 200 mm in the north of the plateau;80% annual precipitation occurs in plant growing season,so it is drought in spring and autumn but rainy in summer.According to the water balance of the forest ecosystem in growing season,the afforestation suitability is calculated using precipitation and ecological water requirement of woodlands.On which four different regions for afforestation are divided by regarding the counties in the Loess Plateau as the units.The results reveal that the area suitable to plant trees is about one quarter of the whole plateau,and another one quarter of the plateau are suitable to plant shrubs.

Estimated Forest Ecological Water Requirements in the Jinghe Watershed—Theory and Case Study

The ecological water requirement of forests is defined as the water resources used to maintain and improve the natural balance of forest ecosystems, which can be expressed by evapotranspiration of trees during the growing season. The relationship of evapotranspiration and soil moisture of forestland with tree growth showed that, if the soil moisture was above the temporary wilting point or the point of growth retardation, the growth of trees can, respectively, be basically or normally sustained. Therefore, they can be taken as the minimum and the suitable ecological water requirements of the forest. These points can be estimated by introducing the soil factor (Ks) and tree species factor (Kt) to potential evapotranspiration with the Penman formula. With geographic information system (GIS), the ecological water requirement for forests in the Jinghe watershed, western China was estimated. The results revealed that the minimum and suitable ecological water requirements of the forests in the Jinghe watershed were approximately 204×107 and 340×107 m3, respectively.

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.