Shen Zhenxi

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.

Species-area relationship within and across functional groups at alpine grasslands on the northern Tibetan Plateau, China

The species-area relationship (SAR) is one of the most fundamental concepts in community ecology and is helpful for biodiversity conservation. However, few studies have systematically addressed this topic for different alpine grassland types on the Tibetan Plateau, China. We explored whether the plant composition of different functional groups affects the manner in which species richness increases with increasing area at scales ≤ 1.0 m2. We also compared species richness (S) within and across forbs, legumes, sedges and grasses, with sampling subplot area (A) increasing from 0.0625 m2 to 1.0 m2 between alpine meadow and steppe communities. We applied a logarithmic function (S = b0 + b1 ln A) to determine the slope and intercept of SAR curves within and across functional groups. The results showed that the logarithmic relationship holds true between species richness and sampling area at these small scales. Both the intercept and slope of the logarithmic forbs-area curves are significantly higher than those for the three other functional groups (P < 0.05). Forb accounts for about 91.9 % of the variation in the intercept and 75.0% of the variation in the slope of the SAR curve when all functional groups’ data were pooled together. Our results indicated that the different SAR patterns should be linked with species dispersal capabilities, environmental filtering, and life form composition within alpine grassland communities. Further studies on the relationship between species diversity and ecosystem functions should specify the differential responses of different functional groups to variations in climate and anthropogenic disturbances.

Modeling Aboveground Biomass Using MODIS Images and Climatic Data in Grasslands on the Tibetan Plateau

Abstract: Accurate quantification of aboveground biomass of grasslands in alpine regions plays an important role in accurate quantification of global carbon cycling. The monthly normalized difference vegetation index (NDVI), enhanced vegetation index (EVI), mean air temperature (Ta), ≥5°C accumulated air temperature (AccT), total precipitation (TP), and the ratio of TP to AccT (TP/AccT) were used to model aboveground biomass (AGB) in grasslands on the Tibetan Plateau. Three stepwise multiple regression methods, including stepwise multiple regression of AGB with NDVI and EVI, stepwise multiple regression of AGB with Ta, AccT, TP and TP/AccT, and stepwise multiple regression of AGB with NDVI, EVI, Ta, AccT, TP and TP/AccT were compared. The mean absolute error (MAE) and root mean squared error (RMSE) values between estimated AGB by the NDVI and measured AGB were 31.05 g m-2 and 44.12 g m-2, and 95.43 g m-2 and 131.58 g m-2 in the meadow and steppe, respectively. The MAE and RMSE values between estimated AGB by the AccT and measured AGB were 33.61g m-2 and 48.04 g m-2 in the steppe, respectively. The MAE and RMSE values between estimated AGB by the vegetation index and climatic data and measured AGB were 28.09 g m-2 and 42.71 g m-2, and 35.86 g m-2 and 47.94 g m-2, in the meadow and steppe, respectively. The study finds that a combination of vegetation index and climatic data can improve the accuracy of estimates of AGB that are arrived at using the vegetation index or climatic data. The accuracy of estimates varied depending on the type of grassland.

Estimation of Daily Vapor Pressure Deficit Using MODIS Potential Evapotranspiration on the Tibetan Plateau

Abstract: Vapor pressure deficit (VPD) is an important parameter in modelling hydrologic cycles and vegetation productivity. Meteorological stations are scarce in remote areas, which often results in imprecise estimations of VPD on the Tibetan Plateau. Moderate Resolution Imaging Spectroradiometer (MODIS) provides evapotranspiration data, which may offer the possibility of scaling up VPD estimations on the Tibetan Plateau. However, no studies thus far have estimated VPD using MODIS evapotranspiration data on the Tibetan Plateau. Therefore, this study used MODIS potential evapotranspiration (PET) to estimate VPD in alpine meadows, alpine steppes, croplands, forests and shrublands for the year, spring, summer, autumn and winter in 2000–2012. A series of root-meansquared-error (RMSE) and mean-absolute-error (MAE) values were obtained for correlating measured VPD and estimated VPD using MODIS PET data for each listed time period and vegetation type: whole year (0.98–2.15 hPa and 0.68–1.44 hPa), spring (0.95–2.34 hPa and 0.72–1.54 hPa), summer (1.39–2.60 hPa and 0.89–1.96 hPa), autumn (0.78–1.93 hPa and 0.56–1.36 hPa), winter (0.48–1.40 hPa and 0.36–0.98 hPa), alpine steppes (0.48–1.39 hPa and 0.36–1.00 hPa), alpine meadows (0.58–1.39 hPa and 0.44–0.90 hPa), croplands (1.10–2.55 hPa and 0.82–1.74 hPa), shrublands (0.98–1.90 hPa and 0.78–1.37 hPa), and forests (1.40–2.60 hPa and 0.98–1.96 hPa), respectively. Therefore, MODIS PET may be used to estimate VPD, and better results may be obtained if future studies incorporate vegetation types and seasons when the VPD data are estimated using MODIS PET on the Tibetan Plateau.

The Effect of Higher Warming on Vegetation Indices and Biomass Production is Dampened by Greater Drying in an Alpine Meadow on the Northern Tibetan Plateau

Abstract: In order to understand whether or not the response of vegetation indices and biomass production to warming varies with warming magnitude, an experiment of field warming at two magnitudes was conducted in an alpine meadow on the northern Tibetan Plateau beginning in late June, 2013. The normalized difference vegetation index (NDVI), green normalized difference vegetation index (GNDVI) and soil adjusted vegetation index (SAVI) data were obtained using a Tetracam Agricultural Digital Camera in 2013–2014. The gross primary production (GPP) and aboveground plant biomass (AGB) were modeled using the surface measured NDVI and climatic data during the growing seasons (i.e. June–September) in 2013–2014. Both low and high warming significantly increased air temperature by 1.54 and 4.00°C, respectively, and significantly increased vapor pressure deficit by 0.13 and 0.31 kPa, respectively, in 2013–2014. There were no significant differences of GNDVI, AGB and ANPP among the three warming treatments. The high warming significantly reduced average NDVI by 23.3% (–0.06), while the low warming did not affect average NDVI. The low and high warming significantly decreased average SAVI by 19.0% (–0.04) and 27.4% (–0.05), respectively, and average GPP by 24.2% (i.e. 0.21 g C m-2 d-1) and 44.0% (i.e. 0.39 g C m-2 d-1), respectively. However, the differences of the average NDVI, SAVI, and GPP between low and high warming were negligible. Our findings suggest that a greater drying may dampen the effect of a higher warming on vegetation indices and biomass production in alpine meadow on the northern Tibetan Plateau.