Song Gu

Estimating aboveground biomass of grassland having a high canopy cover: an exploratory analysis of in situ hyperspectral data

To improve the estimation of aboveground biomass of grassland having a high canopy cover based on remotely sensed data, we measured in situ hyperspectral reflectance and the aboveground green biomass of 42 quadrats in an alpine meadow ecosystem on the Qinghai–Tibetan Plateau. We examined the relationship between aboveground green biomass and the spectral features of original reflectance, first-order derivative reflectance (FDR), and band-depth indices by partial least squares (PLS) regression, as well as the relationship between the aboveground biomass and narrow-band vegetation indices by linear and nonlinear regression analyses. The major findings are as follows. (1) The effective portions of spectra for estimating aboveground biomass of a high-cover meadow were within the red-edge and near infrared (NIR) regions. (2) The band-depth ratio (BDR) feature, using NIR region bands (760–950 nm) in combination with the red-edge bands, yields the best predictive accuracy (RMSE = 40.0 g m−2) for estimating biomass among all the spectral features used as independent variables in the partial least squares regression method. (3) The ratio vegetation index (RVI2) and the normalized difference vegetation index (NDVI2) proposed by Mutanga and Skidmore (Mutanga, O. and Skidmore, A.K., 2004a, Narrow band vegetation indices solve the saturation problem in biomass estimation. International Journal of Remote Sensing, 25, pp. 1–6) are better correlated to the aboveground biomass than other VIs (R 2 = 0.27 for NDVI2 and 0.26 for RVI2), while RDVI, TVI and MTV1 predicted biomass with higher accuracy (RMSE = 37.2 g m−2, 39.9 g m−2 and 39.8 g m−2, respectively). Although all of the models developed in this study are probably acceptable, the models developed in this study still have low accuracy, indicating the urgent need for further efforts.

Short‐term variation of CO2 flux in relation to environmental controls in an alpine meadow on the Qinghai‐Tibetan Plateau

[1] The alpine meadow ecosystem on the Qinghai-Tibetan Plateau may play a significant role in the regional carbon cycle. To assess the CO2 flux and its relationship to environmental controls in the ecosystem, eddy covariance of CO2, H2O, and energy fluxes was measured with an open-path system in an alpine meadow on the plateau at an elevation of 3,250 m. Net ecosystem CO2 influx (Fc) averaged 8.8 g m(-2) day(-1) during the period from August 9 to 31, 2001, with a maximum of 15.9 g m(-2) day(-1) and a minimum of 2.3 g m(-2) day(-1). Daytime Fc averaged 16.7 g m(-2) day(-1) and ranged from 10.4 g m(-2) day(-1) to 21.7 g m(-2) day(-1) during the study period. For the same photosynthetic photon flux density (PPFD), gross CO2 uptake (Gc) was significantly higher on cloudy days than on clear days. However, mean daily Gc was higher on clear days than on cloudy days. With high PPFD, Fc decreased as air temperature increased from 10degreesC to 23degreesC. The greater the difference between daytime and nighttime air temperatures, the more the sink was strengthened. Daytime average water use efficiency of the ecosystem (WUEe) was 8.7 mg (CO2)(g H2O)(-1); WUEe values ranged from 5.8 to 15.3 mg (CO2)(g H2O)(-1). WUEe increased with the decrease in vapor pressure deficit. Daily albedo averaged 0.20, ranging from 0.19 to 0.22 during the study period, and was negatively correlated with daily Fc. Our measurements provided some of the first evidence on CO2 exchange for a temperate alpine meadow ecosystem on the Qinghai-Tibetan Plateau, which is necessary for assessing the carbon budget and carbon cycle processes for temperate grassland ecosystems.

Characterizing evapotranspiration over a meadow ecosystem on the Qinghai‐Tibetan Plateau

To characterize evapotranspiration (ET) over grasslands on the Qinghai-Tibetan Plateau, we examined ET and its relevant environmental variables in a Kobresia meadow from 2002 to 2004 using the eddy covariance method. The annual precipitation changed greatly, with 554, 706, and 666 mm a(-1) for the three consecutive calendar years. The annual ET varied correspondingly to the annual precipitation with 341, 407, and 426 mm a(-1). The annual ET was, however, constant at about 60% of the annual precipitation. About 85% annual ET occurred during the growing season from May to September, and the averaged ET for this period was 1.90, 2.23, and 2.22 mm/d, respectively for the three consecutive years. The averaged ET was, however, very low (< 0.40 mm/d) during the nongrowing season from October to April. The annual canopy conductance (gc) and the Priestley-Taylor coefficient (a) showed the lowest values in the year with the lowest precipitation. This study first demonstrates that the alpine meadow ecosystem is characterized by a low ratio of annual ET to precipitation and that the interannual variation of ET is determined by annual precipitation.

High positive correlation between soil temperature and NDVI from 1982 to 2006 in alpine meadow of the Three-River Source Region on the Qinghai-Tibetan Plateau

Using satellite-observed Normalized Difference Vegetation Index (NDVI) data and Rotated Empirical Orthogonal Function (REOF) method, we analyzed the spatio-temporal variation of vegetation during growing seasons from May to September in the Three-River Source Region, alpine meadow in the Qinghai-Tibetan Plateau from 1982 to 2006. We found that NDVI in the centre and east of the region, where the vegetation cover is low, showed a consistent but slight increase before 2003 and remarkable increase in 2004 and 2005. Impact factors analysis indicted that among air temperature, precipitation, humid index, soil surface temperature, and soil temperature at 10 cm and 20 cm depth, annual variation of NDVI was highly positive correlated with the soil surface temperature of the period from March to July. Further analysis revealed that the correlation between the vegetation and temperature was insignificant before 1995, but statistically significant from 1995. The study indicates that temperature is the major controlling factor of vegetation change in the Three-River Source Region, and the currently increase of temperature may increase vegetation coverage and/or density in the area. In addition, ecological restoration project started from 2005 in Three-River Source Region has a certain role in promoting the recovery of vegetation

Photosynthetic depression in relation to plant architecture in two alpine herbaceous species

The Qinghai-Tibet Plateau is characterized by extremely high radiation, which may induce down-regulation of photosynthesis in plants living in this alpine ecosystem. To clarify whether photoinhibition occurs in the alpine environment and to discern its underlying mechanisms, we examined photosynthetic gas exchange and fluorescence emission in response to the changes in photosynthetic photon flux density (PPFD) and leaf temperature under natural regimes for two herbaceous species: prostrate Saussurea superba and erect-leaved Saussurea katochaete from altitude 3250 m on the Qinghai-Tibet Plateau. S. superba intercepted a higher maximum PPFD and experienced much higher leaf temperature than the erect-leaved S. katochaete. S. superba exhibited a much higher light saturation point for photosynthesis than S. katochaete. Under controlled conditions, the former species had higher CO2 uptake rates and neither species showed obvious photosynthetic down-regulation at high PPFD. Under natural environmental conditions, however, apparent photoinhibition, indicated by reduced electron transport rate (ETR), was evident at high PPFD for both species. After a night frost, the photochemistry of S. katochaete was depressed markedly in the early morning and recovered by mid-day. After a frost-free night, it was high in the morning and low at noon due to high radiation. S. superba did not respond to the night frost in terms of daytime photochemical pattern. In both species, photochemical depression was aggravated by high leaf temperature and the erect species was more sensitive to high temperature. This study suggests that the high radiation on the Qinghai-Tibet Plateau is likely to induce rapidly reversible photoinhibition, which is related closely to plant architecture. Photochemistry in the prostrate species seems able to tolerate higher PPFD, without obvious suppression, than the erect species

Radiation partitioning and its relation to environmental factors above a meadow ecosystem on the Qinghai‐Tibetan Plateau

Understanding the energy balance on the Qinghai-Tibetan Plateau is essential for better prediction of global climate change. To characterize the energy balance on the plateau, we examined the radiation partitioning over a Kobresia meadow, the most widely distributed vegetation on the plateau, for the period from 2002 to 2005. The incident solar radiation (R(s)) and net radiation (R(n)) averaged 6298 and 2779 MJ m(-2) yr(-1), respectively. The albedo averaged 0.220 annually, with a slightly low value of 0.202 in the growing season from May to September. An increase in soil water or leaf area index was correlated with a decrease of albedo over the meadow. The annual solar radiation lost 34% as longwave radiation, which was higher than values reported for lowland grasslands. The annual radiation efficiency (R(n)/R(s)) over the meadow, at an average of 0.44, was, however, much lower than that for lowland grasslands. The net longwave radiation (L(n)) and the normalized effective radiation (L(n)/R(s)) over the meadow were much higher than that for the global surface or for lowland grasslands, indicating that the longwave exchange between alpine meadow and atmosphere is the most important component of energy losses. A path analysis suggests that the water vapor pressure, air temperature, and cloud cover are the major factors governing the variations of both the net radiation and the net longwave radiation in the alpine meadow ecosystem.

Leaf Orientation, Incident Sunlight, and Photosynthesis in the Alpine Species Suassurea superba and Gentiana straminea on the Qinghai-Tibet Plateau

Abstract The extremely high level of solar radiation on the Qinghai-Tibet Plateau may induce photoinhibition and thus limit leaf carbon gain. To assess the effect of high light, we examined gas exchange and chlorophyll fluorescence for two species differing in light interception: the prostrate Saussurea superba and the erect-leaved Gentiana straminea. In controlled conditions with favorable water and temperature, neither species showed apparent photoinhibition in gas exchange measurements. In natural environment, however, their photosynthetic rate decreased remarkably at high light. Photosynthesis depression was aggravated under high leaf temperature or soil water stress. Relative stomatal limitation was much higher in S. superba than in G. straminea and it remarkably increased in the later species at midday when soil was dry. Fv/Fm as an indicator for photoinhibition was generally higher in S. superba than in the other species. Fv/Fm decreased significantly under high light at midday in both species, even when soil moisture was high. F0 linearly elevated with the increment of leaf temperature in G. straminea, but remained almost constant in S. superba. Electron transport rate (ETR) increased with photosynthetically active photon flux density (PPFD) in S. superba, but declined when PPFD was high than about 1000 μmol m−2 s−1 in G. straminea. Compared to favorable environment, the estimated daily leaf carbon gain at PPFD above 800 μmol m−2 s−1 was reduced by 32% in S. superba and by 17% in G. straminea when soil was moist, and by 43% and 53%, respectively, when soil was dry. Our results suggest that the high radiation induces photoinhibition and significantly limits photosynthetic carbon gain, and the limitation may further increase at higher temperature and in dry soil.