Rain-use efficiency (RUE) acts as a typical indicator of ecosystem function. Land surface phenology (LSP) assesses the vegetation activity during the growing season at the ecosystem level. The Songnen Plain (SNP) is located in semi-humid to semi-arid transition ecological fragile zone in Northeast China. RUE in growing season (May-September) was calculated using time series GIMMS NDVI3g images and precipitation data for the period of 1983-2012. The phenology metrics including the start (SOS) and end (EOS) dates of growing season for each year was extracted. Spatial trends of RUE and LSP were examined by applying a linear regression model with time. The correlation analysis was used to analyze the effects of RUE on LSP. The results showed that RUE increased slightly with an undulating trend. Spatially, the highest positive slopes indicating increased trend of RUE were observed in northern and eastern forest. The advanced in SOS was mainly distributed in northern forest areas. 12.2% of the landscape experienced highly increase trend in EOS with a rate of 0.38 days per year. The length of growing season (LOS) was prolonged in 14.2% of the total land. EOS dates in the southern salinized grassland and cropland were mainly negatively correlated with RUE. The results of the significance test show that 2.95% of the pixels were significantly and positively correlated with RUE, indicating that an increase in the RUE would delay the EOS. Increasing RUE promoted the extension of the LOS, particularly in the forest areas.
Effectively assessing cadmium (Cd) contamination in crops is crucial for the sustainable development of an agricultural ecosystem and for environmental security. We developed an integrated stress index (SI) based on two phenological metrics to effectively evaluate Cd stress in rice crops. The selected four experimental areas are located in Zhuzhou City, Hunan Province, China. Six Sentinel-2 images were acquired in 2017, and heavy metal concentrations in soil were measured. The change rate of CIre (CRCIre) and the time-integrated CIre (TICIre) were obtained from daily red-edge chlorophyll index (CIre) time-series using Sentinel-2 data. The CRCIre and TICIre were used to characterize the photosynthetic rate and biomass, respectively. SI was calculated by Fisher discriminant analysis based on CRCIre and TICIre from two experimental areas, and it was verified using another two experimental areas. The results were the following: (i) when SI ≥ 0, rice was under mild stress and when SI < 0, rice was under severe stress. (ii) The SI effectively evaluated Cd stress levels with an overall discriminatory accuracy of 86.02%. This research provides a potential new method to evaluate Cd stress in rice by remote sensing through phenology.
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