基于数字相机的冬小麦物候和碳交换监测

利用数字相机自动、连续监测植被冠层物候变化,逐渐引起人们的广泛关注。依托中国陆地生态系统通量观测研究网络(ChinaFLUX),探讨了数字相机在监测冬小麦生长状况及生态系统碳交换方面的作用,得到如下结果:(1)利用数字相机图像提取的比值绿度指数G/R能较好地反映冬小麦冠层物候变化,通过分析比值绿度指数G/R的时间序列,得到了较为准确的冬小麦关键生育日期(与人工观测数据比较,误差<3 d),表明数字相机可以作为物候监测的一种有效手段;(2)数字相机图像获取的比值绿度指数能较好地模拟冬小麦总生态系统碳交换量GEE,R2为0.66,叶片最大光合同化速率与比值绿度指数G/R变化趋势基本一致。表明利用数字相机技术在一定程度上能够表征作物生理生态过程。从而为我国开展不同陆地生态系统自动连续物候监测,深入研究不同生态系统物候和碳循环的关系提供支持。

Using digital repeat photography to model winter wheat phenology and photosynthetic CO2 uptake

Continuous and automatic monitoring of canopy phenology is of increasing scientific interest as a tool to investigate the multiple implications of vegetation dynamics on ecosystem carbon fluxes. For this purpose, we initiated research on winter wheat ecosystem northern China in 2011, belonging to Chinese Terrestrial Ecosystem Flux Observation and Research Network (ChinaFLUX), and evaluated the applicability of digital camera imagery for monitoring and modeling crop phenology and physiology. A standard, commercially available webcam was mounted at the top of the eddy covariance tower at the YuCheng ChinaFLUX site. Images were collected in growing season every 30 min from 9:30 a.m to 5:00 p.m each day. Red, green, and blue color channel brightness data for a region-of-interest (ROI) were extracted from each image (ROI is the subset of image, can better describe the target's characters. The size of ROI for winter wheat is 380*260 pixels). Since a temporal series of original brightness data can't capture the changes at different canopy development stages, we compared different indices (ratio greenness index, excess greenness index, and relative greenness index) composed of different channel brightness data. Ratio greenness index (G/R), can reflect the size of leaf area index and variations in chlorophyll content of winter wheat, proved to be the index best describing the green-up signals of the vegetation, was calculated by dividing green channel brightness by red channel brightness, and extracted from chronically digital images. Phenological date was defined as the date on which the curvature of G/R reaches its peak and compared to field-measured phenological date. The results confirmed that G/R was useful to monitor canopy phenology. We further investigated the relationship between G/R and carbon exchange capacity (i.e., gross ecosystem exchange ) of vegetation using eddy covariance CO₂ flux data. A strong relationship (GEE: r²=0.66) between ratio greenness and photosynthesis was observed for winter wheat. At the same time, we used the daily NEE and PAR data collected from Eddy covariance technique, employed the Michaelis-Menten equationt, and obtained the curve of Amax. The result demonstrated a positive relationship between G/R and Amax, and suggested that G/R is able to reflect the plant phenological activity in physiological level. We concluded that digital camera images not only provide a reliable measure of plant phenology at high tempo-spatial resolutions, but also act as a complementary role of CO₂ flux measurements, and improve our knowledge of ecosystem processes. Digital cameras have been installed at eight typical terrestrial ecosystem sites of ChinaFLUX for phenological observations, constituting of the Chinese Digital Camera Phenology Observation Network. This platform will provide an unprecedented opportunity to obtain an improved understanding of vegetation responses to climate change in China. With high tempo-spatial resolution phenological data, we can bring insights into the process of biologically mediated carbon sources and sinks, and better understand the uncertainties of the role of terrestrial ecosystems in the global carbon cycle.