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森林物候在低成本定时拍摄相机监测的差异性研究

Research on Differences in Forest Phenology Monitoring with Low Cost Timing Cameras

  • 摘要: 为研究森林物候的差异性,以地面凋落叶为参考,使用3种类型数码相机(美国Stardot生产的NetCam、日本尼康生产的Coolpix和中国猎科生产的Ltl-5610)评估颜色指数时间序列和物候期与凋落叶的差异;评估数码相机的白平衡模式设置(晴天、阴天、自动)对采集图像的颜色指数和物候期提取的影响;对比设置为固定(或晴天)白平衡模式的NetCam、Coolpix和Ltl-5610之间以及Ltl-5610与华为智能手机之间观测冠层物候的差异。研究结果表明,1)3种相机的绿度指数均与凋落叶季节动态显著相关,且相机与凋落叶估计的物候期没有显著差异;但红度指数不能表征凋落叶动态,其估计的变色高峰日比凋落叶的凋落高峰日晚6~31 d; 2)同一台华为智能手机在不同白平衡模式下的绿度指数估计的秋季物候期(叶衰老开始(start of fall, SOF)、落叶高峰(middle of fall, MOF)、落叶结束(end of fall, EOF))差异不显著,但固定或晴天模式估计值的不确定性更低;3)3种数码相机间绿度指数季节变化基本一致,6个关键物候期(展叶开始(start of spring, SOS)、展叶高峰(middle of spring, MOS)、展叶结束(peak of spring, POS)、SOF、MOF和EOF)和2种生长季长度(MOS-MOF和SOS-EOF)普遍差异不显著。MOS和MOF最为稳定,适合用于定义生长季的开始和结束。此研究结果证明低成本国产定时拍摄相机绿度指数监测森林物候的可行性,有助于实现更大覆盖面的物候联网观测,但使用红度指数表征秋季物候需慎重。

     

    Abstract: In order to study the difference of forest phenophases, using the leaf-litter as the reference, three types of digital cameras(Stardot NetCam made in America, Nikon Coolpix made in Japan, and Ltl acorn Ltl-5610 made in China) were used to evaluate the differences in the time series of color indices and phenophases. The influence of white balance mode of camera(sunny, cloudy, auto) on color index and phenology extraction of captured images were evaluated. Compared the differences in observed canopy phenology between NetCam, Coolpix and Ltl-5610 set in fixed(or sunny) white balance mode, and between Lti-5610 and Huawei smartphone. The results showed that: 1) the seasonal variations in greenness indices by all the three cameras tightly correlated with that of leaf-litter, and their phenophases had no significant differences. However, the redness cannot track the dynamics of leaf-litter, and the estimated peaks of color were later than the leaf fall peak by 6~31 d. 2) There was no significant difference in the estimated autumn phenophases of the greenness index of the same Huawei smartphone under different white balance modes(start of fall SOF, middle of fall MOF, and end of fall EOF). 3) The seasonal variations in greenness index of 3 cameras were consistent approximately. There were no significant differences in the six phenophases(start of spring SOS, middle of spring MOS, peak of spring POS, SOF, MOF, and EOF) and two length of growing seasons(the duration from the MOS to MOF, and from SOS to EOF). The MOS and MOF were mostly stable and suitable to define as the start and end of growing season. This study demonstrates that the greenness index of time-lapse camera is able to monitor vegetation phenology and contributes to expand the global phenology network, while the redness to track the autumn phenology should be carefully used.

     

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