Junker Laura Verena, Ensminger Ingo
Department of Biology, University of Toronto, 3359 Mississauga Road, Mississauga, L5L1C6 ON, Canada Graduate Department of Cell and Systems Biology, University of Toronto, Toronto M5S 3G5, ON, Canada
Department of Biology, University of Toronto, 3359 Mississauga Road, Mississauga, L5L1C6 ON, Canada Graduate Department of Cell and Systems Biology, University of Toronto, Toronto M5S 3G5, ON, Canada Graduate Department of Ecology and Evolutionary Biology, University of Toronto, Toronto M5S 1A1, ON, Canada.
Tree Physiol. 2016 Jun;36(6):694-711. doi: 10.1093/treephys/tpv148. Epub 2016 Feb 29.
The ability of plants to sequester carbon is highly variable over the course of the year and reflects seasonal variation in photosynthetic efficiency. This seasonal variation is most prominent during autumn, when leaves of deciduous tree species such as sugar maple (Acer saccharum Marsh.) undergo senescence, which is associated with downregulation of photosynthesis and a change of leaf color. The remote sensing of leaf color by spectral reflectance measurements and digital repeat images is increasingly used to improve models of growing season length and seasonal variation in carbon sequestration. Vegetation indices derived from spectral reflectance measurements and digital repeat images might not adequately reflect photosynthetic efficiency of red-senescing tree species during autumn due to the changes in foliar pigment content associated with autumn phenology. In this study, we aimed to assess how effectively several widely used vegetation indices capture autumn phenology and reflect the changes in physiology and photosynthetic pigments during autumn. Chlorophyll fluorescence and pigment content of green, yellow, orange and red leaves were measured to represent leaf senescence during autumn and used as a reference to validate and compare vegetation indices derived from leaf-level spectral reflectance measurements and color analysis of digital images. Vegetation indices varied in their suitability to track the decrease of photosynthetic efficiency and chlorophyll content despite increasing anthocyanin content. Commonly used spectral reflectance indices such as the normalized difference vegetation index and photochemical reflectance index showed major constraints arising from a limited representation of gradual decreases in chlorophyll content and an influence of high foliar anthocyanin levels. The excess green index and green-red vegetation index were more suitable to assess the process of senescence. Similarly, digital image analysis revealed that vegetation indices such as Hue and normalized difference index are superior compared with the often-used green chromatic coordinate. We conclude that indices based on red and green color information generally represent autumn phenology most efficiently.
植物固碳能力在一年中变化很大，反映了光合效率的季节性变化。这种季节性变化在秋季最为显著，此时糖枫（Acer saccharum Marsh.）等落叶树种的叶子会衰老，这与光合作用下调和叶片颜色变化有关。通过光谱反射测量和数字重复图像对叶片颜色进行遥感，越来越多地用于改进生长季节长度模型和碳固存的季节性变化模型。由于与秋季物候相关的叶片色素含量变化，从光谱反射测量和数字重复图像得出的植被指数可能无法充分反映秋季红衰树物种的光合效率。在本研究中，我们旨在评估几种广泛使用的植被指数在捕捉秋季物候以及反映秋季生理和光合色素变化方面的有效性。测量了绿色、黄色、橙色和红色叶片的叶绿素荧光和色素含量，以代表秋季叶片衰老，并用作参考来验证和比较从叶片水平光谱反射测量和数字图像颜色分析得出的植被指数。尽管花青素含量增加，但植被指数在跟踪光合效率和叶绿素含量下降的适用性方面存在差异。常用的光谱反射指数，如归一化差异植被指数和光化学反射指数，由于叶绿素含量逐渐下降的表示有限以及高叶片花青素水平的影响，存在主要限制。过量绿度指数和绿红植被指数更适合评估衰老过程。同样，数字图像分析表明，诸如色调和归一化差异指数等植被指数比常用的绿色色度坐标更优越。我们得出结论，基于红色和绿色颜色信息的指数通常最有效地代表秋季物候。
