Remote Sensing of Environmental Dynamics Laboratory, DISAT, University of Milano-Bicocca, Milan, Italy.
ESA-ESRIN Earth Observations, Frascati, Italy.
Glob Chang Biol. 2018 Jul;24(7):2980-2996. doi: 10.1111/gcb.14097. Epub 2018 Mar 25.
Leaf fluorescence can be used to track plant development and stress, and is considered the most direct measurement of photosynthetic activity available from remote sensing techniques. Red and far-red sun-induced chlorophyll fluorescence (SIF) maps were generated from high spatial resolution images collected with the HyPlant airborne spectrometer over even-aged loblolly pine plantations in North Carolina (United States). Canopy fluorescence yield (i.e., the fluorescence flux normalized by the light absorbed) in the red and far-red peaks was computed. This quantifies the fluorescence emission efficiencies that are more directly linked to canopy function compared to SIF radiances. Fluorescence fluxes and yields were investigated in relation to tree age to infer new insights on the potential of those measurements in better describing ecosystem processes. The results showed that red fluorescence yield varies with stand age. Young stands exhibited a nearly twofold higher red fluorescence yield than mature forest plantations, while the far-red fluorescence yield remained constant. We interpreted this finding in a context of photosynthetic stomatal limitation in aging loblolly pine stands. Current and future satellite missions provide global datasets of SIF at coarse spatial resolution, resulting in intrapixel mixture effects, which could be a confounding factor for fluorescence signal interpretation. To mitigate this effect, we propose a surrogate of the fluorescence yield, namely the Canopy Cover Fluorescence Index (CCFI) that accounts for the spatial variability in canopy structure by exploiting the vegetation fractional cover. It was found that spatial aggregation tended to mask the effective relationships, while the CCFI was still able to maintain this link. This study is a first attempt in interpreting the fluorescence variability in aging forest stands and it may open new perspectives in understanding long-term forest dynamics in response to future climatic conditions from remote sensing of SIF.
叶片荧光可用于追踪植物的生长和胁迫状况，被认为是遥感技术中可直接测量光合作用活性的最有效方法。本研究利用 HyPlant 机载高光谱仪，在美国北卡罗来纳州的同龄火炬松人工林上采集高空间分辨率图像，生成红边和远红边太阳诱导叶绿素荧光（SIF）图谱。计算红边和远红边峰值处的冠层荧光产率（即荧光通量与吸收光标准化）。该方法量化了与冠层功能更直接相关的荧光发射效率，与 SIF 辐亮度相比更具优势。研究了荧光通量和产率与树龄的关系，以推断这些测量方法在更好地描述生态系统过程方面的潜力。结果表明，红边荧光产率随林龄变化而变化。幼龄林的红边荧光产率比成熟林高出近两倍，而远红边荧光产率保持不变。我们将这一发现解释为衰老火炬松林光合作用气孔限制的结果。当前和未来的卫星任务以粗空间分辨率提供 SIF 全球数据集，导致像素内混合效应，这可能是荧光信号解释的一个混杂因素。为了减轻这种影响，我们提出了荧光产率的替代指标，即冠层覆盖荧光指数（CCFI），通过利用植被分数覆盖来解释冠层结构的空间变异性。研究发现，空间聚集往往掩盖了有效的关系，而 CCFI 仍然能够保持这种联系。本研究首次尝试解释衰老林分中的荧光变化，可能为从 SIF 遥感角度理解未来气候条件下森林的长期动态提供新的视角。
