Using spectral reflectance signatures and chlorophyll fluorescence to investigate plant physiology and ecosystem functions
Plant physiological metrics such as photosynthetic rates and the seasonality of leaf phenology respond sensitively to micro-climate and global climate change. Digital cameras have been developed to monitor the seasonal and annual changes of forest canopy phenology, because camera images may be acquired quickly and frequently to track the critical phenological events. However, most of these digital cameras record red, green and blue bands, and these RGB images cannot accurately reflect important plant physiological characteristics. Employing infrared bands significantly expands our ability to record leaf physiology. In addition, chlorophyll fluorescence provides a new tool to indirectly measure photosynthetic assimilation. Correlation between chlorophyll fluorescence and carbon fixation helps us better understand photosynthetic processes and assess plant responses to environmental stresses.
We used a tower-based NDVI (normalized difference vegetation index) camera as a simple, accurate and inexpensive technique to monitor vegetation reﬂectance in visible and near-infrared bands to quantify canopy structural and functional properties at the Harvard Forest, Massachusetts, USA. We measured leaf chlorophyll fluorescence along with gas exchange measurements using LI-COR 6400. In addition, we developed a new technique to measure canopy solar-induced fluorescence (SIF) from a spectrometric system and obtained gross primary production (GPP) estimated by eddy covariance flux data.
The NDVI camera that combines visible and near infra-red bands provides important information for canopy phenology, structure and leaf function (leaf chlorophyll and nitrogen content) during the growing season. We found high linear relationships between camera-NDVI and physiological metrics such as leaf chlorophyll concentration, leaf nitrogen contents, and leaf area index. We conclude that (1) camera-NDVI provides a powerful tool to identify key phenological events, such as leaf-out, greenness peak and leaf-off dates as well as vegetation structural and functional properties, while RGB camera-based metrics are not sufficient to interpret vegetation biochemistry characteristics; and (2) tower-based NDVI data serve well as ground verification for the satellite vegetation indices application.
We found a strong seasonal pattern of leaf chlorophyll fluorescence in a temperate deciduous forest, and the variations of steady fluorescence (Fs) were correspondent with leaf chlorophyll concentrations. There was a significant correlation between chlorophyll fluorescence and carbon fixation, indicating that leaf chlorophyll fluorescence is a powerful tool to study leaf photosynthetic capacities. On the canopy scale, strong correlation between ground-based SIF and GPP demonstrated that SIF has the potential to estimate GPP and canopy photosynthesis on the canopy and up to regional and global scales.