Abstract
The effect that the canopy structure and the viewing geometry have on the intensity and the spatial distribution of passively measured sun-induced chlorophyll fluorescence at canopy scale is still not well understood. These uncertainties constrain the potential use of fluorescence to quantify photosynthesis at this level. Using a novel technique, we evaluated the diurnal changes in the spatial distribution of sun-induced fluorescence at 760 nm (F760) within the canopy as a consequence of the spatial disposition of the leaves and the viewing angle of the sensor. High resolution spectral and stereo images of a full sugar beet canopy were recorded simultaneously in the field to estimate maps of F760 and the surface angle distribution, respectively. A dedicated algorithm was used to align both maps in the post-processing and its accuracy was evaluated using a sensitivity test. The relative angle between sun and the leaf surfaces primarily determined the amount of incident Photosynthetic Active Radiation (PAR), which in turn was reflected in different values of F760, with the highest values occurring in leaf surfaces that are perpendicularly oriented to the sun. The viewing angle of the sensor also had an impact in the intensity of the recorded F760. Higher viewing angles generally resulted in higher values of F760. We attribute these changes to a direct effect of the vegetation directional reflectance response on fluorescence retrieval. Consequently, at leaf surface level, the spatio-temporal variations of F760 were mainly explained by the sun–leaf–sensor geometry rather than directionality of the fluorescence emission. At canopy scale, the diurnal patterns of F760 observed on the top-of-canopy were attributed to the complex interplay between the light penetration into the canopy as a function of the display of the various leaves and the fluorescence emission of each leaf which is modulated by the exposure of the individual leaf patch to the incoming light and the functional status of photosynthesis. We expect that forward modeling can help derive analytical simplified skeleton assumptions to scale canopy measurements to the leaf functional properties.
Highlights
Understanding the dynamic changes of canopy photosynthesis is crucial for many scientific disciplines
There are still uncertainties related to the effect that canopy structure and the observation geometry may have on the intensity of fluorescence emission that is recorded at canopy level, limiting partly the capability to link large scale measurements with leaf photosynthesis and the actual physiological status of the vegetation [13,24,25]
The φl and θl could be retrieved for about 30% to 40% of the total pixels of each imaging spectroscopy data, from which approximately 80% to 90% showed a good correlation between the corresponding pixels of the two imaging systems
Summary
Understanding the dynamic changes of canopy photosynthesis is crucial for many scientific disciplines. Reported measurements of sun-induced fluorescence from ground [15,16,17], airborne [18,19,20] and spaceborne [21,22,23] sensors have proven the feasibility of retrieving this signal on top-of-canopy (TOC), providing new insights on the spatial distribution of vegetation fluorescence across a wide range of spatial scales, ranging from single leaves to the ecosystem scales Despite these achievements, there are still uncertainties related to the effect that canopy structure and the observation geometry may have on the intensity of fluorescence emission that is recorded at canopy level, limiting partly the capability to link large scale measurements with leaf photosynthesis and the actual physiological status of the vegetation [13,24,25]. Understanding the effect of canopy structure on the distribution of light, plant photosynthesis and fluorescence emission within the canopy is a critical issue, when sun-induced fluorescence is measured at coarse spatial resolution
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