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Remote sensing of vegetation has largely been revolving around the measurement of passive or active electromagnetic radiation of the top of the canopy. Nevertheless, plants hold a vertical structure and different processes and intensities take place within a plant organism depending on the environmental conditions. One of the main inputs for photosynthesis is photosynthetic active radiation (PAR) and a few studies have taken into account the effect of the qualitative and quantitative changes of the available PAR within the plants canopies. Mostly large plants (trees, shrubs) are affected by this phenomena, while signs of it could be observed in dense monocultures, too. Lake Balaton is a large lake with 12 km² dense reed stands, some of which have been suffering from reed die-back; consequently, the reed density and stress condition exhibit a vertical PAR variability within the canopy due to the structure and condition of the plants but also a horizontal variability attributed to the reedbed’s heterogeneous density. In this study we investigate the expression of photosynthetic and spectroscopic parameters in different PAR conditions. We concentrate on chlorophyll fluorescence as this is an early-stage indicator of stress manifestation in plants. We first investigate how these parameters differ across leaf samples which are exposed to a higher degree of PAR variability due to their vertical position in the reed culm (sun and shade leaves). In the second part, we concentrate on how the same parameters exhibit in reed patches of different densities. We then look into hyperspectral regions through graphs of coefficient of determination and associate the former with the physiological parameters. We report on the large variability found from measurements taken at different parts of the canopy and the association with spectral regions in the visible and near-infrared domain. We find that at low irradiance plants increase their acclimation to low light conditions. Plant density at Phragmites stands affects the vertical light attenuation and consequently the photophysiological response of basal leaves. Moreover, the hyperspectral response from the sun and shade leaves has been found to differ; charts of the coefficient of determination indicate that the spectral region around the red-edge inflection point for each case of sun and shade leaves correlate strongly with ETRₘₐₓ and α. When analysing the data cumulatively, independent of their vertical position within the stand, we found correlations of R² = 0.65 (band combination 696 and 651) and R² = 0.61 (band combination 636 and 642) for the ETRₘₐₓ and α, respectively.

Photophysiology and Spectroscopy of Sun and Shade Leaves of Phragmites australis and the Effect on Patches of Different Densities