Biotic and physical mediation of whole-canopy solar-induced chlorophyll fluorescence in eastern deciduous trees

Abstract:

Photosynthesis is an important regulator of CO₂ fluxes within the global carbon cycle. Remotely-sensed observations of vegetation provide a means for estimating photosynthesis globally. Spaceborne measurements of solar-induced chlorophyll fluorescence (SIF), a process within the light-harvesting component of the photosynthetic machinery, correlate with eddy-covariance derived estimates of gross primary productivity (GPP) at the landscape scale. However, spaceborne measurements are made at coarse spatial (> 1 km²) and temporal (≥ 1 day) resolutions, making them less useful for identifying the physiological regulators of photosynthetic variations, which operate at scales of minutes and meters.

We present diurnal observations of SIF at the scale of individual tree canopies in a temperate mixed forest of the Piedmont region of Virginia. These measurements were made with a FluoSpec 2 system installed onto an unmanned aerial vehicle, and were collected during the middle of the 2020 growing season. FluoSpec 2 provides measurements of canopy SIF at 760 nm and reflectance (415 nm – 935 nm). To examine the influence of canopy structure on SIF, we use the escape probability, calculated both optically through NIR_v and from point clouds from terrestrial lidar scanning. We use measurements of leaf gas exchange and variable chlorophyll fluorescence to characterize the influence of canopy physiology on SIF. Our results show that both canopy structure and physiology drive the spatiotemporal variability in SIF retrievals.