Influences of fractional vegetation cover on the spatial variability of canopy SIF from unmanned aerial vehicle observations

Solar-induced chlorophyll fluorescence (SIF) has provided novel methods for monitoring vegetation growth and the carbon cycle of terrestrial ecosystems. However, the effects of spatial heterogeneity on canopy SIF measurements remain unclear. The unmanned aerial vehicle (UAV) platform provides a unique opportunity to assess the impact of spatial heterogeneity on SIF variability due to its adjustable observational height at the intermediate canopy scale. In this work, we used a UAV-based SIF system to investigate the influences of fractional vegetation cover (FVC) on SIF measurements over (1) a homogeneous rice paddy field and (2) a heterogeneous planted forest characterized by unevenly distributed differing plant species. We first simultaneously conducted an experiment with UAV- and tower-based SIF systems in a homogeneous paddy field to test the reliability of SIF measurements from UAV-based system. The results showed that the SIF measured by UAV- and tower-based systems have a strong linear relationship (R2 = 0.98), demonstrating that UAV-based SIF system is capable of capturing the diurnal variations of canopy SIF. Then, we operated the UAV flying over the heterogeneous planted forest field in five flights, with each flight observing the same plant species across different heights to investigate the influences of FVC on the spatial variability of SIF. We found that FVC exerted substantial effects on the spatial variability of SIF, with the coefficient of variation (CV) of SIF observations at different flying heights increasing from 5% (high FVC) to 30% (low FVC), which was consistent with DART (Discrete Anisotropic Radiative Transfer) model simulations. Furthermore, our results indicated that the escaping probability of SIF and total emitted SIF showed nonlinear responses to FVC at individual observational heights. In particular, the escaping probability reached its lowest value when FVC was at an intermediate level (FVC = 0.6). These findings highlight the significant effect of spatial heterogeneity on canopy SIF measurements, especially at low FVC. Therefore, an exhaustive consideration of the SIF measurement footprint on different underlying surfaces (homogeneous or heterogeneous) is essential to advance SIF applications in terrestrial vegetation science.