Abstract
Solar-induced chlorophyll fluorescence (SIF) has been shown to be a suitable remote sensing proxy of photosynthesis at multiple scales. However, the relationship between fluorescence and photosynthesis observed at the leaf level cannot be directly applied to the interpretation of retrieved SIF due to the impact of canopy structure. We carried out a SIF modelling study for a heterogeneous forest canopy considering the effect of canopy structure in the Discrete Anisotropic Radiative Transfer (DART) model. A 3D forest simulation scene consisting of realistic trees and understory, including multi-scale clumping at branch and canopy level, was constructed from terrestrial laser scanning data using the combined model TreeQSM and FaNNI for woody structure and leaf insertion, respectively. Next, using empirical data and a realistic range of leaf-level biochemical and physiological parameters, we conducted a local sensitivity analysis to demonstrate the potential of the approach for assessing the impact of structural, biochemical and physiological factors on top of canopy (TOC) SIF. The analysis gave insight into the factors that drive the intensity and spectral properties of TOC SIF in heterogeneous boreal forest canopies. DART simulated red TOC fluorescence was found to be less affected by biochemical factors such as chlorophyll and dry matter contents or the senescent factor than far-red fluorescence. In contrast, canopy structural factors such as overstory leaf area index (LAI), leaf angle distribution and fractional cover had a substantial and comparable impact across all SIF wavelengths, with the exception of understory LAI that affected predominantly far-red fluorescence. Finally, variations in the fluorescence quantum efficiency (Fqe) of photosystem II affected all TOC SIF wavelengths. Our results also revealed that not only canopy structural factors but also understory fluorescence should be considered in the interpretation of tower, airborne and satellite SIF datasets, especially when acquired in the (near-) nadir viewing direction and for forests with open canopies. We suggest that the modelling strategy introduced in this study, coupled with the increasing availability of TLS and other 3D data sources, can be applied to resolve the interplay between physiological, biochemical and structural factors affecting SIF across ecosystems and independently of canopy complexity, paving the way for future SIF-based 3D photosynthesis models.
Highlights
Solar-induced chlorophyll fluorescence (SIF) is electromagnetic radiation emitted by plants during daylight in the red and near-infrared wavelengths
Previous studies, based on simulations (Verrelst et al, 2015; Zhang et al, 2016) as well as measurements (Cheng et al, 2013; Rossini et al, 2015), suggested that red SIF is more sensitive to photosynthesis than far-red SIF. Both red and far-red SIF were influenced by the overall quantum yield of fluorescence (Fig. 11f), a parameter shown to respond to seasonal variation in photosynthesis (Ensminger et al, 2004; Soukupová et al, 2008; Springer et al, 2017), far-red fluorescence was more responsive to variations in leaf area index (LAI), FCover, understory LAI and especially foliar chlorophyll content (Fig. 13). These results suggest that far-red top of canopy (TOC) SIF may better capture the full space of factors affecting photosynthesis, whereas red TOC SIF is perhaps less sensitive to variations in absorbed photosynthetically active radiation (APAR), albeit with a greater sensitivity to fluorescence quantum efficiency (Fqe)
We presented a study modelling TOC SIF with consideration of the fine 3D structural heterogeneity found in a forest ecosystem
Summary
Solar-induced chlorophyll fluorescence (SIF) is electromagnetic radiation emitted by plants during daylight in the red and near-infrared wavelengths. The magnitude of SIF is very small compared to the radiation reflected by plant canopies being less than 5% of reflected sunlight in the near-infrared (Meroni et al, 2009). Under natural illumination, SIF cannot be measured directly due to the spectral overlap with reflected radiation. Spectral fluorescence is influenced by the dynamics of photosynthesis, and by chlorophyll a + b (Cab) content and leaf structure (Van Wittenberghe et al, 2015; Atherton et al, 2017; Magney et al, 2019a). At the canopy scale, SIF is influenced by 1) sun and view geometry, 2) canopy structure, 3) instrumental effects, and 4) the selection of SIF retrieval algorithm (Guanter et al, 2010; Damm et al, 2015a; Liu et al, 2016)
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