Abstract
Abstract. Global terrestrial ecosystems control the atmospheric CO2 concentration through gross primary production (GPP) and ecosystem respiration processes. Chlorophyll fluorescence is one of the energy release pathways of excess incident light in the photosynthetic process. Over the last 10 years, extensive studies have revealed that canopy-scale Sun-induced chlorophyll fluorescence (SIF), which potentially provides a direct pathway to link leaf-level photosynthesis to global GPP, can be observed from satellites. SIF is used to infer photosynthetic capacity of plant canopy; however, it is not clear how the leaf-level SIF emission contributes to the top-of-canopy directional SIF. Plant canopy radiative transfer models are useful tools to understand the mechanism of anisotropic light interactions such as scattering and absorption in plant canopies. One-dimensional (1-D) plane-parallel layer models (e.g., the Soil Canopy Observation, Photochemistry and Energy fluxes (SCOPE) model) have been widely used and are useful to understand the general mechanisms behind the temporal and seasonal variations in SIF. However, a 1-D model does not explain the complexity of the actual canopy structures. Three-dimensional models (3-D) have a potential to delineate the realistic directional canopy SIFs. Forest Light Environmental Simulator for SIF (FLiES-SIF) version 1.0 is a 3-D Monte Carlo plant canopy radiative transfer model to understand the biological and physical mechanisms behind the SIF emission from complex forest canopies. The FLiES-SIF model is coupled with leaf-level fluorescence and a physiology module so that users are able to simulate how the changes in environmental and leaf traits as well as canopy structure affect the observed SIF at the top of the canopy. The FLiES-SIF model was designed as three-dimensional model, yet the entire modules are computationally efficient: FLiES-SIF can be easily run by moderate-level personal computers with lower memory demands and public software. In this model description paper, we focused on the model formulation and simulation schemes, and showed some sensitivity analysis against several major variables such as view angle and leaf area index (LAI). The simulation results show that SIF increases with LAI then saturated at LAI>2–4 depending on the spectral wavelength. The sensitivity analysis also shows that simulated SIF radiation may decrease with LAI at a higher LAI domain (LAI>5). These phenomena are seen in certain Sun and view angle conditions. This type of nonlinear and nonmonotonic SIF behavior towards LAI is also related to spatial forest structure patterns. FLiES-SIF version 1.0 can be used to quantify the canopy SIF in various view angles including the contribution of multiple scattering which is the important component in the near-infrared domain. The potential use of the model is to standardize the satellite SIF by correcting the bidirectional effect. This step will contribute to the improvement of the GPP estimation accuracy through SIF.
Highlights
Global terrestrial ecosystems control the atmospheric CO2 concentration through gross primary production (GPP) and ecosystem respiration processes (Canadell et al, 2007; Richardson et al, 2009; Piao et al, 2013)
Over the last 10 years, extensive studies have revealed that canopyscale Sun-induced chlorophyll fluorescence (SIF) can be observed from satellites, such as the Greenhouse gases Observation Satellite (GOSAT) (Frankenberg et al, 2011), Orbiting Carbon Observatory-2 (OCO-2) (Li et al, 2018; Norton et al, 2019), Global Ozone Monitoring Experiment-2 (GOME-2) (Joiner et al, 2013), and TROPOspheric Monitoring Instrument (TROPOMI) (Köhler et al, 2018) using Fraunhofer lines in the near-infrared spectral domain
We have described the structure of Forest Light Environmental Simulator (FLiES)-SIF version 1.0 and the simulation algorithm for canopy-scale Sun-induced chlorophyll fluorescence emissions
Summary
Global terrestrial ecosystems control the atmospheric CO2 concentration through gross primary production (GPP) and ecosystem respiration processes (Canadell et al, 2007; Richardson et al, 2009; Piao et al, 2013). SIF can be used to infer the photosynthetic capacity of the plant canopy (Zhang et al, 2018) It is not clear how leaf-level SIF emissions contribute to the top-of-canopy directional SIF, because satellite-observed SIF uses the near-infrared spectral domain, in which multiple scatterings on the leaf surface are dominant. 3-D models are required to evaluate realistic SIF features, the model’s input variables should be created or accessible from existing databases This is because, without sufficient input data, it is difficult to extend the model simulations to the various ecosystems around the world. This paper describes a 3-D Monte Carlo plant canopy radiative transfer model, the Forest Light Environmental Simulator (FLiES), for simulating canopy-scale directional SIF radiance and evaluates the performance of the model by analyzing the angular and multiple-scattering effects on SIF
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