Abstract
Abstract. Recent successes in passive remote sensing of far-red solar-induced chlorophyll fluorescence (SIF) have spurred the development and integration of canopy-level fluorescence models in global terrestrial biosphere models (TBMs) for climate and carbon cycle research. The interaction of fluorescence with photochemistry at the leaf and canopy scales provides opportunities to diagnose and constrain model simulations of photosynthesis and related processes, through direct comparison to and assimilation of tower, airborne, and satellite data. TBMs describe key processes related to the absorption of sunlight, leaf-level fluorescence emission, scattering, and reabsorption throughout the canopy. Here, we analyze simulations from an ensemble of process-based TBM–SIF models (SiB3 – Simple Biosphere Model, SiB4, CLM4.5 – Community Land Model, CLM5.0, BETHY – Biosphere Energy Transfer Hydrology, ORCHIDEE – Organizing Carbon and Hydrology In Dynamic Ecosystems, and BEPS – Boreal Ecosystems Productivity Simulator) and the SCOPE (Soil Canopy Observation Photosynthesis Energy) canopy radiation and vegetation model at a subalpine evergreen needleleaf forest near Niwot Ridge, Colorado. These models are forced with local meteorology and analyzed against tower-based continuous far-red SIF and gross-primary-productivity-partitioned (GPP) eddy covariance data at diurnal and synoptic scales during the growing season (July–August 2017). Our primary objective is to summarize the site-level state of the art in TBM–SIF modeling over a relatively short time period (summer) when light, canopy structure, and pigments are similar, setting the stage for regional- to global-scale analyses. We find that these models are generally well constrained in simulating photosynthetic yield but show strongly divergent patterns in the simulation of absorbed photosynthetic active radiation (PAR), absolute GPP and fluorescence, quantum yields, and light response at the leaf and canopy scales. This study highlights the need for mechanistic modeling of nonphotochemical quenching in stressed and unstressed environments and improved the representation of light absorption (APAR), distribution of light across sunlit and shaded leaves, and radiative transfer from the leaf to the canopy scale.
Highlights
Our ability to estimate and measure photosynthesis beyond the leaf scale is extremely limited
An important caveat in the analysis of Biosphere Energy Transfer Hydrology (BETHY) simulations is that, at the time of this writing, the prescribed meteorological forcing at NR1 is only available for 2015. While this degrades comparison to diurnal and synoptic variation observed by PhotoSpec in 2017, we find that the analysis of magnitude, light sensitivities, and within-model experiments still provides useful insight for the interpretation of other terrestrial biosphere models (TBMs)– solarinduced fluorescence (SIF) models and future modeling requirements in general
The primary shortcoming across TBM–SIF models and Soil Canopy Observation Photosynthesis Energy model (SCOPE) is a systematic high bias in absorbed PAR (APAR) magnitude (129 %), with most models exceeding the upper range of observed APAR and high model spread
Summary
Our ability to estimate and measure photosynthesis beyond the leaf scale is extremely limited This inhibits the ability to evaluate the performance of terrestrial biosphere models (TBMs) that are designed to quantify the direct impact and feedbacks of the carbon cycle with climate change. The emission of SIF represents a byproduct of two primary de-excitation pathways, photochemical (PQ) and nonphotochemical (NPQ) quenching. Plants have evolved these regulatory mechanisms to prevent damage to photosynthetic machinery when the amount of absorbed radiation is greater than that which can be used to drive photochemistry. SIF is fundamentally different than steady-state fluorescence yield typically measured at the leaf scale, as it is sensitive to both changes in photochemistry as well as absorbed PAR (APAR; related to incident light, canopy structure, and biochemical content). Magney et al (2019b) showed that seasonal changes in canopy SIF for cold-climate evergreen systems is influenced by changes in needle physiology and photoprotective pigments (Magney et al, 2019b)
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