Assessing the sensitivity of shoot-level chlorophyll fluorescence to scalable proxies of absorbed radiation in an evergreen needleleaf forest

Photosynthesis from evergreen needleleaf forests (ENFs) may mediate global efflux of CO₂ from the biosphere to the atmosphere. Chlorophyll fluorescence (ChlF), a process physically linked to photosynthesis that is sensitive to both physical (e.g., absorbed radiation) and physiological constraints (e.g., light use efficiency), may be optically observed from remote sensing as solar-induced fluorescence (SIF), providing opportunity to observe photosynthetic dynamics at global scales. However, interpretation of this signal requires a mechanistic understanding of the sensitivity of ChlF to fine-scale spatiotemporal variance in absorbed radiation. ENF canopies are characterized by a high degree of variation in absorbed radiation as modulated by heterogeneity in canopy structure, pigment content, and foliage illumination. The objective of this study was to assess the sensitivity of in situ shoot-level ChlF to absorbed radiation in order to disentangle physical from physiological drivers of photosynthetic dynamics of ENF canopies. We collected seven rounds of observations of ChlF from six shoots on each of 10 grand fir trees (Abies grandis; n = 420) in a montane forest in Idaho. Contemporaneously collected terrestrial lidar data were used to characterize canopy structure for parameterizing a dynamic shadow-casting model to estimate fine-scale (5 cm pixel size), spatially explicit irradiance of sampled shoots. For the duration of sampling, pyranometers were deployed in several light conditions (always shaded, always sunlit, and mixed shaded/sunlit) to validate the direct and diffuse components of modeled irradiance. Shoots were harvested following ChlF observations for pigment content analysis. We developed a linear mixed-effects model using modeled shoot level irradiance, wet-chemistry-derived foliar pigment content, and time of sampling as fixed effects and sampling round and shoot ID as random effects to predict F_t (steady-state fluorescence). Preliminary results indicated that modeled shoot-level irradiance and time of sampling were significant predictors of F_t (p < 0.05). These results suggest that the relationship between physically and physiologically driven variation in shoot-level ChlF must be considered to interpret canopy SIF for tracking photosynthetic dynamics.