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Uncertainties in future climate projections are, in large part, due to an incomplete understanding of terrestrial carbon and ecosystem feedbacks (Friedlingstein et al., 2014). Among the most poorly understood ecosystems is the boreal forest, which stores a significant amount of carbon and is one of the regions most sensitive to environmental change (

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Recent advancements in understanding remotely sensed solar-induced chlorophyll fluorescence often suggest a linear relationship with gross primary productivity at large spatial scales. However, the quantum yields of fluorescence and photochemistry are not linearly related, and this relationship is largely driven by irradiance. This raises questions about the mechanistic basis of observed linearity from complex canopies that experience heterogeneous irradiance regimes at subcanopy scales. We present empirical data from two evergreen forest sites that demonstrate a nonlinear relationship between needle-scale observations of steady-state fluorescence yield and photochemical yield under ambient irradiance. We show that accounting for subcanopy and diurnal patterns of irradiance can help identify the physiological constraints on needle-scale fluorescence at 70-80% accuracy. Our findings are placed in the context of how solar-induced chlorophyll fluorescence observations from spaceborne sensors relate to diurnal variation in canopy-scale physiology.Plain Language Summary Chlorophyll fluorescence is a faint signal emitted by plants that can provide information about photosynthesis and other processes important for plant growth. However, fluorescence is governed by complex chemical reactions that depend on light, and it is not linearly related to photosynthetic carbon uptake. Ecosystems with complex canopy structure, such as evergreen needleleaf forests, experience dynamic sunlit and shaded conditions, which make fluorescence observations challenging to interpret. However, by accounting for incoming light at fine spatial scales in studies using fluorescence, we can track the conditions under which canopies are partitioned by light-saturated and light-limited physiological constraints at 70-80% accuracy. Findings from our field-based study are relevant for interpreting satellite-based measurements of fluorescence as a proxy of photosynthetic carbon uptake. Furthermore, our study underscores the need for further research on how data from leaf-scale studies can be scaled up to shed light on ecosystem responses to changing climatic conditions.