# Parallel Seasonal Patterns of Photosynthesis, Fluorescence, and Reflectance Indices in Boreal Trees
## 1 Introduction
The NDVI can be used to estimate light absorption as f\({}_{\text{PAR}}\)[32], whereas the PRI can be used to estimate \(\varepsilon\)[33]. The precise role of the CCI in the LUE model is still unclear, although it is similar to the PRI in that it is sensitive to changing pigment pool size [15]. Like SIF, the CCI may be sensitive to both APAR and \(\varepsilon\), and is therefore a direct indicator of GPP [15; 31]. Recent reviews [31; 34] have proposed that different vegetation, or optical types, have contrasting structural and physiological controls influencing productivity and optical signatures, which leads to varying relationships between the NDVI, PRI and GPP across ecosystems; we describe this as the \"complementarity hypothesis\". Accordingly, photosynthetic phenology of deciduous vegetation should relate to f\({}_{\text{PAR}}\), and be strongly detectable by the NDVI, whereas the NDVI of evergreens, with little temporal variation in f\({}_{\text{PAR}}\), should relate poorly to primary productivity. Conversely, deciduous vegetation should have little seasonal variation in \(\varepsilon\) detectable by the PRI, whereas changes in \(\varepsilon\) in evergreens should be largely driven by changing pigment pool sizes [13; 14; 19], leading to strong relationships between the PRI and primary productivity. As a newly defined index, the CCI has not previously been considered in the complementarity hypothesis [15]. We predict that, on seasonal timescales, the CCI will be similar to SIF, sensitive to both canopy structure-influencing absorbed radiation (APAR) and photosynthetic downregulation [10]. Both metrics should be indicators of GPP in both deciduous and evergreen vegetation (Figure 1). Figure 1: Representation of the light-use efficiency (LUE) model in black, stating that gross primary productivity (GPP) is a function of absorbed photosynthetically active radiation (APAR: f\({}_{\text{PAR}}\times\text{PAR}\)) and efficiency (\(\varepsilon\)). Red text shows optical measurements useful for model parameterization and validation, including solar-induced fluorescence (SIF), the chlorophyll/carotenoid index (CCI), the normalized difference vegetation index (NDVI), and the photochemical reflectance index (PRI) (modified from Gamon 2015) [31]. Further understanding of the mechanisms underlying reflectance indices and fluorescence in different vegetation types and natural environments is essential to the proposed FLuorescence EXplorer (FLEX) mission, which will enable both reflectance indices and SIF to assess photosynthetic phenology from satellites [35]. This mission will provide insight into ecosystem phenology and productivity beyond that of satellite missions providing reflectance (MODIS) or SIF (e.g., Orbiting Carbon Observatory-2: OCO-2) by allowing concurrent measurements of reflectance indices and SIF at finer scales. In support of this mission, ground-based studies are needed to clarify the utility of both reflectance indices and fluorescence as indicators of photosynthesis for different vegetation types. The goals of this study were to evaluate the efficacy of reflectance indices (NDVI, PRI, and CCI) and fluorescence (F\({}_{\text{S}}\)) in tracking photosynthetic phenology, by comparing the responses of these metrics in boreal trees that undergo large seasonal swings in photosynthetic activity. We also evaluated the complementarity hypothesis by considering the efficacy of these metrics in estimating photosynthetic activity in evergreen and deciduous species. We demonstrate that reflectance indices and fluorescence can be used to as indicators of photosynthetic phenology, showing parallel seasonal patterns to photosynthetic activity, but with noticeable differences across and within these functionally different vegetation types.