# Parallel Seasonal Patterns of Photosynthesis, Fluorescence, and Reflectance Indices in Boreal Trees
## 1 Introduction
The photochemical reflectance index (PRI), on the other hand, detects subtle changes in regulatory processes related to photosynthetic activity [21]. The PRI can detect pigment responses to environmental cues, primarily in evergreen species, that the NDVI can miss [13]. Over diurnal time-scales, PRI responses are driven by changes in the xanthophyll cycle, a facultative response, while responses over seasonal timescales reflect changes in pigment pool size (carotenoid/chlorophyll ratios), a constitutive response [22]. This index utilizes reflectance in the 531 nm waveband, compared to 570 nm as a reference waveband [23]. A variety of other wavebands have been used to calculate the PRI. Some sensors provide the PRI using the 532 nm instead of the 531 nm waveband; this formulation is functionally similar to the PRI calculated using the original 531 nm waveband, and detects both facultative and constitutive responses depending upon the sampling period [19]. On the other hand, a number of studies report other PRI formulas using different reference wavebands (e.g., Moderate Resolution Imaging Spectroradiometer PRI: MODIS PRI) [24; 25], which primarily detect pigment pool size variation when applied over seasonal timescales or across canopies [19]. Thus, the proper interpretation of the PRI can depend upon the formula and sampling scale used. The chlorophyll/carotenoid index (CCI) provides another indicator of photosynthetic activity, particularly in evergreen. Like the PRI, this index is sensitive to changes in pigment pools at both stand- and leaf-levels, and can accurately track seasonally changing chlorophyll/carotenoid levels indicating an important role of carotenoid pigments in winter downregulation [15]. The CCI can be applied over larger spatial scales using satellites, and can be derived from NASA's satellite-based MODIS sensor using bands 1 (645 nm, a terrestrial band) and 11 (531 nm, an ocean band). Unlike the xanthophyll cycle, which affects a narrow waveband, pigment pool size changes detectable with the CCI have a broad spectral response [15]. While the CCI has been shown to be a potent index of seasonal photosynthetic activity at leaf- and canopy-scales for evergreens [15], similar studies on deciduous species have not yet been reported. Chlorophyll fluorescence can also provide information on photosynthetic performance [26], and can reflect changing photosynthetic activity driven by both internal and external factors. Chlorophyll fluorescence has often been measured using the pulse-amplitude modulation (PAM) technique, which is restricted to the leaf-level due to the requirement of a saturating pulse [10]. With high-spectral resolution spectrometers that can resolve atmospheric absorption bands (Fraunhofer lines), fluorescence can now be passively detected from a distance as a small signal present in the \"gaps\" of the solar spectrum [27]). This solar-induced fluorescence (SIF) is closely linked to gross primary productivity (GPP) [28; 29; 30], however, the coarse spatial and temporal scales of many satellite-based SIF measurements cannot resolve detailed seasonal dynamics or explain underlying mechanisms driving changes in GPP. At leaf-scales, the PAM method can measure steady-state chlorophyll fluorescence (F\({}_{\text{S}}\)) without delivering a saturating pulse of light, which is an analogous measurement to SIF measured at large scales, as both can be measured under ambient illumination, allowing for more mechanistic studies of individual leaves and canopies. Together, the NDVI and PRI can provide complementary information in estimating the two main terms in the light-use efficiency (LUE) model [31] (Figure 1). This model expresses GPP as a function of APAR, and the efficiency (\(\varepsilon\)) of converting absorbed radiation into fixed carbon [18; 32]. APAR is the product of photosynthetically active radiation (PAR) or photosynthetic photon flux density (PPFD) and the fraction of PAR that is absorbed for photosynthesis (f\({}_{\text{PAR}}\)).