# Parallel Seasonal Patterns of Photosynthesis, Fluorescence, and Reflectance Indices in Boreal Trees
## 5 Conclusions  
Our results show the varying efficacy of F\({}_{\rm S}\) and reflectance indices (NDVI, PRI, and CCI) in tracking photosynthetic phenology for evergreen and deciduous species. As expected, the NDVI was primarily sensitive to canopy structural changes associated with leaf development and senescence, and detected changing photosynthetic activity in deciduous, but not in evergreen, species. The PRI was able to track photosynthetic activity driven by changes in pigment pool size, which can be observed in both functional groups, but was better seen in evergrerens. The CCI was able to track photosynthetic phenology in both deciduous and evergreen species, reflecting seasonal changes in both pigment pool size and canopy structural changes. In many cases, F\({}_{\rm S}\) showed similar responses to the CCI, but with weaker correlations with photosynthetic phenology, partly due to the limitations of smaller sample sizes. Our findings support the complementary nature of the reflectance indices NDVI and PRI, with respect to the LUE model. As expected, the NDVI and PRI provided information relevant to the A\({}_{\rm PAR}\) and \(\varepsilon\) terms in the model, respectively. By contrast, the CCI and F\({}_{\rm S}\) seem to have provided similar information regarding photosynthetic activity, and may have been sensitive to both A\({}_{\rm PAR}\) (via chlorophyll content and canopy structure) and \(\varepsilon\) (mediated via relative pigment concentrations and NPQ). We also showed that the CCI correlated well with actual rates of photosynthesis and could perhaps transcend the LUE model by providing a direct metric of productivity, as proposed for SIF. F\({}_{\rm S}\) (a leaf-level analog for SIF) may have a similar potential, but further work at larger scales is needed to explore the cause of these differences apparent in our leaf (F\({}_{\rm S}\)) and canopy (CCI) comparisons. The development of new instruments for canopy-level SIF measurements in conjunction with reflectance should further clarify these relationships and their mechanistic underpinnings. Different functional groups (evergreen and deciduous) showed distinct optical and photosynthetic phenology, outlining the importance of considering optical types when sampling at the ecosystem-level. With similar responses in both types, the CCI is well suited for detecting changes representative of both functional groups, and can presumably be applied to mixed boreal stands, where deciduous and evergreen species both contribute to canopy optical properties and photosynthetic fluxes. Whenconsidering reflectance indices for evaluating photosynthetic phenology of different functional groups, the limitations of the NDVI and PRI, which responded differently for evergreen and deciduous species, are readily apparent. The CCI and SIF both offer means of monitoring large-scale photosynthesis and productivity, and the tandem ability of these metrics to monitor photosynthetic phenology in both deciduous and evergreen vegetation will be a particularly valuable contribution to the planned FLEX mission and supporting studies. Relative to other metrics, the stronger ability of the CCI and SIF in tracking photosynthetic activity of different vegetation types, by detecting structural and physiological contributions to photosynthetic phenology, appear to offer a powerful means of estimating photosynthesis and primary productivity across ecosystems. The following are available online at www.mdpi.com/2072-4292/9/7/691/s1, Figure S1: Correlations between daily average temperature, photosynthetic rate (umol CO2 m\({}^{-2}\) s\({}^{-1}\)), NDVI, PRI, CCI, or fluorescence for evergreen (a-e) and deciduous (f-j) species. This study was supported by iCORE/AITF (G224150012 & 200700172), NSERC (RGPIN-2015-05129), CFI (26793), and NASA ABoVE (NNX15 AT78A) grants to [PERSON], and a China Scholarship Council fellowship to [PERSON]. The authors thank [PERSON] for assistance with plant care.