# Parallel Seasonal Patterns of Photosynthesis, Fluorescence, and Reflectance Indices in Boreal Trees
## 4 Discussion  
### Seasonal Kinetics  
Photosynthetic activity, fluorescence, and reflectance indices often exhibited seasonal responses that paralleled changing environmental conditions. However, the indices showed subtle differences from each other that indicated variation in their ability to track photosynthetic phenology. In parallel \begin{table}
\begin{tabular}{c c c c c c} \hline  & **Species** & **n** & **NDVI** & **PRI** & **CCI** \\ \hline b) & _P. mariana_ & 14 & 0.001 & 0.873 *** & 0.799 *** \\  & _P. banksimana_ & 14 & 0.642 *** & 0.827 *** & 0.885 *** \\  & _P. glauc_ & 14 & 0.104 & 0.900 *** & 0.854 *** \\ a) & _L. laricina_ & 9 & 0.590 * & 0.601 * & 0.707 ** \\  & _P. tremuloides_ & 9 & 0.501 * & 0.393 & 0.471 * \\  & _P. balsamifera_ & 9 & 0.763 ** & 0.710 ** & 0.696 ** \\ \hline \end{tabular}  
* \(p<0.05\); ** \(p<0.01\); *** \(p<0.001\); *** \(p<0.0001\).  
\end{table} 
Table 2: Linear correlation coefficients (\(p^{2}\)) between canopy-level optical indices NDVI, PRI, CCI and Fs for evergreen (**a**) and deciduous (**b**) species. Data points from the spring transition in 2016 for _P. glauca_ and _P. mariana_ were excluded from analyses. Figure 5: Correlations between Fs and canopy-level optical indices NDVI, PRI, and CCI for evergreen (**a**\(-\)**c**) and deciduous (**d**\(-\)**f**) species. Data points were obtained near solar noon from January 2016 to December 2016. Error bars denote \(\pm\)SE of the mean. to seasonally changing temperature, optical and photosynthetic metrics in evergreen species (with the exception of the NDVI) changed gradually with temperature, whereas metrics in deciduous species showed abrupt changes that were typically limited to the spring and fall transition periods, and did not track temperature as closely as in evergreens. In particular, we noted that seasonal CCI patterns exhibited a close correlation with temperature for evergreen and deciduous species (supplemental Figure S1). The strong relationship between the CCI and temperature suggested a strong temperature driver in the CCI signal. Gradual seasonal changes in photosynthetic activity for evergreen species, compared to more abrupt spring activation and fall declines of photosynthesis for deciduous species, reflect a longer overall photosynthetic season than for deciduous species [38]. While evergreens in boreal forests do not photosynthesize year-round, they can typically begin photosynthesis earlier in the spring and persist later in the fall than deciduous species [39]. The abrupt drop in photosynthetic rate for _P. mariana_ and _P. glauc_ during the spring transition was likely an artefact of leaf-level sampling. Budburst near branch tips for spruce trees made accurate sampling of mature needles difficult, leaving only young leaves to be sampled, which are less productive than fully mature leaves [40]. The more abrupt transitions during spring and fall for deciduous species were expected as, unlike evergreens, deciduous species shed foliage during unfavorable seasons [38]. The higher overall photosynthetic capacity observed in deciduous species compared to evergreens during the growing season was consistent with deciduous species having a higher rate of photosynthesis per unit leaf mass than evergreens [38]. Deciduous species have thinner leaves with a greater surface area than evergreens, allowing them to be more productive during favorable conditions, whereas evergreens have thick leaves that avoid damage brought on by frost or drought during unfavorable seasons [41]. The conifer _L. laricina_, though deciduous, showed very similar photosynthetic rates to evergreen species, and typically has low summer rates of photosynthetic activity and growth [42]. When evaluating the efficacy of canopy reflectance indices in tracking photosynthetic phenology, we found that the NDVI was capable of tracking midday photosynthetic activity of deciduous species associated with the development and loss of green foliage across seasons, as previously reported [17].