# Parallel Seasonal Patterns of Photosynthesis, Fluorescence, and Reflectance Indices in Boreal Trees
## 3 Results
### Seasonal Patterns of Photosynthesis and Remote Metrics  
Photosynthetic activity, fluorescence, and reflectance indices often exhibited parallel responses to seasonally changing environmental conditions. However, indices showed subtle differences from each other that indicated variation in their ability to track photosynthetic phenology. Key differences in optical behavior emerged between evergreen and deciduous species, and further interspecific differences were also evident between some species within each group. Photosynthetic rate and most optical metrics followed similar seasonal patterns as temperature for both evergreen (Figure 3a-e) and deciduous (Figure 3f-j) species. Rapid changes in canopy index values in the winter months for deciduous and evergreen species coincided with the presence of snow on the canopies. Deciduous species showed rapid changes in both photosynthesis and optical indices during transitions, while more gradual seasonal transitions were observed in evergreen species for both optical metrics and gas exchange. For deciduous species, leaf-level sampling methods were limited to periods when fully formed foliage was present, and canopy-level optical sampling was influenced by bare soil and remaining stems in the absence of foliage. Small dips in index values in April, during the spring transition, were related to the repotting and increased spacing of trees. In evergreens, photosynthetic rate responded gradually to seasonally changing temperature (Figure 3a). Spring activation began in March, with photosynthetic rates increasing to summer maxima  
Figure 2: Seasonal dynamics of photosynthetic photon flux density (PPFD; \(\mu\)mol photons m\({}^{-2}\) s\({}^{-1}\)) and temperature (\({}^{\circ}\)C) from December 2015 to January 2016. PPFD is expressed as a midday average, taken as the period between 13:00 and 14:00 (UTC-06), and temperature is expressed as daily averages. Long-term normal (1981–2010) daily average temperature by month is also shown. Error bars denote \(\pm\)SD of the mean. Between 20 November and 20 January, the sun did not clear buildings to the south due to low solar elevation, causing anomalously low PPFD values during this period. (ca. 10 \(\upmu\)mol CO\({}_{2}\) m\({}^{-2}\) s\({}^{-1}\)) by mid-June. During the spring transition, a large drop in photosynthetic rate was observed for the spruce species (_P. marianna_ and _P. glauca_) that coincided with the sampling of newly emerged branches following budburst (also indicated by open symbols; Figure 3a); this drop was not seen in _P. banksiana_ as mature needles were sampled through the transition. Following the growing season, photosynthetic rate gradually decreased, reaching near-zero again by November. Deciduous species displayed more rapid photosynthetic changes in spring and fall, during early leaf development and senescence, respectively, with winter periods lacking foliage for sampling  
Figure 3: Annual patterns of midday average temperature (grey line), optical indices and photosynthetic metrics for evergreen and deciduous species. Evergreen species (**a\(-\)e**) included _P. mariana_ (black), _P. banksiana_ (red), and _P. glauca_ (blue). Deciduous species (**f\(-\)j**) included _L. laricina_ (black), _P. tremuloides_ (red), and _P. balsamifera_ (blue). Optical indices (**b\(-\)d.g\(-\)i**) were calculated from reflectance measurements at the canopy-level. Fluorescence (F\({}_{S}\)) was measured at the leaf-level. Data points shown were obtained near solar noon from December 2015 to January 2017. Open points (**a**) denote dates during the spring transition, where new branches were sampled for _P. mariana_ and _P. glauca_. The winter period is indicated by the grey regions (November–March). Annotations on figures indicate the effects of snow (S), repotting (R), and new-leaf expansion (E). For deciduous trees, data were restricted to periods with fully formed leaves during the growing season. Error bars denote \(\pm\)SE of the mean. (Figure 3f).