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Evergreen conifer forests are the most prevalent land cover type in North America. Seasonal changes in the color of evergreen forest canopies have been documented with near-surface remote sensing, but the physiological mechanisms underlying these changes, and the implications for photosynthetic uptake, have not been fully elucidated. Here, we integrate on-the-ground phenological observations, leaf-level physiological measurements, near surface hyperspectral remote sensing and digital camera imagery, tower-based CO2 flux measurements, and a predictive model to simulate seasonal canopy color dynamics. We show that seasonal changes in canopy color occur independently of new leaf production, but track changes in chlorophyll fluorescence, the photochemical reflectance index, and leaf pigmentation. We demonstrate that at winter-dormant sites, seasonal changes in canopy color can be used to predict the onset of canopy-level photosynthesis in spring, and its cessation in autumn. Finally, we parameterize a simple temperature-based model to predict the seasonal cycle of canopy greenness, and we show that the model successfully simulates interannual variation in the timing of changes in canopy color. These results provide mechanistic insight into the factors driving seasonal changes in evergreen canopy color and provide opportunities to monitor and model seasonal variation in photosynthetic activity using color-based vegetation indices.

Keywords: AmeriFlux; dual isotope approach; Péclet effect; precipitation; stable oxygen isotopes; stomatal conductance; tree rings; vapour pressure deficit (VPD) EN AmeriFlux dual isotope approach Péclet effect precipitation stable oxygen isotopes stomatal conductance tree rings vapour pressure deficit (VPD) 809 812 4 10/13/22 20221101 NES 221101 A reply to Lin et al. (2022) 'Do changes in tree-ring 18 O indicate changes in stomatal con... There is growing scientific consensus that rising atmospheric CO SB 2 sb concentrations have caused an increase in forest water-use efficiency (WUE), but the magnitude of this effect and the relative roles of enhanced photosynthesis vs reduced stomatal conductance ( I g i SB s sb ) are still under debate (Walker I et al i ., 2020). Detecting long-term changes in stomatal conductance: challenges and opportunities of tree-ring 18O proxy Dual isotope approach, Péclet effect, AmeriFlux, precipitation, stable oxygen isotopes, stomatal conductance, tree rings, vapour pressure deficit (VPD) Simulating oxygen isotope ratios in tree ring cellulose using a dynamic global vegetation model. [Extracted from the article]