Abstract
Summary 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.
Highlights
Evergreen conifer forests are the dominant land cover type across much of the mid-latitudes of North America (Pielou, 2011)
Our study shows that changes in evergreen conifer canopy color that occur on seasonal timescales are associated with variation in leaf pigment ratios
The seasonality of canopy color variation is directly linked to photochemical processes We found that two visible-wavelength spectral indices, green chromatic coordinate (Gcc) and green–red vegetation index (GRVI), reveal strong seasonal variation in color of evergreen conifer canopies
Summary
Evergreen conifer forests are the dominant land cover type across much of the mid-latitudes of North America (Pielou, 2011) These forests account for a substantial portion of continental-scale CO2 uptake and carbon sequestration, influence water cycling and the ecohydrology of some of the continent’s largest watersheds, and have major effects on land–atmosphere interactions that are relevant in the context of the global Earth system (Bonan, 2008). They play a critical role in the response of the terrestrial biosphere to environmental change.
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