Evaluation of Carotenoid Pigment Indices from High Resolution Satellites to Track the Photosynthetic Seasonality in Evergreen Forests

The magnitude and seasonality of carbon uptake by evergreen forests is a challenging but critical piece to robustly quantifying the terrestrial carbon budget. Eddy covariance techniques have been measuring the land-atmosphere carbon exchange successfully at the canopy scale. However, the limited spatial coverage and uneven distribution of eddy covariance towers hamper the upscaling of measurements to the global scale. From their vantage point in space, satellites might fill this gap. Current satellite products of gross primary productivity utilizing the classical vegetation indices (VIs), such as Normalized Difference Vegetation Index (NDVI), have large uncertainties for estimating the seasonal variation of evergreens as they only parameterize the light use efficiency from plant functional types and meteorological conditions. Newly developed indices, such as the Green Chromatic Coordinate (GCC), photochemical reflectance index and chlorophyll/carotenoid index, show promise to track the seasonal variations in evergreen forests by measuring changes in photoprotective pigment concentrations, which help regulate light use efficiency in evergreen forests. Among these VIs, the GCC holds much promise as it uses the RGB channels that many satellites provide. High spatial resolution satellites, such as Landsat and Sentinel 2, reduce the uncertainties caused by surface heterogeneity. Here, we generate GCC and NDVI timeseries from the Landsat and Sentinel 2 surface reflectance at a subalpine evergreen forest in Niwot Ridge, CO, USA to evaluate the evergreen seasonality and its spatial variations observed from space. Satellite generated timeseries related to changes in photoprotective carotenoid pigments are evaluated against eddy covariance measurements and continuous tower-based hyperspectral reflectance measurements at Niwot Ridge. To evaluate the overall carbon cycling of evergreens, we extend the method to larger spatial scales at high latitudes and compare seasonality against measurements of solar induced chlorophyll fluorescence. We will discuss the general applicability of the method and its ability of tracking the seasonality of evergreens by accounting for background effects (i.e., snow, topography) which impact reflectance in the visible spectrum.