Using digital repeat photography and eddy covariance data to model grassland phenology and photosynthetic CO2 uptake

The continuous and automated monitoring of canopy phenology is of increasing scientific interest for the multiple implications of vegetation dynamics on ecosystem carbon and energy fluxes. For this purpose we evaluated the applicability of digital camera imagery for monitoring and modeling phenology and physiology of a subalpine grassland over the 2009 and 2010 growing seasons.We tested the relationships between color indices (i.e. the algebraic combinations of RGB brightness levels) tracking canopy greenness extracted from repeated digital images against field measurements of green and total biomass, leaf area index (LAI), greenness visual estimation, vegetation indices computed from continuous spectroradiometric measurements and CO2 fluxes observed with the eddy covariance technique. A strong relationship was found between canopy greenness and (i) structural parameters (i.e., LAI) and (ii) canopy photosynthesis (i.e. Gross Primary Production; GPP). Color indices were also well correlated with vegetation indices typically used for monitoring landscape phenology from satellite, suggesting that digital repeat photography provides high-quality ground data for evaluation of satellite phenology products.We demonstrate that by using canopy greenness we can refine phenological models (Growing Season Index, GSI) by describing canopy development and considering the role of ecological factors (e.g., snow, temperature and photoperiod) controlling grassland phenology. Moreover, we show that canopy greenness combined with radiation use efficiency (RUE) obtained from spectral indices related to photochemistry (i.e., scaled Photochemical Reflectance Index) or meteorology (i.e., MOD17 RUE) can be used to predict daily GPP.Building on previous work that has demonstrated that seasonal variation in the structure and function of plant canopies can be quantified using digital camera imagery, we have highlighted the potential use of these data for the development and parameterization of phenological and RUE models, and thus point toward an extension of the proposed methodologies to the dataset collected within PhenoCam Network.

► In this work is showed the strong correlations between color indices derived from digital camera imagery and both the radiometric properties and biological activity of the surface vegetation.
► It is possible to refine phenological models using color indices derived from digital camera imagery as model constrain.
► With a modeling approach the role of ecological factors in controlling phenology at the site was clarified. A significant role of day length was observed during spring, particularly relevant when snow-melt occurred earlier.
► Color indices combined with an estimation of radiation use efficiency can be used effectively for the prediction of gross primary production.
► These results show the usefulness of automated, near-surface remote sensing of canopy phenology and point towards an extension of the proposed methodology to the dataset collected within phenological networks. In this context further work is necessary first to overcome limitations related to the image quality, in particular for continuous monitoring of vegetation status, and second to improve the use of color indices for monitoring the interannual and spatial (i.e. across site) variability of canopy structural parameters.