Interpreting canopy development and physiology using a European phenology camera network at flux sites

[摘要] Plant phenological development is orchestrated through subtle changes inphotoperiod, temperature, soil moisture and nutrient availability.Presently, the exact timing of plant development stages and their responseto climate and management practices are crudely represented in land surfacemodels. As visual observations of phenology are laborious, there is a needto supplement long-term observations with automated techniques such as thoseprovided by digital repeat photography at high temporal and spatialresolution. We present the first synthesis from a growing observationalnetwork of digital cameras installed on towers across Europe above deciduousand evergreen forests, grasslands and croplands, where vegetation andatmosphere CO₂ fluxes are measured continuously. Using colour indicesfrom digital images and using piecewise regression analysis of time series,we explored whether key changes in canopy phenology could be detectedautomatically across different land use types in the network. The piecewiseregression approach could capture the start and end of the growing season,in addition to identifying striking changes in colour signals caused byflowering and management practices such as mowing. Exploring the dates ofgreen-up and senescence of deciduous forests extracted by the piecewiseregression approach against dates estimated from visual observations, wefound that these phenological events could be detected adequately (RMSE < 8 and 11 days for leaf out and leaf fall, respectively). We alsoinvestigated whether the seasonal patterns of red, green and blue colourfractions derived from digital images could be modelled mechanisticallyusing the PROSAIL model parameterised with information of seasonal changesin canopy leaf area and leaf chlorophyll and carotenoid concentrations. Froma model sensitivity analysis we found that variations in colour fractions,and in particular the late spring `green hump' observed repeatedly indeciduous broadleaf canopies across the network, are essentially dominatedby changes in the respective pigment concentrations. Using the model we wereable to explain why this spring maximum in green signal is often observedout of phase with the maximum period of canopy photosynthesis in ecosystemsacross Europe. Coupling such quasi-continuous digital records of canopycolours with co-located CO₂ flux measurements will improve ourunderstanding of how changes in growing season length are likely to shapethe capacity of European ecosystems to sequester CO₂ in the future.

[学科分类] 地球化学与岩石