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A new possibility to improve photosynthesis modelling in vegetation models

A new possibility to improve photosynthesis modelling in vegetation models

New observations of chlorofyll fluorescence enable to detect photosynthesis of plants from space. Adding chlorofyll fluorescence to vegetation models provides means to understand photosynthesis better at a global scale and improve its modelling.

Advanced remote sensing methods provide observations of chlorophyll fluorescence from space. Chlorophyll fluorescence takes place in the plant leaves simultaneously with photosynthesis, therefore a direct link between the two exists and chlorophyll fluorescence can be used to track photosynthesis. Obtaining reliable estimates of photosynthesis at global scale has been challenging and these new observations thus provide a new, valuable data source.

In order to use chlorophyll fluorescence in model development, a process-based description for it is required. In this work we implemented a chlorophyll fluorescence model developed at leaf scale to a global vegetation model JSBACH and we evaluated the model performance in terms of photosynthesis and chlorophyll fluorescence. In the comparison we used leaf level observations from Hyytiälä, CO2 flux observations from four coniferous forests in Finland as well as observations from GOME2 satellite. Additionally we compared the skill of chlorophyll fluorescence to commonly used vegetation index, fraction of photosynthetically active radiation that is more connected with the amount of green biomass than directly with photosynthesis.

Comparison at the CO2 flux sites revealed the capability of the chlorophyll fluorescence to track photosynthesis and possibilities for data assimilation applications. Larger scale simulation was done for the Fenno-Scandinavian region. The model was successful in simulating the observed chlorophyll signal (figure 1). In the northernmost region the seasonal cycle of the remotely sensed signal differed from both model simulations as well as from a data-based upscaled photosynthesis product. This highlights the challenges that the remote sensing products have at high latitudes.

The Finnish Meteorological Institute implemented the chlorophyll fluorescence model into a global vegetation model in collaboration with researchers from the Max Planck Institute. Additionally CO2 flux observations from the Finnish Meteorological Institute were used in the study. Also Dutch and Italian researchers were involved in this work. The study was funded by e.g. The Academy of Finland.

Picture 1. The average of five years (07-11) for (a) chlorophyll fluorescence observed from space, (b) modelled chlorophyll fluorescence, (c) modelled photosynthesis, (d) photosynthesis from upscaled data-based product.

More information:

Reseacher Tea Thum,

Thum, T., Zaehle, S., Köhler, P., Aalto, T., Aurela, M., Guanter, L., Kolari, P., Laurila, T., Lohila, A., Magnani, F., Van Der Tol, C., and Markkanen, T.: Modelling sun-induced fluorescence and photosynthesis with a land surface model at local and regional scales in northern Europe, Biogeosciences, 14, 1969-1987, doi:10.5194/bg-14-1969-2017, 2017.



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