Tower based measurement of solar induced fluorescence
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Current remote sensing approaches for estimating carbon and water exchange of vegetation are indirect measures of time integrated net photosynthesis as they are based on biomass estimates. Other approaches rely on models which often badly represent reality due to either model or input variable deficiencies. Remote sensing of solar induced fluorescence is a revolutionary new approach for extensive measurement of instantaneous photosynthetic activity of vegetation. Therefore it promises to hugely advance our understanding of the terrestrial carbon cycle and its role in climate change.
Light energy absorbed by the photosynthetic apparatus goes into one of three energy pathways. It is either converted to chemical energy (photosynthesis), heat energy or emitted as fluorescence light. The distribution between these three pathways is determined by internal (e.g. pigment concentration) and external (e.g. water availability, leaf temperature and solar load) factors. Therefore fluorescence is a very direct measure of the state of the photosynthetic apparatus and as such is closely correlated with photosynthetic rate. Thus fluorescence stimulated using light emitting diodes as light source has been used to investigate photosynthesis for many years. Systems using lasers as light source have been introduced in the 1980s. These systems allow the measurement of chlorophyll fluorescence over a distance of a few metres. They can therefore measure fluorescence at the plant level. For measurements at stand and landscape level the use of artificial light sources is obviously not practical though measuring solar induced steady-state fluorescence has been proposed as method to improve estimates of canopy gross primary productivity (GPP) over large areas using remote sensing. While methods to measure solar induced fluorescence using air- and space-borne sensors have been developed (Maier 2001, 2002, 2006, Maier et al. 2003) we have limited understanding of the exact link between solar induced fluorescence and photosynthesis. To advance our understanding of chlorophyll fluorescence this project will install automatic spectroradiometers on the flux tower at the Savanna Supersite in Litchfield National Park to continuously measure solar induced fluorescence over a savanna woodland.