Computer Science
Scientific paper
Jun 2002
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2002esasp.474e..21r&link_type=abstract
Proceedings of the SPECTRA Workshop - The concept of a space-borne Earth Observation Mission addressing the terrestrial componen
Computer Science
Scientific paper
The gross primary productivity of natural ecosystems (GPP) can be expressed as the product of the amount of radiation absorbed by the green canopy (APAR) by the radiation use efficiency (). The fraction of incoming radiation that is intercepted by the canopy can be readily determined from remotely sensed data by means of spectral indexes such as NDVI (Normalized Difference Vegetation Index; Waring &Running 1998; Raddi, Magnani &Pippi 1998). Light use efficiency, however, is also known to be highly variable among species and as a result of environmental conditions. A proper determination of ɛ is therefore a key precondition for the realistic assessment of ecosystem productivity. Several different approaches have been proposed over the years to estimate light use efficiency. Because of the relationship between protein content and leaf photosynthetic potential, the remote sensing of foliar nitrogen content has been applied to estimate maximum assimilation rates, as an input for ecosystem models. Chlorophyll content, which can be more easily determined in the visible range, is also often used as a proxy for nitrogen concentration. This approach takes into account only the effects of soil fertility on ɛ. In contrast, the effects on ɛ of microclimatic factors can be estimated from complex forest ecosystem models, driven by records of local environmental conditions and species-specific parameters.In order to estimate regional productivity from RS data, models can be run for each pixel of interest, or they can be applied over a limited number of representative areas to obtain a robust empirical relationship between ɛ and key environmental variables. Finally, foliar photosynthesis can be directly estimated from leaf reflectance in the blue-green region, through indexes such as the Photochemical Reflectance Index (PRI; Gamon, Penuelas &Field 1992). The index has a clear functional basis, because of the well-known correlation between nonphotochemical quenching of absorbed radiation and the de-epoxydation state of xanthophyll pigments (Müller , Li &Niyogi 2001). Such a direct estimate, of course, will include the effects of both fertility and the environment. In the following, we will explore the potential of two of these approaches, based (i) on the development of robust relationships from detailed ecosystem models and (ii) on the direct application of the PRI index.
Magnani Federico
Pippi Ivan
Raddi Sabrina
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