Use of vegetation indices to estimate indices to estimate intercepted solar radiation and net carbon dioxide exchange of a grass canopy

Abstract

Photosynthesis, respiration, and decomposition in terrestrial plant communities consume and produce large amounts of carbon dioxide (CO2) and play an important role in the global cycle of carbon. Recent studies have suggested that satellite remote sensing may be useful in providing the extensive data sets needed to quantify these processes over large areas, but many of the factors which potentially limit the accuracy of the analysis in natural vegetation have not been examined experimentally. We have conducted field experiments relating spectral reflectance to intercepted photosynthetically active radiation (PAR) and net CO2 exchange in a natural canopy composed of the tidal wetland grass Spartina alterniflora (marsh cordgrass). Reflectance measurements made by a hand-held radiometer with Landsat Thematic Mapper spectral wavebands were used to compute remote sensing indices such as the normalized difference vegetation index (NDVI). Net CO2 exchange was measured in a clear, climate-controlled chamber placed over square meter plots of the canopy. The NDVI was near-linearly related to the proportion of solar PAR intercepted by green plant material. The relationship was not as strong as has been reported for growing agricultural canopies, but the regression model form persisted throughout the growing season (March through October) and was applicable to a wide variety of growth forms exhibiting different proportions of green foliage. NDVI was also near-linearly related to net CO2 exchange rates throughout the growing season and in plots subjected to varying levels of chronic stress. However, significant uncertainty in the relationship was produced by changes in photosynthetic efficiency in response to varying environmental conditions. Estimates of CO2 exchange based on NDVI were improved if the effect of ambient air temperature on net photosynthesis was assessment of photosynthesis and net gas exchange in natural vegetation is feasible, particularly if the analysis incorporates information on biological responses to environmental variables.