project description in detail from proceedings of 2011 TERRECO Science Conference GAP
Abstract 2013: The carbon cycle in terrestrial ecosystems has become increasingly important due to its role in regulating CO2 exchange between the atmosphere and the biosphere and potential contributions to global warming. Quantifying CO2 exchange of croplands, which cover 38% of the global terrestrial surface, is made difficult by management activities that create a mosaic with respect to phenological development, e.g., in uptake of CO2 by the vegetation planted and harvested at different times. Local direct measurements of CO2 exchange are, however, useful for evaluations of landscape level fluxes and agricultural yields when they are used to parameterize simulation models.
In this study, we use the physiologically-based model PIXGRO which is parameterized on the basis of CO2 exchange observations from eddy covariance towers at agricultural field sites. The estimated GPP from tower sites is used to derive plant physiological capacity of CO2 exchange (Vcuptake) for crops important in Haean Catchment. Vcuptake changes with phenological development of the crops. Phenology observed via remote sensing, however, reflects changes in both the amount of aboveground chlorophyll (LAI) and in the associated physiology of photosynthesis.
In order to estimate seasonal variation of plant phenology and fluxes for the landscape mosaic, we link Vcuptake to the normalized difference vegetation index (NDVI) from MODIS remote sensing. NDVI together with a land cover map will allow description of crop stand development at many different locations. We aim to relate NDVI to a single model parameter that describes seasonal structural as well as functional change in the carbon uptake capacity of crop canopies and can be effectively used to describe landscape level GPP in Haean Catchment.
The steps carried out in this research are: 1) to utilize existing eddy covariance determinations of GPP to determine seasonal courses of carboxylation capacity (Vcuptake) of major crops grown in the Haean Catchment; 2) estimate for each major crop an approximate fixed seasonal description of canopy physiology; 3) estimate a “mixed“ parameter to account for all remaining seasonal change in carbon uptake capacity (LAIad); 4) relate LAIadj to NDVI at daily time scale; and 5) determine seasonal change in NDVI for the Haean Catchment and estimate GPP with highest possible resolution. Current results demonstrate that the annual pattern of Vcuptake for major crops is both characteristic and different. We are investigating the patterns found in NDVI for forests, dry land farm fields and rice paddies of the Haean Catchment. Development of statistical routines provide MODIS smoothed vegetation indices on a daily time scale. Applied at 250 m resolution, we will obtain a picture of changes in landscape physiology (GPP) in Haean Catchment which can be related to overall carbon and water balances.
Keywords: Carboxylation capacity, CO2 exchange, process-based canopy model, PIXGRO, MODIS, NDVI
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