Site‐level evaluation of satellite‐based global terrestrial gross primary production and net primary production monitoring

Abstract

Operational monitoring of global terrestrial gross primary production (GPP) and net primary production (NPP) is now underway using imagery from the satellite-borne Moderate Resolution Imaging Spectroradiometer (MODIS) sensor. Evaluation of MODIS GPP and NPP products will require site-level studies across a range of biomes, with close attention to numerous scaling issues that must be addressed to link ground measurements to the satellite-based carbon flux estimates. Here, we report results of a study aimed at evaluating MODIS NPP/GPP products at six sites varying widely in climate, land use, and vegetation physiognomy. Comparisons were made for twenty-five 1 km 2 cells at each site, with 8-day averages for GPP and an annual value for NPP. The validation data layers were made with a combination of ground measurements, relatively high resolution satellite data (Landsat Enhanced Thematic Mapper Plus at $ 30 m resolution), and process-based modeling. There was strong seasonality in the MODIS GPP at all sites, and mean NPP ranged from 80 g C m À2 yr À1 at an arctic tundra site to 550 g C m À2 yr À1 at a temperate deciduous forest site. There was not a consistent over-or underprediction of NPP across sites relative to the validation estimates. The closest agreements in NPP and GPP were at the temperate deciduous forest, arctic tundra, and boreal forest sites. There was moderate underestimation in the MODIS products at the agricultural field site, and strong overestimation at the desert grassland and at the dry coniferous forest sites. Analyses of specific inputs to the MODIS NPP/ GPP algorithm -notably the fraction of photosynthetically active radiation absorbed by the vegetation canopy, the maximum light use efficiency (LUE), and the climate datarevealed the causes of the over-and underestimates. Suggestions for algorithm improvement include selectively altering values for maximum LUE (based on observations at eddy covariance flux towers) and parameters regulating autotrophic respiration.

Monitoring Grassland Seasonal Carbon Dynamics, by Integrating MODIS NDVI, Proximal Optical Sampling, and Eddy Covariance Measurements

“…While these models can depict broad global patterns, their agreement with local field measurements varies considerably across ecosystems or with different model formulations [67,68]. In fact, the selection of input to key parameters (e.g., PAR, fAPAR, APAR, ε) or their precise definitions (e.g., whether APAR and f APAR are based on total or green canopy material) can broadly affect the results [34,69].…”

Remote estimation of crop gross primary production with Landsat data

“…The current MODIS GPP algorithm provides reasonable spatial patterns and logical temporal variability across a diverse range of biomes and climate regimes. However, continued efforts are needed to resolve significant problems in certain biomes, especially in croplands, where an accurate MODIS estimation of GPP is still elusive (e.g., Heinsch et al, 2006;Turner et al, 2005Turner et al, , 2006. A recent evaluation of satellite-based MODIS products revealed that the estimates of GPP and net primary production (NPP) were particularly poor for maize and soybean test sites (Turner et al, 2005).…”

“…However, continued efforts are needed to resolve significant problems in certain biomes, especially in croplands, where an accurate MODIS estimation of GPP is still elusive (e.g., Heinsch et al, 2006;Turner et al, 2005Turner et al, , 2006. A recent evaluation of satellite-based MODIS products revealed that the estimates of GPP and net primary production (NPP) were particularly poor for maize and soybean test sites (Turner et al, 2005). Even though the annual NPP for the maize and soybean fields was among the most accurate available ancillary data of all biomes studied, the uncertainties of the MODIS-derived estimates for the agricultural sites were the highest; the MODIS product strongly underestimated NPP.…”

Woody plant encroachment into grasslands leads to accelerated erosion of previously stable organic carbon from dryland soils

“…Primary productivity may also change, with implications for the dynamic replacement or depletion of SOC in landscapes undergoing erosion [Berhe et al, 2007;Nadeu et al, 2012]. Previous research has shown no significant change in annual net primary following creosote shrub encroachment but an Journal of Geophysical Research: Biogeosciences 10.1002/2014JG002635 increase in piñon-juniper woodland [Turner et al, 2005;Muldavin et al, 2008]. Gaseous CO 2 fluxes show a large degree of interannual variability, being highly responsive to meteorological variables [Mielnick et al, 2005] but with Emmerich [2003] also showing a higher CO 2 flux from woody-dominated sites.…”

“…Nonetheless, it is only one component of the ecosystem C cycle. Woody encroachment into drylands will also alter biomass [Fang et al, 2001;Asner et al, 2003] with an increase in aboveground biomass being reported for piñon-juniper woodland [Turner et al, 2005] but a decrease reported for creosote shrubland [Cross and Schlesinger, 1999]. Primary productivity may also change, with implications for the dynamic replacement or depletion of SOC in landscapes undergoing erosion [Berhe et al, 2007;Nadeu et al, 2012].…”