Focus Area on Gross/Net Primary Production Product Validation
Arthur Endsley, University of Montana, USA
Álvaro Moreno, University of Valencia, Spain
GPP and NPP Definitions
Gross Primary Production (GPP) is the total carbon (C) taken up by an ecosystem through plant photosynthesis. The C acquired through photosynthesis comes from atmospheric carbon dioxide (CO2) and may ultimately be returned to the atmosphere through cellular respiration or stored long-term in plant tissues, the tissues of consumers of plants, and as dead wood or litter.
Long-term storage is quantified by Net Primary Production (NPP) and represents the initial conversion of CO2 into plant biomass. NPP is the difference between GPP and the total amount of C returned to the atmosphere from plant cellular respiration.
GPP and NPP are the primary input fluxes of C to terrestrial ecosystems. As such, they are used to assess biomass accumulation in forests (i.e., forest C sequestration) and croplands (i.e., crop production and yields). Terrestrial plant production constitutes the fundamental basis of the biospheric carbon cycle, wherein atmospheric CO2 is converted into carbohydrate matter within plants. This plant matter provides humanity with essential resources such as food, fiber, and fuel.
Units: GPP and NPP fluxes are typically measured in terms of a mass of carbon (C) per unit area per unit time; for example, grams of carbon per meter squared per day. A molar flux of C is sometimes used for individual plants, e.g., umol C per second.
Highest Validation Stage Currently Reached for Satellite-Derived GPP and NPP Products
Validation stage 2 (LPV validation stage hierarchy) - The highest LPV validation stage reached for satellite-derived GPP and NPP products.
Validation Good Practices
Validation of NPP generally involves destructive biomass harvesting, based on the assumption of peak biomass (prior to senescence) and no carryover of carbohydrates from one year to the next. There are also methods that rely on empirical relationships (allometry) between non-destructive measures (e.g., of stem diameter, basal area, or leaf-area index) and biomass; when estimates are made over time, the annual increase in biomass is related to annual NPP. Sala and Austin (2000) review these methods:
- Sala, O. E., and A. T. Austin. 2000. Methods of Estimating Aboveground Net Primary Productivity. In Methods in Ecosystem Science, eds. O. E. Sala, R. B. Jackson, H. A. Mooney, and R. W. Howarth, 31–43. New York, NY: Springer New York
For validation of GPP at ecosystem scale, the eddy covariance technique is widely used, as it provides a direct measure of the net exchange of CO2 between an ecosystem and the atmosphere. By assuming that cellular respiration (including soil respiration) can be modeled, GPP is estimated by differencing respiration and net exchange. Reichstein et al. (2005) describes how this is done:
- Reichstein, M., E. Falge, D. Baldocchi, D. Papale, M. Aubinet, P. Berbigier, C. Bernhofer, N. Buchmann, T. Gilmanov, A. A. Granier, T. Grünwald, K. Havránková, H. Ilvesniemi, D. Janous, A. Knohl, T. Laurila, A. Lohila, D. Loustau, G. Matteucci, T. Meyers, F. Miglietta, J.-M. M. Ourcival, J. Pumpanen, S. Rambal, E. Rotenberg, M. Sanz, J. Tenhunen, G. G. Seufert, F. Vaccari, T. Vesala, D. Yakir, R. Valentini, T. Grunwald, K. Havrankova, H. Ilvesniemi, D. Janous, A. Knohl, T. Laurila, A. Lohila, D. Loustau, G. Matteucci, T. Meyers, F. Miglietta, J.-M. M. Ourcival, J. Pumpanen, S. Rambal, E. Rotenberg, M. Sanz, J. Tenhunen, G. G. Seufert, F. Vaccari, T. Vesala, D. Yakir, and R. Valentini. 2005. On the separation of net ecosystem exchange into assimilation and ecosystem respiration: review and improved algorithm. Global Change Biology 11 (9):1424–1439.
Recent papers that present comprehensive validation of GPP and/or NPP estimates include:
- Nelson, J. A., Walther, S., Gans, F., Kraft, B., Weber, U., Novick, K., ... & Jung, M. (2024). X-BASE: the first terrestrial carbon and water flux products from an extended data-driven scaling framework, FLUXCOM-X. Biogeosciences, 21(22), 5079-5115.
- Endsley, K. A., M. Zhao, J. S. Kimball, and S. Devadiga. 2023. Continuity of global MODIS terrestrial primary productivity estimates in the VIIRS era using model‐data fusion. Journal of Geophysical Research: Biogeosciences 128 (9):e2023JG007457.
- Jones, M. O., N. P. Robinson, D. E. Naugle, J. D. Maestas, M. C. Reeves, R. W. Lankston, and B. W. Allred. 2021. Annual and 16-day rangeland production estimates for the Western United States. Rangeland Ecology & Management 77:112–117.
Validation Reference Datasets
- FLUXNET: https://fluxnet.org/
- AmeriFlux: https://ameriflux.lbl.gov/
- Integrated Carbon Observing System (ICOS): https://www.icos-cp.eu/data-products
- National Ecological Observatory Network (NEON) eddy covariance tower data
- The European Eddy Fluxes Database Cluster: https://www.europe-fluxdata.eu/
- Australian and New Zealand Flux Research and Monitoring (OzFlux): https://www.ozflux.org.au/
LPV Focus Areas
Meetings
Land Product Validation and Evolution, June 12 -14, 2023 ESA/ESRIN, Italy.
IGARRS 2023, 16-21 July 2023, Pasadena, CA. Abstract submission due 18 Nov 2022.
EGU 2023, 23-28 April 2023, Vienna, Austria. Abstract submission due 10 Jan 2023.
