Net Primary Production
Primary production is the rate of organic biomass growth or accumulation by plants. Primary production is commonly split into two components, gross primary productivity (GPP) and net primary productivity (NPP). Gross primary productivity is the overall rate of biomass production by producers, whereas net primary productivity is the remaining fraction of biomass produced after accounting for energy lost due to cellular respiration and maintenance of plant tissue. Thus,
NPP = GPP – respiration.
NPP is an important component of the global carbon budget and is used as an indicator of ecosystem function. NPP can be directly assessed by measuring plant traits or harvesting plant material on the ground, but across large areas remotely sensed images can be used to estimate NPP. NPP is often calculated as a product of fPAR (fraction of photosynthetically active radiation) and light use efficiency (also called radiation use efficiency). Common inputs to NPP models include land cover, phenology, surface meteorology, and leaf area index (LAI).
NPP models for remote sensing vary widely depending on the ecosystem type and desired output, but there are two main ways to estimate vegetation productivity using remotely-sensed images:
See Lu et al. (2006) for a review of various approaches to biomass estimation using remotely sensed data.
These methods require correction for atmospheric variation and sometimes require bidirectional reflectance normalization. The input images are composited over multiple days (i.e., the value for any given pixel in the final image is taken from the highest-quality readings for that pixel across multiple images) to minimize the impact of atmosphere and screening by clouds or snow. The relationship between satellite measures of reflectance and estimates of NPP will vary depending on the type of vegetation being considered, and thus major land cover type is an important input to calculating NPP.
NPP models produce a map of vegetation biomass for a particular spatial and temporal resolution determined by the input data. The nature of the information depicted on the map (e.g., whether the map shows only live biomass or also includes senescent biomass, the units used, etc.) depends on the NPP model chosen.
Net primary productivity estimates have been used for many purposes:
Specific rangeland applications include:
Remotely sensed NPP estimates are only approximations of true biomass values. The mathematical models used to calculate NPP vary widely, and each model contains assumptions and requires specific inputs. It is important to understand the model assumptions and assess the suitability of the model based on the available data, how well the model characterizes the vegetation compared to field measurements, and the desired output. Most models work optimally at a particular scale and in a particular ecosystem type, and the application of an existing model to a new location may require changes to the model. NPP is often derived from spectral vegetation indices, such as NDVI, but there is no single equation with a set of coefficients that can be applied to images of different surface types. Estimation of NPP by satellite imaging requires corrections for atmospheric effects, topography and diurnal variations, and values change rapidly throughout the season with changing phenology. NPP estimates from visible/near-infrared images require a cloudless, clear image, and thus NPP values are typically chosen from the best quality images over a multiple day period (often an 8 or 10-day window). For areas that are continually cloudy, the use of radar or lidar may be necessary to assess vegetation characteristics.
The remote sensing data inputs for NPP will vary depending on the method that is used to estimate it. Generally, an image with visible and near infrared bands is required. If an empirical approach is being used, ground measurements will be needed in order to derive estimates of NPP.
Calculating NPP requires image processing and statistical/mathematical modeling software.
Map of global net primary productivity (g C m-2yr-1) from the International Global Biosphere Programme
Net primary productivity map of the Sahel from MODIS satellite images (source Fensholt et al. 2006)
High correlation (R2=0.91) between the predicted NPP from NASA-CASA models based on EVI (enhanced vegetation index) and field measurements of NPP (source: Potter et al. 2007)
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