Note that the data published in the 2002 State of the Nation’s Ecosystems Report as well as the 2003 and 2005 Web-Only Updates have been superseded by the 2008 Report and thus should be used with caution. For the most recent data, purchase the 2008 Report from Island Press.

The Indicator

The plant growth index is based on data collected by the Advanced Very High Radiation Radiometer (AVHRR) aboard the National Oceanographic and Atmospheric Administration’s (NOAA) polar-orbiting satellites. Each 1.1 km² mapping area (pixel) has been measured twice a day. Daytime measurements in the visible wavelengths (0.58–0.68 m) and near-infrared wavelengths (0.725–1.1 m) are transformed into a Normalized Difference Vegetation Index (NDVI), which has a near-linear relationship to absorbed photosynthetically active radiation for a given land cover type. NDVI also correlates well with net uptake of carbon dioxide and plant biomass production.

For this indicator, NDVI is calculated at 2-week intervals and summed throughout the growing-season; only values that exceed non-growing-season, background NDVI are included. Growing season start dates, end dates, and background NDVI were calculated for each land cover type and region. (For a detailed explanation of calculating growing-season accumulated NDVI, see Reed and Yang 1997).

Because the relationship between NDVI and absorbed photosynthetically active radiation varies by cover type, the growing-season accumulated NDVI was calculated separately for the forest, farmland, and grassland/shrubland areas in each county of the conterminous 48 states, for each year between 1989 and 2004 (except for 1994, when the satellite failed). The 15-year average growing-season accumulated NDVI was also calculated for each of the three land cover types in each county. The values in each county segment for each year were then normalized by using the corresponding 15-year average for that county segment to produce a plant growth index where a value of 1.0 equals the long-term average. Areas with plant growth indices greater than 1.0 have higher-than-average accumulated NDVI; within the same cover type and in an area as small as a county, this implies higher-than-average plant growth for that year. The regional and system-specific plant growth indices are the area-weighted averages of the segments contained within the region and system.

Land cover for each 1.1 km² pixel for the growing season was identified from the National Land Cover Dataset. These data are derived from the Multi-Resolution Land Characteristization Consortium, which is a partnership between the U.S. Geological Survey, the U.S. Forest Service, NOAA, and the Environmental Protection Agency. (See the Core National Indicator: Ecosystem Extent technical note for more details.)

The Data

Data Sources: Data on accumulated NDVI and analysis of those data are from the USGS’s Center for Earth Resources Observation Science (EROS), Sioux Falls, South Dakota (see http:edc2.usgs.gov/phenological/).

Data Quality/Caveats: In 2000, the NOAA-14 orbit drifted to a late afternoon overpass time. The effects of this on the plant growth index have yet to be fully understood. However, because the index is accumulated from the beginning of the growing season—a point that is identified each year from the inherent seasonal patterns in the NDVI data—scientists at EROS believe the 2000 estimates are comparable to those of previous years.

The Data Gaps

Data for 1994 are unavailable because of satellite failure. Data are available only for the land area of the conterminous 48 states. The Coasts and Oceans chapter of this report includes an indicator of chlorophyll concentrations in coastal waters, a measure related to algal growth. That measure is based on maximum rather than accumulated concentrations, and thus is not directly comparable.

2003 Web Site Update: Data were provided for this update by USGS's EROS. Analysts at EROS have developed a protocol for removing the influence of water vapor on NDVI data. The water vapor reduces the observed near infrared reflectance observed at the satellite. In addition, the longer path length from the sun - to the surface - to the satellite, the greater the effect that water vapor has on lowering the observed near-infrared reflectance measured at the satellite. NVDI values computed from atmospherically corrected data are generally higher (greener) than NDVI from uncorrected data. This is due primarily to the effect of water vapor on AVHRR channel 2. Hence a composite produced from corrected daily observations will have a different characteristics than a composite produced from uncorrected data, most notably a higher average NDVI for each composite.

For this reason, the entire time series was re-analyzed. There was a substantial change in the time series due to re-analysis.

2005 Web Site Update: Data for 2003-2004 were provided for this update by EROS.

References

B.C. Reed and L. Yang. 1997. Seasonal vegetation characteristics of the United States. Geocarto International 12(2):65–71.)