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published in the 2002 State of the Nation’s Ecosystems Report as well as the 2003 and 2005 Web-Only Updates have been superseded by the 2008 Report and thus should be used with caution.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.

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During 2004, the plant growth index nationwide was equal to the 15-year average. The index was somewhat below average in the Pacific and Rocky Mountain regions, higher than the 15-year average in the Northeast/Mid-Atlantic and Southeast, and about average in the remaining regions.

Discussion The energy brought into an ecosystem is an overall measure of its performance. How much energy a system absorbs can be affected by factors such as climate and weather, pollution, and how farms, forests, and other areas are managed, to name a few. Long-term changes in the amount of energy absorbed can have significant implications for the way an ecosystem functions.

Some ecosystem types naturally capture more energy than others; that is, they are more productive. Rather than comparing the absolute amount of energy captured, the plant growth index compares each year’s growth at a particular location with the long-term average at that location.

Given natural year-to-year variability, the 15 years for which data are available are not enough to determine whether there are any regional or system-specific trends (data for 1994 are not available because of satellite failure). The particular satellite measurement used for this analysis, Normalized Difference Vegetation Index (NDVI), correlates well, but by no means perfectly, with ground measurements of plant productivity. Values are calculated every two weeks and summed over the entire growing season.

Data for this measure are available only for the land area of the lower 48 states. The Coasts and Oceans section of this report includes a measure related to productivity of algae in coastal waters, but that indicator focuses on seasonal peaks rather than annualized measurements, as reported here. In addition, it is possible to measure the plant growth or productivity of freshwater lakes, but these data are not available on a consistent basis nationwide.