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What Is This Indicator, and Why Is It Important? This indicator reports a plant growth index, based on satellite measurements of the amount of solar energy absorbed by vegetation and potentially used for photosynthesis.

The index shows, for any given year, whether plant growth in a region or for an ecosystem type was above or below the 15-year average (1989 through 2004, with one missing year). An index value of 1.0 in any year means that the amount of solar energy captured by vegetation and used for photosynthesis in that region or system during that year was the same as the 15-year average.

Plants use energy from the sun to turn carbon dioxide from the air, plus water and nutrients, into plant matter. This process, photosynthesis, drives and sustains virtually all life on earth. The amount of sunlight absorbed by plants is a key factor in determining the amount of photosynthesis and thus the amount of plant growth that occurs in a year. Changes in the amount of energy captured by plants over very large regions, as reported in this measure, may signal significant changes in ecosystem functioning. These changes could lead to increases or decreases in yield of products such as crops or wood and possibly changes in the number and types of species that live in a region. Changes in climate (including temperature and timing and amount of precipitation), as well as factors such as ground-level ozone, increased atmospheric deposition of nitrogen, and increased levels of carbon dioxide, might cause or contribute to changes in plant growth.

What Do The Data Show? No overall trend in plant growth can be seen for this 15-year period, either nationally or within any of the regions or ecosystem types. The similarity in year-to-year variation among regions and systems, however, is striking. For example, in 1993 all regions and systems had higher than average growth index values; in 1996, the opposite was true. The reason for this is not clear.

Year to year variability of the plant growth index is high nationally, within all six regions, and within all the ecosystem types; no overall trends are apparent.