( A team led by a University of Minnesota...)Of mice, men, trees and the global carbon cycle

A team led by a University of Minnesota researcher has found a universal rule that regulates the metabolism of plants of all kinds and sizes and that may also offer a key to calculating their carbon dioxide emissions, a number that must be known precisely in order to construct valid models of global carbon dioxide cycling.

Emissions of the gas occur in both plants and animals through the process of respiration; Peter Reich, a professor of forest resources, and his colleagues have found that plant emissions can be deduced from the nitrogen content of any plant. The study also reveals that the respiration, or metabolic, rates of plants and animals follow different laws of scaling with respect to body size. The work will be published in the Jan. 26 issue of the journal Nature.

In revealing nitrogen content as the key to plant metabolic rates, the work uncovered a fundamental difference between plants and animals in how their metabolism varies with size. The larger an animal, the slower its metabolism on a per-weight basis. Thus, although an elephant burns many more calories per hour than a mouse, the mouse has a much higher rate per pound of body weight. An elephant with the same rate per pound as a mouse would generate so much heat it would have serious problems maintaining body temperature and eating fast enough to keep up. Instead of a one-to-one ratio between body size and metabolic rate, as an animal's body weight quadruples, its respiration rate only triples.

In contrast, when Reich and his colleagues studied 500 plants from 43 species, they found that within a wide range of plant sizes, a quadrupling of weight leads to a quadrupling of respiration rate. The important variable was nitrogen content: The more nitrogen in a plant, the more it respired and the more carbon dioxide the plant emitted. Similarly, if two plants were the same size but had different concentrations of nitrogen in their tissues, the one with the higher nitrogen concen
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Source:University of Minnesota

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