COLLEGE STATION – When it comes to trees' metabolic rates, size doesn't matter. Nitrogen does, said Dr. Mark Tjoelker, Texas Agricultural Experiment Station forestry researcher.

Though seemingly simple, this finding, by Tjoelker and an international team of scientists, has the potential to topple some long-standing theories of how an organism's metabolic rate is inversely related to its size.

An article co-authored by Tjoelker and the team, appeared in this week's edition of the science journal Nature.

The team's findings could have implications affecting a wide range of issues, including the role that carbon sequestration plays in current global warming models, said Tjoelker.

For more than a century, Tjoelker said, biologists have been intrigued by the relationship between an organism's size and the rate at which it consumes energy. For example, a mouse may have to eat half its body weight daily, while an elephant only 5 percent.

"Likewise, a small seedling has a higher metabolic rate than a large redwood tree," Tjoelker said.

This disparity was once thought to be simply due to the ratio of an organism's surface area to its weight. A mouse may be small compared to an elephant, but its ratio of surface area to mass is much higher than an elephant's. Thus, it loses body heat in proportion to its weight faster. Similar simple relationships were posited between mass and metabolic rate in plants and animals. This is called the "three-quarter-power law" from the mathematical formula used to calculate the relationship. Until recently, the three-quarter-power law was considered a fundamental law of nature.

Though the three-quarter-power law seemed to work well within a class of organisms, observations suggested, further investigation showed it broke down across plants and animals, or even mammals and microbes. A scientific debate arose on the "validity and universality of the model," Tjoelker said.

"Some argue that the variation is important to the point of arguing that the three-quarter-power law is not supported; others argue that the three-quarter-power law remains the fundamental starting point," he said.

As a result of the debate, the relatively simple original three-quarter-power law has been expanded beyond mere ratios of bulk and breadth. Now it involves, for example, explanations of how capillary and other vascular networks work in plants and animals. The theorists argued that with more miles of circulatory network, larger organisms take longer to transport nutrients throughout their bodies and thus better utilize them. Poorer utilization means slower metabolic rates. In the case of trees and their effect on carbon dioxide levels in the atmosphere, slower metabolic rates could mean less carbon dioxide released to the atmosphere.

But whatever refinements, the trend has been one of moving toward a sort of unified theory of metabolic ecology that would universally apply to all organisms, flora or fauna.

Enter Tjoelker, worked with Peter B. Reich of the University of Minnesota, Jose-Luis Machado of Swarthmore College in Pennsylvania, and Jacek Oleksyn of the Polish Academy of Sciences.

Tjoelker and the team measured the whole-plant dry mass, nitrogen content and respiration rate of perennial species. They looked at seedlings and saplings of various sizes of trees in the field and in the greenhouse. Data from some grasses and broad-leaved herbs were also included. The plants ranged from 1-month-old greenhouse seedlings to 25-year-old trees. The scientists conducted experiments with temperature, light, nitrogen supply and atmospheric carbon dioxide concentration. In all, more than 500 observations of 43 species were used in the study, yielding what Tjoelker characterized as "conclusive results."

"In contrast to the lack of a single universal relationship between plant respiration and plant size, the data for all plants from all studies, including field and laboratory, are described by a single, common relationship between total respiration and total plant nitrogen content," Tjoelker and the team wrote for the Nature article.

Though his team's research was pure not applied, it's not too much of a stretch to see some practical applications, Tjoelker said.

For example, global warming models are just that: "models." Large-scale studies have been done on the metabolic rates of many species and sizes of trees. But it's still difficult to extrapolate such models to include all trees, all forests on a global or even regional scale, he noted.

Yet with a better understanding of what rules to use for extrapolation, there's an improved chance that the models will more closely mirror the real world, Tjoelker said.

"These findings suggest that plants and animals likely follow different metabolism/size relations, driven by distinct mechanisms," he said.