Rain-forest trees exhibit high turnover.

For trees in tropical forests, life has sped up since the 1950s.

Even in areas largely untouched by natural calamities or humans, the annual death rate of these trees has accelerated over the past 45 years, particularly since the 1980s, researchers write in the Feb. 18 SCIENCE. However, the number of trees has remained stable at the 40 sites studied because the pace of new tree growth has also quickened, assert Oliver L. Phillips and the late Alwyn H. Gentry of the Missouri Botanical Garden in St. Louis. Gentry died in an airplane crash in Ecuador last year.

The two estimated annual death and growth rates using data that they and other scientists collected between 1934 and 1993 in Latin America, Africa, and Asia. They calculated the annual turnover at a site by averaging the percent of living trees with trunks over 10 centimeters in diameter and the percent of such trees that had died, Phillips says.

The 19 plots inventoried by researchers during two or more different periods best demonstrate that trees are coming and going faster than before, they say, At nine of these sites, turnover rates more than doubled between the first and last inventories, which spanned 6 to 38 years, says Phillips.

"Phillips and Gentry have discovered a worldwide increase in forest turnover," Stuart L. Pimm of the University of Tennessee in Knoxville and Andrew M. Sugden, editor of TRENDS IN ECOLOGY & EVOLUTION in Cambridge, England, write in SCIENCE.

The increase "is really huge," says ecologist Stephen P. Hubbell of Princeton University, But he warns that some of Phillips and Gentry's data came from forest plots that initially had fewer dead trees than normal. More trees probably died on those plots than on others while they were being studied. This may have inflated slightly the reported increases in turnover, Hubbell says.

The Missouri-based researchers speculate that the increase in carbon dioxide in the world's atmosphere may be causing the trend they uncovered. Carbon dioxide may make some trees, as well as some tree-killing vines, grow more quickly, they write.

Also, they warn that the rapid turnover may decrease biodiversity in the forests. With more trees dying, space opens up in the forest that fast-growing, light-loving trees and vines may quickly fill. Those that grow slowly and like deep shade would fare less well, Phillips says.

The work by Phillips and Gentry "moves us considerably closer to understanding the suspected links between global change and the loss of diversity," write Pimm and Sugden. However, the reasons given for the acceleration neglected some possibilities, such as how changes in global weather patterns affect turnover, they write.

Although Hubbell agrees that increases in carbon dioxide may decrease biodiversity, he adds, "It's highly unpredictable what species will be favored by carbon dioxide increases."