The research by an international team took a new look at what makes a forest.

Rather than defining it simply as the area covered with trees, the scientists also considered how big the trees were -- how many of them were large enough to be considered timber, also known as growing stock -- how thickly they grew and how much atmospheric carbon was tied up in them.

Releasing carbon dioxide into the atmosphere spurs global warming. Trees absorb carbon dioxide.

In addition, the scientists took into account the amount of organic material, known as biomass, present in the forest.

By this standard, the researchers found that despite widespread concerns about deforestation, the number of timber-size trees increased from 1990 to 2005 in 22 of the 50 countries with the most forest.

The reasons to care about healthy forests are legion: forests foster biodiversity, anchor soil, slow erosion, and, when trees are allowed to grow to timber size, contribute to the economy in the form of lumber and paper.

In the United States, the transition from deforestation to reforestation took place first in the northeastern state of Connecticut in the early 1900s. Some states, including Texas, made the change only in 2002.

FROM DEFORESTING TO REFORESTING

"The United States is doing quite well, but we've done quite well for a period of time," said co-author Roger Sedjo of the Washington-based group Resources for the Future. "Our forests have been more or less stable for the last 100 years."

Other countries made the transition much earlier: Denmark's shift came in 1810, France in the 1830s, Switzerland in the 1860s, Portugal by 1870, Scotland in the 1920s and European Russia in the 1930s.

Sedjo said in a telephone interview that the transition often comes when countries begin to prosper, and are better able to put policies in place that preserve forested land.

Almost every country with a per capita gross domestic product over US$4,600 has moved to reforestation, the study found.

Why are the forests returning? Co-author Paul Waggoner of the Connecticut Agricultural Experiment Station noted a set of events that seem to feed on each other.

When countries protect forests, they can grow; at the same time, when farmland is preserved, farmers are less likely to encroach on forests, Waggoner said in an answer to e-mailed questions.

In Europe, timber imports, energy technology, and economic development that sent country people to the cities also played a role, Waggoner said.