Old-Growth Forests Dying Off in U.S. West

Jan. 23 -- THURSDAY, Jan. 22 (HealthDay News) -- Trees in old-growth forests in the Western United States are dying at twice the rate they were a few decades ago, and experts suspect regional warming is to blame.

The report, led by the U.S. Geological Survey (USGS), found that the increase in tree deaths has included trees in a variety of forests, elevations and sizes. Species have included pine, fir, hemlock and other coniferous trees. In addition, the rate of new tree growth has not changed, according to the report in the Jan. 23 issue of Science.

"If current trends continue, forests will become sparser over time, and average tree ages will decrease by half," study co-author Phillip van Mantgem, a USGS research ecologist, said during a teleconference Wednesday.

In the future, forests will store less carbon than they do now, van Mantgem said. "It introduces the possibility that Western forests could become net sources of carbon dioxide, further speeding up the pace of global warming," he explained.

In addition, fewer trees could result in a loss of habitat for animals that depend on old-growth forests, van Mantgem said, and there might also be an increased risk of forest fires, with increasing drought and more fallen trees.

To determine the causes of increased tree death, the researchers considered problems in the forest themselves, such as overcrowding. "Every way we cut the data and examined it, it looks like internal dynamics are not a significant source of the increase in mortality rates," Nathan Stephenson, a USGS research ecologist and co-author of the study, said during the teleconference.

The researchers also looked at external causes, such as air pollution. However, they concluded that these were unlikely causes of the troubling trend, Stephenson said.

"What we were left with was temperature," he said. "Increasing temperature was correlated with the increase in mortality rates."

Rising temperatures in the Western United States have changed weather patterns, Stephenson said. Summers are getting longer, increasing drought conditions. "It is possible that trees are under more drought stress," he said.