GRANTS PASS — Trees in old growth forests across the West are dying at a small, but increasing rate that scientists conclude is probably caused by longer and hotter summers from a changing climate.

While the death rate is not noticeable to someone walking through the forests, it is doubling every 17 to 29 years, hitting levels of 0.5 percent to 1.7 percent a year, and was seen in trees of all ages, species and locations, according to a study published in the Friday edition of the journal Science.

"If current trends continue, forests will become sparser over time," said lead author Phillip J. van Mantgem of the U.S. Geological Survey's Western Ecological Research Center.

"Eventually this will lead to decreasing tree size," he said in an interview. "This is important because it indicates future forests might store less carbon than present. Western forests could be a net source of carbon dioxide, further speeding up global warming."

The rising death rate could also produce a cascading decline in forests that leads to less habitat for fish and wildlife, an increased risk of wildfires, and a vulnerability to sudden forest die-offs, he said.

The likely cause of death for the trees is the increasing average temperature across the West, about 1 degree over the study period, said co-author Nathan L. Stephenson of the U.S. Geological Survey Western Ecological Research Center. That results in greater stress on the trees from lack of water, leaving them vulnerable to disease and insects.

Even if the precipitation remains the same, warmer temperatures mean more rain that runs off than snow that soaks in. Longer summers, typically dry in the West, also mean more moisture in the soil is lost to evaporation.

"So you could conclude that if there is indeed a rising rate of temperature and temperatures continue to increase, very likely mortality rates will continue to rise," Stephenson said.

The study examined data between 1955 and 2007 in 76 research plots in British Columbia, Washington, Oregon, California, Idaho, Colorado and Arizona. The average age of the forests examined was about 450 years, with some as old as 1,000 years. Of the 59,736 trees counted, 11,095 died over the study period. They included trees that were young, old, at high, medium and low elevations, in wet and dry climates, and of a variety of species, including hemlock, pine and fir.

The death rate was highest in California's Sierras, starting at about 0.9 percent in 1980 and rising to about 1.3 percent. It rose fastest in the Northwest, starting at about 0.7 percent in the 1970s and rising to about 1.3 percent. In the Rockies it started at about 0.2 percent in 1955 and rose to about 0.5 percent.

— The Associated Press