Prolonged droughts are weakening trees and causing more of them to die in forest fires in the western United States, a study indicates.

Trees are being weakened as climate change warms and dries out the U.S. West, making them less able to cope with the effects of fire, the study authors said.

"What we're interested in is trying to understand what the climate might be in relation to a fire event," research leader Phil van Mantgem, an ecologist with the U.S. Geological Survey, told LiveScience.

Previous studies have show a temperature increase of as much as 0.9 degrees Fahrenheit per decade since the late 1980s in high altitudes of the western United States.

Fires have been increasing during the same time period as higher air temperatures, coupled with lower humidity, makes forests drier and better fuel for wildfires, climatologists said.

"A lot of [previous] studies were trying to understand, or make predictions, about what the fire regime will look like going forward with climate change, but they were focused on the physical aspects," Van Mantgem said. "We were interested more in the biological context of this."

Analyzing tree fire injuries and fire-related deaths between 1984 and 2005 in controlled burns by the National Park Service, the researchers found a tree burned in dry conditions was more likely to die than a tree burned in wet conditions.

The researchers say they believe xylem tubes -- which carry water from the ground through the tree -- are damaged by the fire so that water cannot be carried efficiently to nurture the tree.