Tree mortality doubled in just 17 years in the Pacific Northwest and 25 years in California. Mortality rates in states farther inland took 29 years to double. The U.S. and Canadian researchers from a variety of agencies and universities studied trees in old-growth forests for more than 50 years to document the die-off, which they say is beginning to outpace replacement by new trees. The report was published in the journal Science.

The study was conducted by a team of 11 researchers from institutions including the USGS and the Forest Service; the University of British Columbia in Vancouver; the University of Washington at Seattle; Northern Arizona University; Oregon State University; the University of Colorado at Boulder; and Pennsylvania State University.

The team spotted the rising death rate trend across a wide range of ages and species in western forest landscapes from regions as diverse as California, Colorado, British Columbia, and Arizona. Temperatures in the region have risen by about 1 degree Fahrenheit during the past 20 years.

“Even in the most resilient old-growth stands, there still is a coherent signal of increased mortality related to warming,” said co-author Thomas Veblen of the University of Colorado.

While no trees are immune, the scientists say, the victims are primarily the conifers whose abundance throughout California's Sierra Nevada range makes the forests famed throughout the world. Varied species of pines, firs and hemlocks are most at risk, they say.

The most likely cause of the increasing deaths, Phillip van Mantgem, a forest expert with the U.S. Geological Survey's Western Ecological Research Center in Arcata (Humboldt County) and a leader of the research team, said is the widespread increase in average temperatures over the past 30 years throughout the region - an increase of a full degree Fahrenheit and an amount consistent with the global warming measurements and models reported by world's experts of the Intergovernmental Panel on Climate Change.

The authors of the new study said in a teleconference that if tree mortality rates continued to increase, it could ultimately cause a drop in the average size of trees because trees would die at younger ages. Smaller trees cannot store as much carbon dioxide as large trees. Habitats could also become less suitable for some species and more welcoming for others, or cause existing species to act in peculiar ways. “Novel behaviors on the part of pests and pathogens are the sort of thing we’ll get surprised by,” Jerry F. Franklin, a professor of ecosystem analysis at the University of Washington and an author of the study.

Higher mortality rates would also mean more dead wood on the ground, which would increase the risk of fire. It's even possible that tree loss could turn some forests into carbon sources, rather than carbon-absorbing "sinks," further feeding the warming cycle. So it’s high time we became more implicated and hope it’s not too late…