On the US West Coast, forests are dying more quickly, according to a study to be published in the journal Science on Friday.
Tree death rates have doubled since the 1970s, and global warming and stress due to lack of water are leading factors for the development, according to the decades-long study by forestry officials, the US Geological Survey, universities and the National Science Foundation, dpa reported.
The study examined forests older than 200 years old in the western states of California, Colorado, Washington and Oregon and the Canadian province of British Columbia.
Researchers ruled out air pollution and other factors for the increasing death rate.
"Average temperature in the West rose by more than 1 degree Fahrenheit over the last few decades," researcher Phil van Mantgem said. "While this may not sound like much, it has been enough to reduce winter snowpack, cause earlier snowmelt, and lengthen the summer drought."
Warmer temperatures could also contribute to pests and diseases.
The situation was particularly stark in the north-west where death rates in coniferous forests had doubled in just 17 years, compared to 25 years in California.
The death of trees could lead to less carbon dioxide being cleaned from the air, resulting in more green houses and gasses and even higher temperatures.