Old growth trees in western parts of the US are probably being killed as a result of regional changes to the climate, a study has suggested.
Analysis of undisturbed forests showed that the trees' mortality rate had doubled since 1955, researchers said.
They warned that the loss of old growth trees could have implications for the areas' ecology and for the amount of carbon that the forests could store.
The findings have been published in the journal Science.
"Data from unmanaged old forests in the western US showed that background mortality rates have increased rapidly in recent decades," the team of US and Canadian scientists wrote.
"Because mortality increased in small trees, the overall increase in mortality rates cannot be attributed to ageing of large trees," they added.
"Regional warming and consequent increases in water deficits are likely contributors to the increase in tree mortality rates."
Water woes
After ruling out a variety of other possible factors, including insect attacks and air pollution, the researchers concluded that regional warming was the dominant contributor.
"From the 1970s to 2006, the mean annual temperature of the western US increased at a rate of 0.3C to 0.4C per decade, even approached 0.5C," they observed.
"This regional warming has contributed to widespread hydrological changes, such as declining fraction of precipitation falling as snow, declining snowpack water content, earlier spring snowmelt and a consequent lengthening of summer drought."
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