Trees and other vegetation take longer than expected to recover from extreme drought, thus storing less carbon dioxide and accelerating global warming, a study said Thursday.

Previous climate models assumed that forests bounced back rapidly after periods of severe water shortage, but a report in the US journal Science found that trees actually took two to four years to resume normal growth rates after droughts ended.

During this prolonged recovery period, forests can store less carbon dioxide than completely healthy forests, according to the study.

The amount of time needed to recover after a drought was previously unknown.

Trees and other plants play an important role in stemming the effects of climate change by storing carbon dioxide and other harmful emissions released by human activity.

"If forests are not as good at taking up carbon dioxide, this means climate change would speed up," said study lead author William Anderegg.

Scientists warned that climate impacts could be detrimental, with more and harsher dry spells predicted.

"This really matters because in the future droughts are expected to increase in frequency and severity due to climate change," said Anderegg, who is also an assistant professor of biology at the University of Utah.

"Some forests could be in a race to recover before the next drought strikes."

The study warned of the impact of higher temperatures.

"The fact that temperatures are going up suggests quite strongly that the western regions of the US are going to have more frequent and more severe droughts, substantially reducing forests' ability to pull carbon from the atmosphere," Anderegg said.

- 'Lethal drought' -

The impacts in some parts of the United States have already been made clear.

Following a drought in the southwestern part of the country from 2000 to 2003, temperatures were three to six degrees Fahrenheit (1.6 to 3.3 Celsius) warmer than the long-term average, according to the study.

"It really seemed to make the drought lethal to vegetation where previous droughts with the same rainfall deficit weren't," Anderegg said.

The researchers examined the recovery of tree stem growth after severe drought since 1948 at 1,300 forest sites globally.

They looked at the International Tree Ring Data Bank to determine the history of wood growth and track carbon uptake of the surrounding ecosystem.

The researchers found that in most forests -- and especially in arid ecosystems -- trees "struggled for years after experiencing drought."

Forests in parts of California and the Mediterranean region were the only exceptions.

Growth was about nine percent slower than expected in the first year, and five percent slower than expected in the second year.

The delay has major implications. Over the course of a century, carbon storage capacity in semi-arid ecosystems could drop by 1.6 billion tonnes of carbon dioxide.

That's more than the total energy-related carbon emissions the US produces in a year.

There were 32.3 billion tonnes of carbon dioxide emissions in the world last year, according to the International Energy Agency.

The researchers were comparing data to virtual climate models.

It is not known how droughts may cause long-lasting harm, though it could be from loss of foliage and carbohydrate reserves that impair growth, according to the study.

Pests and diseases can also accumulate in drought-stricken trees.

As a third hypothesis, the researchers said lasting damage to vascular tissues could impair water transport.