Australian researchers have found that climate change is having a bigger impact on the Amazon rainforest than first thought.

A joint study conducted by the Australian National University (ANU) shows that as soil under the rainforest canopy dries, air bubbles enter the internal plumbing of the taller trees, making it difficult for nutrients to get through to the leaves.

The Amazon rainforest in Brazil, the largest on earth, helps offset the carbon dioxide in the atmosphere, but global warming is starting to take a toll on its effectiveness, according to Professor Meir, from ANU Research school of Biology.

"With droughts in some regions predicted to intensify and lengthen with global warming, the increased death of large trees in rainforests could significantly increase carbon dioxide emissions from Amazonia, and influence the global carbon cycle," Meir said on Tuesday.

The project, which was conducted over 13 years, is the longest-running ecosystem-scale experimental drought study in a tropical rainforest.

It found that the older, taller trees are the ones most affected by drought.

"The tallest trees appear to be most vulnerable to this process. Smaller trees are more likely to survive," Meir said.

The researchers initially thought that lack of water might prevent the trees from producing enough sugars to survive. But they found that they all had similar sugar levels, with the taller trees' faulty plumbing preventing the sugars from reaching the extremes, leading to their premature death.

This would add unwanted amounts of carbon dioxide to the atmosphere as the dead trees decomposed, according to the first author of the study, Lucy Rowland, of the University of Edinburgh's School of GeoSciences.