Over the course of the 20th century, the so-called water use efficiency has risen nearly 20 percent from the increase in atmospheric CO2 concentrations.

Trees take up carbon dioxide from the air through tiny pores on their leaves called stomata and they lose water through these same pores.

When the CO2 concentration in the air increases, the size of the stomatal opening reduces to regulate the amount of carbon acquired which minimises the water lost. As a result, the so-called water use efficiency increases, the researchers explained.

"The observed water use efficiency increase, in response to atmospheric CO2 increase, is something we are able to reproduce with global vegetation models giving us more confidence in the whole ecosystem response to CO2," said one of the authors of the report Pierre Friedlingstein from University of Exeter in Britain.

In this study the researchers used measurements of carbon from tree-rings and computer models to quantify tree and forest responses to both climate variation and increased atmospheric CO2 concentrations.

"Tree-ring data provide one of the unique opportunities to obtain long-term records of ecosystem responses to climate change," David Frank from University of Bern in Switzerland noted.

The researchers used data from 23 tree ring sites spanning Morocco to Norway to quantify variation in water use efficiency -- the amount of water required to produce a given amount of carbon - and a basic measure of plant and ecosystem economy.

The study appeared in the journal Nature Climate Change.