(CNN) -- Scientists have known for some time that climate change and the impacts of longer droughts and higher temperatures could pose a problem for forests. But many thought it would only affect a minority of trees, perhaps just those in extremely arid regions.
However, new research is showing that a large majority of tree species around the world are operating on the brink of collapse. If the predicted pace of climate change continues, many may not be able to adapt in time and large numbers could die off.
The authors of the study, whose findings were published in the scientific journal Nature, looked at 226 different tree species from 81 sites around the world, covering the full range of climatic conditions, from Mediterranean-type arid to the tropical Amazon rainforest.
They found that 70% of the trees studied adapt closely to the local environment, whether arid or tropical, absorbing just enough water in order to survive, but leaving them highly vulnerable to minor shifts in rainfall and drought stress.
"We thought that in the dry areas, plants would have adapted to survive more than ones growing in the wet, but we found they were all equally vulnerable. It was a big surprise," says Steven Jansen, from Germany's Ulm University and co-author of the study.
In periods of drought, the vascular network (xylem) distributing water and nutrients around the tree develops air bubbles which hinder the passage of water. As drought stress increases, these blockages, or "embolisms," accumulate eventually causing the tree to dry out and die.
"We don't want to say all trees are doomed. Some species may migrate to higher locations, but that's not always possible in some regions where there are no highlands or where the forest landscape is already fragmented," Jansen said.
Identifying more drought-tolerant tree species to introduce into forest plantations could help, he says, and trials in Europe have worked on reducing vegetation density by introducing sheep or goats to eat shrubs.
"Less density and competition amongst plants enables trees to tolerate more drought-stress," he says.
For Craig Allen from the U.S. Geological Survey, the study's findings are further "strong evidence" that all forest types globally are at growing risk from climate change.
"It suggests that if temperature projections are at all correct that we are now entering a period that could be called 'the end of old trees', as current tree individuals increasingly die from rapidly growing drought/heat stress, perhaps in wholesale fashion in coming years/decades," Allen says.
Giving evidence at the U.S. Senate in August this year, Allen said temperature rises and drought-stress were leaving trees in the American southwest increasingly vulnerable to insect outbreaks and wildfires.
In the U.S., more than 1.5 million acres of forest in Colorado have been damaged by pine beetles, according to a 2009 report by the U.S. Global Change Research Program. Less severe winters and longer summers are allowing the beetle to thrive, while at the same time weakening trees that struggle in the long periods of drought.
As well as being an important habitat for wildlife, forests are also a crucial carbon sink. They absorb more carbon dioxide (CO2) from the atmosphere than they produce, meaning they help reduce greenhouse gas emissions in the atmosphere and mitigate global warming.
However, drought-stressed trees absorb less CO2 and, if they die or burn in wildfires, release far more CO2 (50% of a tree is carbon).
The result is what scientists call a negative feedback loop. If trees take up less CO2 because they are stressed, then the CO2 levels in the atmosphere will increase, putting more forests at risk.
In 2005, severe drought in the Amazon region saw widespread wildfires that turned the rainforest into a carbon source. Another severe drought in 2010 is still being analyzed but researchers believe it could have been even worse.
"If extreme droughts like these become more frequent, the days of the Amazon rainforest acting as a natural buffer to man-made carbon emissions may be numbered," forest ecologist Simon Lewis, from University College London, has said.
Trees, plants and soils currently absorb approximately one quarter of the CO2 emissions produced by humans, but climate scientists such as Lewis believe that may soon come to an end.
"The free subsidy from nature that forests provide in reducing climate change may be less secure than many think. This sobering conclusion should weigh on the minds of governments as we enter the annual United Nations (U.N.) climate change talks in Doha," says Lewis.
So far, the U.N. discussions have yielded little progress, with major polluting countries unwilling to commit to legally binding cuts in carbon emissions.
This stalemate is also holding up a deal on providing funding for countries to protect forests, known as REDD+ (Reducing Emissions from Deforestation and forest Degradation).
Deforestation and forest degradation already accounts for about 12% of global carbon emissions says the Center for International Forestry Research (CIFOR). But while the majority of countries support the REDD+ proposals, funding and monitoring for projects has still to be agreed.
Louis Verchot, director of CIFOR's Forests and Environments Program has urged larger nations to act on CO2 emissions.
"Without emissions reductions commitments from the big players like the U.S. and China, the big funding won't be coming in for REDD+ anytime soon," Verchot said.
"If these big players are willing to get involved, then other countries will be willing to take on firm emissions reductions and to commit financially."