Tree death rates could increase globally because of rising temperatures and prolonged droughts linked to climate change, according to multiple studies.
The reasons for tree mortality in a warmer, drier world have been narrowed down to three main scenarios -- greater prevalence of insects and diseases in a warmer world, the drying out of plants, and a third mechanism where water-stressed trees stop photosynthesizing, called carbon starvation.
While researchers are still debating the relative merits of the three, scientists from Los Alamos National Laboratory have pointed to the last scenario as the most relevant for tree mortality.
Researchers have made a concerted effort over the past few years to document tree deaths around the world. With climate models predicting up to a 4-degree-Celsius increase in global temperatures in the coming years, the impact of warmer weather on forests is unknown.
"It is likely that overall warmer temperatures, together with drought, has the potential to kill trees," said Craig Allen, an ecologist with the U.S. Geological Survey.
The ability of plants to absorb carbon dioxide from the atmosphere is a vital component in global climate change talks because forests act as natural sinks that trap carbon from the atmosphere.
Tree death episodes have been noted on all continents since the 1970s because of seasonal and longer-term droughts, according to a recent study co-authored by Allen. The atlas cedar has declined markedly in North Africa following extreme drought. In Russia, 76 million hectares (187.8 million acres) of forests have been labeled as having high risk of health problems. In the United States, pine trees have declined across millions of hectares, and their mortality has been linked to droughts, according to the study.
All these data should allow for modeling of the world's forests with future climate predictions if it were not for a significant problem in the field of plant physiology: No one quite knows what it takes to kill a tree.
What triggers death in a tree?
"Ecologists know how to grow trees, but we don't know how to kill them realistically," said Allen. "We don't know the thresholds of tree mortality."
The particular reasons a plant dies are difficult to isolate. A given tree may first encounter drought stress, then become prone to disease or pest infestation before finally dying. The link between recent tree die-offs and climate change is even more tenuous, except for one study in the western United States.
In 2009, a report by the U.S. Geological Survey in the journal Science revealed that trees were dying out at double the background rate than previously, and the researchers identified climate stress as the most likely contributor to the decline.
"Every year, if people die in your hometown, and then the death rate doubles, you'd notice," said Nathan Stephensen, a co-author of the study.
A 0.3- to 0.5-degree Celsius warming every decade since the 1970s has caused less snowfall, declining snowpack water content, and longer summer drought periods, the study said. Both old and new trees were affected.
Droughts can trigger insect invasions, or drying out of the plant. The 2005 piñon pine decline in the Southwest was initially attributed to bark beetles. But studies by Los Alamos National Laboratory last year revealed a mechanism tied to carbon starvation. The hypothesis suggests that trees die when they literally starve of carbon.
"If plants don't photosynthesize, they will die," said Nate McDowell, a researcher at Los Alamos and a strong proponent of the hypothesis. "The process is universal."
A drought-protective mechanism that causes self-starvation
During times of prolonged drought, trees close tiny pores called stomata on the surface of leaves through which they breathe. The response is triggered as the plant goes into emergency mode to conserve water and prevent evaporation. Since plants also inhale carbon dioxide through their leaves, closing the pores means that the plant starves itself by blocking the entry of carbon, an essential input for photosynthesis.
At the same time, they continue burning carbohydrates during respiration. At one point, the available store of food gets exhausted and the tree starts starving. It can then become vulnerable to insect attacks and other stressors, McDowell said.
"It is the law of conservation of energy which plants must obey," McDowell said. "An analogy is, if we stop earning money but keep spending it, eventually our bank account will hit zero."
Carbon starvation is a reasonable hypothesis when the tree is water-starved, according to scientists. Craig and Allen said the mechanism could act in concert with insect infestation during warmer weather, and drying out of trees to cause declines.
According to Anna Sala at the University of Montana Division of Biological Sciences, the hypothesis requires more evidence before being entirely accepted. She stressed the need to keep all research options open.
"Carbon starvation is very logical, but if it is accepted too easily, it can have a negative effect on the advancement of science," Sala said.