From Water Deeply (Jane Braxton Little):
New research into how drought kills trees has helped reveal a potentially huge climate consequence of an increase in dead and dying forests that one scientist cautioned could result in a “carbon death spiral.”
“If we understand the mechanisms of how trees die, we can make predictions and prioritize areas for management,” he said.
First we need to understand how healthy trees function. They get water from the soil, pulling it up through their roots through xylem, one of two specialized transportation tissues in vascular plants. The water flows into tree trunks and branches to leaves or needles. Sunlight turns the nutrients in water to sugar and sap, which nourish new growth.
When there is less water in the soil, trees pull harder. When they pull too hard they break the column of water in the xylem, creating air bubbles known as embolisms. This causes “hydraulic failure.” And this is what killed all of the trees studied by the research team of 62 scientists, said Adams.
But another process also causes trees to die when their response to drought deprives them of carbon.
“Trees aren’t stupid,” Adams said. To prevent losing precious water to the atmosphere through their needles and leaves, trees close the pores scientists call stomata. That keeps the water in the tree, but stomata do more than prevent water loss. They also let carbon dioxide in, allowing trees to carry out photosynthesis.
When trees close their stomata to wait out a drought, they are no longer making any food through photosynthesis. Instead, they are relying on stored sugars and starches. If the drought lasts long enough, they use up everything they have stored, and they die.
Carbon starvation contributed to tree mortality in at least 60 percent of the cases the researchers studied. They still don’t understand the critical threshold for carbon starvation but know it plays a significant role in trees dying.
Scientists also know bark beetles contribute to tree mortality. That sticky resinous sap used to pitch out beetles is made from carbon. When trees become carbon-starved they are not only unable to metabolize the nutrients for survival; they are also unable to muster the carbon they need to defend themselves against beetles.
The connection between physiological stress caused by drought and defending against bark beetle attack is still an active area of research, Adams said. Entomologists have tended to approach tree mortality in the West through beetles, while physiologists have come at it from the drought perspective…
He and his colleagues analyzed drought-induced mortality in 26 tree species. They examined data from 19 recent experimental and observational studies from around the globe. The research will help scientists and land managers more accurately predict how trees will respond to environmental stresses that include insect damage and disease as well as drought, said Lina Patino, a section head at the National Science Foundation’s Division of Earth Sciences, which cofunded the study.