[MCN] Drought kills trees directly, but precisely how???

Lance Olsen lance at wildrockies.org
Mon Aug 21 11:54:11 EDT 2017

Excerpt from National Science Foundation news release: “In some cases, the deaths are a result of carbon starvation, in which trees close their pores, essentially starving themselves by blocking the entry of carbon, which is needed for photosynthesis. Or the culprit is hydraulic failure: the inability of a plant to move water from roots to leaves.

"Adams explains that 99 percent of the water moving through a tree is used to keep stomata open. Stomata are the pores that let in carbon dioxide, allowing a tree to carry out photosynthesis.

"Trees respond to the stress of drought by closing these pores. They then need to rely on stored sugars and starches to stay alive, and will die if these run out before a drought ends.

"If a tree loses too much water too quickly, an air bubble (embolism) forms. The tree then has hydraulic failure and cannot transport water from the roots to the leaves, causing it to dry out and die.

"The scientists found that hydraulic failure is universal when trees die, while carbon starvation is a contributing factor roughly half the time."

Full NSF release:

“Serious thresholds are crossed when forests convert to vegetation types without trees 
and, as a result, lose valued forest ecosystem services. ”
Constance I. Millar and Nathan L. Stephenson. Temperate forest health in an era of 
emerging megadisturbance. Science  21 August 2015
"… the race between climate dynamics and  climate policy will be a close one …."

Hans Joachim Schellenhuber. “Global warming: Stop worrying and start panicking?” 
Proceedings of the National Academy of Sciences 2008

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