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Drought Puts Trees the World Over ‘At the Edge’

As the climate warms, scientists expect an increase in droughts around the world, causing all sorts of problems for water supplies, agriculture, and energy production. Forests will be affected too, naturally, but a new study released Wednesday in Nature shows just how widespread the effects could be.

After looking at 226 tree species at 81 locations around the world, two dozen experts from around the world have determined that fully 70 percent of trees are likely to suffer if conditions get drier — and it doesn’t matter whether those trees live in wet or dry habitats.

Aftermath of a drought-fueled fire in a Wisconsin pine forest.
Credit: Wisconsin DNR    

That’s a big surprise, according to Bettina Engelbrecht of the University of Bayreuth, in Germany, who wrote a commentary on the new research in Nature. “I would have expected trees in dry areas would have a wider safety margin,” she said in an interview. “But the majority of species appear to be right on the edge. Just a little more drought will push them over.”

What actually kills a drought-stressed tree is something called “hydraulic failure.” Normally, water evaporates (or more accurately, transpires) from a tree’s leaves or needles through tiny pores called stomata. As the water exits, it creates a suction that pulls more water up from the roots through tiny channels in the trunk, in the woody tissue known as xylem. As Engelbrecht wrote in her commentary, it is “much like sucking water through a straw.”

During a drought, however, the rate of transpiration goes up, increasing the suction, while the amount of available water at the roots goes down. The extra suction can pull air bubbles in through pores in the sides of the channels, and when that happens, the channels become clogged. “How vulnerable the tissue in a particular species is to the formation of bubbles is a measure of its drought resistance,” Engelbrecht said.

It turns out that while trees in dry areas do have greater drought tolerance in absolute terms than those in wet areas, they still operate on the brink of death. The reason, Engelbrecht explained, “makes compete sense in hindsight.” The bigger the water-carrying channels that pass through the xylem, the bigger the pores in leaves or needles where water escapes. Those bigger stomata give also allow the tree to take in more carbon for photosynthesis and thus grow faster. Faster growth means that the trees with the biggest channels and stomata in both dry and wet areas tend to out-compete those with smaller openings.

Evolution dictates, therefore, that trees will tend to have the biggest possible channels, no matter what conditions they live under. The downside: bigger channels are more prone to air leakage, so even a little extra drought will push the trees into hydraulic failure relatively easily.

“We might have hoped that some forests would be relatively immune to drought, and that we didn’t have to worry so much about those,” Engelbrecht said. “Now we’re finding that we have to worry about all of them.”

Unfortunately, there doesn’t seem to be much anyone can do to prevent forest die-offs in a warming, drought-prone world. “Basically, this tells us that we should keep climate change in check as much as possible,” Engelbrecht said.

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By Gail Zawacki (Oldwick, NJ 08858)
on November 22nd, 2012

Trees are naturally resilient to drought.  They evolved that way - they can’t pick up and move when conditions are adverse.  Of course, climate change induced megadroughts are another story.  However this study completely neglects the well-established fact that plants that absorb toxic ozone pollution are more vulnerable to droughts because their roots shrink.  It also ignores the fact that trees in the UK are dying off at absolutely dizzying rates and that area isn’t dry, if anything it’s wetter.  Well, they blame it on a “tidal wave of pathogens” from imported plants - ignoring the fact that English arborists have been enthusiastic importers of exotic trees and other botanical treasures for centuries.

So why all the sudden are trees dying off by the millions - all species of trees?  Perhaps it is because plants injured by ozone lose their natural immunity to insects, disease and fungus.

The Americans like to blame warmer temperatures for the bark bee devastation even though one of the first and most classic case studies proved that pines above the Los Angeles basin were killed by bark beetles because they were weakened by pollution - ozone and NOx deposition.  And it NEVER freezes there!

Trees are rotting inside everywhere around the world, whether dry or wet, old or young.  Their foliage looks like acid was thrown on it - their bark is corroded.

There is more going on than regional climate disruption.

Photos and links to research here: and my book can be downloaded for free here:

There is an excellent earlier book that can be bought used on Amazon: An Appalachian Tragedy.

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By Robert Callaghan (Toronto Canada)
on February 7th, 2013

we are standing on the shore of the showstopper of mass extinction events.
ocean acid caused 4 of 5 mass extinction events.
the oceans are turning into acid 10 faster than earth’s
greatest event 300 million years ago

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