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| Dying forest in northern Canada |
By Justin Gillis, The New York Times, November 23, 2012
One of the great scientific tasks of the day is to
understand how and why trees die. It may seem like a question that would have
been answered many decades ago, but it was not — at least not at a detailed
physiological level. Now, amid growing signs worldwide that forests are at risk
as the climate changes, scientists are trying to catch up to events.
Lately, more and more evidence is pointing toward a
mechanism known as hydraulic failure as the culprit in many large-scale forest
die-backs. This occurs when drought reduces the flow of water into tree roots.
The trees take measures to limit the loss of water through their leaves, but
trees need water flowing through them as much as humans need blood. Eventually,
if the drought is bad enough, the tiny tubes that carry water up the trunk of
the plant can fill with air bubbles.
Detailed understanding of this mechanism may still be
developing, but anybody who has forgotten to water a house plant has seen the
consequences. The flow of water through the body of the plant is interrupted,
and unless moisture is restored to the soil, it can droop and eventually die.
Now comes a surprising new paper from an international
research team presenting ominous findings about the risks to forests from
global warming and its accompanying water stress.
For the study,
released online on Thursday by the journal Nature, Brendan Choat of the University of Western
Sydney in Australia, Steven Jansen of Ulm University in Germany,
and a large group of their colleagues compiled data from 226 forest species at
81 sites worldwide. They found that around 70 percent of the species operate
with only a narrow margin of safety when it comes to their water supply. In
other words, many of the world’s important forest species are vulnerable to
hydraulic failure.
In effect, the trees have adopted an aggressive
evolutionary strategy, creating robust water-moving machinery that allows them
to grow quickly and out-compete other trees during times of adequate rainfall,
but putting them at risk of dying when water is scarce.
That means that virtually all types of forests, even in
regions that seem to get plenty of rain today, are vulnerable to increased
drought and increased evaporation driven by higher temperatures. If the changes
in rainfall and soil moisture in coming decades turn out to be as big as many
scientists fear, the Choat-Jansen paper implies that the result could be
massive die-backs, shifts in the composition of forests, and a transition from
forest to grassland in many regions.
That may sound alarmist, but a developing body of
evidence suggests that it is already starting to happen. Last year, for
example, I wrote about the large forest die-backs that
are being seen in the American West and the Pacific Northwest because of
mountain pine beetles, an insect pest that is moving farther north because of
global warming.
We are also seeing huge impacts on forests from water stress in the
Mediterranean, the Amazon and many other regions.
William R.L. Anderegg, a Stanford University
researcher who was uninvolved in the new paper but is doing related work, told me he saw the new research
as “a major step forward” in gaining a more complete global understanding of
the risks to forests from climate change.
The new paper “tells us that many, many tree species
live close to the dry edge of what they can tolerate, even if they live in a
very wet area,” Mr. Anderegg said in an e-mail. It makes evolutionary sense, he
added, because “no matter your environment as a tree, you would want to
maximize your growth in order to compete with other trees, while still narrowly
avoiding death from water stress. The practical and critical outcome of this is
that trees and forests, globally, appear to all be relatively vulnerable to
drought-induced mortality.”
Climate change puts at risk not only the rich diversity
of life in the world’s forests, but also the ability of those forests to suck
carbon dioxide out of the air, as they do today in immense volume, helping to
limit global warming. In other words, if forests start dying from global
warming, that means the warming will get worse, presumably killing more forests
— a dangerous feedback loop.
“The consequences of longer droughts and higher
temperatures are potentially dramatic,” Dr. Choat, Dr. Jansen and their
colleagues wrote in the new paper. “For example, rapid forest collapse as a
result of drought could convert the world’s tropical forests from a net carbon
sink into a large carbon source during this century.”
The big question now is how much ability the world’s
trees have to adapt. In theory, one might imagine that young trees growing
under drought stress would shift their architecture in ways that would limit
their risk. But whether they really have the genetic capacity to do this, or to
do it quickly enough to keep up with the rapid climatic shifts projected for
coming decades, is an open issue.
A distinct possibility, the scientists wrote, is that
“the rapid pace of climate change may outstrip the capacity of populations to
adapt.”
In a commentary accompanying the paper, Bettina
M.J. Engelbrecht of the University of Bayreuth in Germany, who was not involved
in the research, writes that the accumulating scientific evidence sounds “a
warning bell that we can expect to see forest diebacks become more widespread,
more frequent and more severe — and that no forests are immune.”
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