by Jeff Tietz
From a tree’s perspective, excessive heat may be as deadly as lack of water.
To photosynthesize, a tree opens pores in its leaves called stomata and inhales CO2. Solar-charged chemical reactions then transform the CO2 into carbohydrates — the raw stuff of leaves and wood. During this process, a fraction of the tree’s internal water supply evaporates through its stomata, creating the negative pressure that pulls water from the soil into the tree’s roots, through its trunk and up to its canopy. But heat juices the rate at which trees lose moisture, and that rate escalates exponentially with temperature — so small temperature increases can cause a photosynthesizing tree to lose dangerous amounts of water.
“Forests notice even a one-degree increase in temperature,” says Park Williams at Los Alamos National Laboratory.
In the death scenario, the sky sucks water from the leaves faster than it can be replaced by water in the soil, and the resulting partial vacuum fatally fractures the tree’s water column. If a tree closes its stomata to avoid this, shutting down photosynthesis, it risks starvation.
Ultimately, the tree’s cellular chemistry will fail, but it will often die before that, as its defenses fall; the complexly toxic sap that repels predatory insects dries up.
Many insects can detect diminished sap levels within tree bark by scent — they smell drought stress and pheromonally broadcast news of deteriorating tree health. Other defenses – against microbes, for example — may also be compromised.
A hotter climate generally means more insects.
It also means more, and more intense, wildfires.
For decades, all over the planet, heat-aggravated drought has been killing trees: mountain acacia in Zimbabwe, Mediterranean pine in Greece, Atlas cedar in Morocco, eucalyptus and corymbia in Australia, fir in Turkey and South Korea.
In 2010 a group of ecologists published the first global overview of forest health. They described droughts whose severity was unequaled in the “last few centuries” and documented “climate-driven episodes of regional-scale forest die-off.”
Because global warming outpaces evolutionary adaptation, the question is: Can trees survive as they are?
The conifer forests of the Southwest United States, if climate projections are even minimally accurate, cannot, but what about the rest of the world’s forests?
That’s a critical question, because forests cover more than a quarter of the planet’s land, and they help stabilize the climate by pulling immense quantities of CO2 out of the air.
In August 2011, a team of scientists led by Dr. Yude Pan, a U.S. Forest Service researcher, reported that between 1990 and 2007, forests sequestered about 25 percent of all greenhouse-gas emissions — everything not in the air or seas.
Climatologists worry that if forests across the planet deteriorate, they could, on balance, begin releasing as much carbon as they absorb.
One of Pan’s collaborators, Dr. Richard Birdsey: “If the carbon sink in forests fails, a simple speculation is that global temperatures would increase proportionally to the increase of CO2 concentration, so about 25 percent above current climate projections.”
“The more forests die, the less carbon they take out of the air, the warmer it gets, the more forests die,”
says Dr. Nate McDowell at Los Alamos. “It’s a thermostat gone bad.”
The better we understand climate change, the more we seem to find that warming begets warming in unexpected and self-amplifying ways: Implacable heat engines materialize and run independently of all human effort.
There are an estimated 1 trillion metric tons of frozen carbon in the soils of the Arctic region — a century’s worth of global emissions, twice the amount stored in the global forest, another few Industrial Revolutions.
As the planet warms, permafrost thaws and decomposes, sending carbon into the air and further warming the planet. Higher temperatures also kindle increasingly intense and frequent wildfires in high-latitude forests, to quadruple effect.
And fire releases carbon directly; it burns off the insulating upper layer of vegetation, exposing more permafrost to warm air; it blackens the trees and land, which consequently absorb more solar radiation; and its soot can settle on and darken snow and ice sheets to the north, which then also absorb more solar radiation.
By the end of the century, the woodlands of the Southwest will likely be reduced to weeds and shrubs. And scientists worry that the rest of the planet may see similar effects.
Trees Cry Out
The Longevity of Trees
A Living Miracle
Du Bon Usage des Arbres