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?
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