“Eventually, if the drought is bad enough, the tiny tubes that carry water up the trunk of the plant can fill with air bubbles,” explains the New York Times Green blog, and the alert reader may pause over those “air bubbles” to inquire, “Wait…like an embolism?”
Yes, just like an embolism.
The drily titled study at the root of this, “Global convergence in the vulnerability of forests to drought,” was published in Nature recently, and I can’t help but think they buried the lede. It’s better to surprise people with the news that trees get embolisms, too, and then move on to the climate change connection. Why? Because once people hear “embolism,” they’re thinking about how to avoid it, and that’s a good frame of mind to be in.
Also, it never hurts to ponder how much we share with plants. In the U.S., by end of July, nearly 100 deaths were attributed to this summer’s nationwide heat wave. Does that sound like a lot or a little? The 2003 heat wave in Europe estimated to have influenced the deaths of more than 70,000 people.
It’s $32 for the full Nature article, so I’ll just quote from the abstract:
Drought stress creates trapped gas emboli in the water transport system, which reduces the ability of plants to supply water to leaves for photosynthetic gas exchange and can ultimately result in desiccation and mortality.
NPR gets it. “An Arbor Embolism? Why Trees Die In Drought” puts it in everyday language: “When drought dries out the soil, a tree has to suck harder. And that can actually be dangerous, because sucking harder increases the risk of drawing air bubbles into the tree’s plumbing.” The story goes on to emphasize another study finding, which is that drought, for trees, is a relative term.
Trees, it turns out, are daredevils rather than the fusty old Ents of Tolkien. They’re so invested in growing as big and as fast as possible, that they have little margin of hydraulic error. It doesn’t matter whether they’re scrubby piñon pines in New Mexico or mossed-over giants in the Olympic National Forest. Yes, trees are more or less “drought-resistant,” in terms of how much water they require. But it turns out that 70 percent of the tree species studied drink as much as they, in particular, can–if there’s less water, they’ll suck up air instead.
The University of Bayreuth’s Bettina Engelbrecht spells out what this means for the NPR audience: “Now, we have to worry about all [the trees],” she says. “We have to really deal with the problem at the global scale.”
“The problem” is multifaceted. Short-term, dried-out trees make for fast-moving wildfires, and more of them. Dried-out and dying trees are also less resistant to insect attack, while climate change has augmented the numbers of pine bark beetles. The destruction of 9 million acres of public western forests in 2009 looks like it may have been the peak of the mountain pine beetle’s infestation–that sounds like good news, but it’s likely because the beetle has run out of the tree it likes to eat.
Then–it sounds like piling on, I know–there’s the work of forests as carbon-dioxide storage lockers. That’s longer term, but thirsty, stressed trees aren’t typically replaced by larger, more robust forests. They get smaller and more sparse. Chronic drought can, over time, convert a forest to grassland. But that extra CO2 has got to go somewhere.
