It’s a hard-knock life on Mount Rainier (Photo: MvB)
“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.
WSDOT's Don Becker caught this marmot observing the clearing of the North Cascades Highway, which reopened May 25, 2011. (Photo: WSDOT)
University of Washington atmospheric scientist Cliff Mass has some debating words for authors of a recent study (“The Unusual Nature of Recent Snowpack Declines in the North American Cordillera“) suggesting the Western snowpack is dramatically shrinking due to climate change.
If you really check out the Science paper and look at the data, the loss of snowpack during the past few decades have not been serious. There is no clear smoking gun of anthropogenic global warming.
That is in stark contrast to the paper’s summary, which claims:
Over the past millennium, late-20th century snowpack reductions are almost unprecedented in magnitude across the northern Rocky Mountains, and in their north-south synchrony across the cordillera. Both the snowpack declines and their synchrony result from unparalleled springtime warming due to positive reinforcement of the anthropogenic warming by decadal variability.
To be clear–as Mass responded to one blog commenter thanking him for exposing the climate change “hoax”–this is not an argument over whether we’re changing the earth’s climate. Mass, along with the vast majority of climate researchers, has concluded that we are. His objection seems to be to statements like “almost unprecedented in magnitude.”
Says Mass, “Folks, we are in the early days of the warming and most of the action is yet to come. We need to be very careful on jumping to conclusions too early, since that only aids the deniers and skeptics who are just looking to pounce on excessive claims.”
A Seattle Times story alludes to the squabbling over how to frame the study’s finding: “The precise amount of those declines, particularly in the Cascades, has been the subject of fierce debate even within UW climate circles. But no more than half of the declines can be explained by natural shifts, the study shows.” In the New York Times, Greenwire’s Laura Petersen says, “The latest findings support conclusions from other studies suggesting that 30 to 60 percent of recent snowpack decline is due to human-induced warming.”
From Mass’s point of view, looking at 30 years of Cascades snowpack increase since the mid-1970s (based on snowpack measurements, not tree rings), the question of climate change effects is less predictable than that. Also, there’s the Pacific Decadal Oscillation, which does to graphs exactly what its name suggests.
His other criticism is that the study’s own data seems to show a trend of snowpack decline beginning around 1900, which is much earlier than you’d expect climate change to have begun messing with our heads–so the question becomes, If some other, unexplained factor was thinning the snowpack, how is it possible to back out those inputs from what we’re observing more recently?
For those of you curious as to whether trees are really that accurate as snow gauges, the study’s authors didn’t include just any tree. First, they focused on collecting samples from areas in which snowfall (and thus snowpack) is known to be the primary source of water for trees, and then, to isolate the “snowpack signal” further, they concentrated on trees whose “seasonal biology (i.e., timing of tree-ring growth) ties them closely to snow.”
Nor do the authors just bleep over the Pacific Decadal Oscillation: In fact, their argument uses long-term PDO observation to single out those unusual moments when, instead of the storm-track shifting from north to south (and vice versa) due to PDO, the whole range of the cordillera either increases or decreases in snowpack. This is rare; the authors say the the last time this occurred was the mid-1300s to early 1400s, a “time of anomalous warmth at regional and hemispheric scales.”
That warmth, says lead author Greg Pederson, is the crux of the “unprecedented magnitude” the paper references. The major declines from 1900 on were not associated with a temperature signal (so Mass is half-right), but with a change in the storm track that left the Northern Rockies “robbed of moisture.” Even so, Pederson emphasizes, that previous instance of decline beginning in the mid-14th century was due to a “period of warming not as high as the last few decades.”
Pederson argues that the change in average temperature due to warming has made the snowpack more temperature sensitive. You may not think half a degree matters that much, but it does when the half-degree is the difference between frozen and melting. Proportionally, things shift: the amount of precipitation that falls as rain vs. snow, the chance that a brief warm front in winter will bring massive melt and floods, and the chance that the snowpack will endure into summer.
Pace Mass, I can’t blame Pederson, et al, for the spin that different media have put on the study’s findings, though I’m sympathetic with his concern about “Sky is falling” messaging that doesn’t hold…um…water. But I do take issue with Mass’s definition of what’s serious. If Pederson’s tree-ring-to-snowpack record is correct, asynchronous declines tied to a global warming trend are simply not a frequent (on the human time scale) occurrence.
If you consider that it’s not unusual to have snowpack averages that diverge substantially for decades–over and over throughout the 800-year survey, you see peaks and valleys that endure for what would have been a lifetime–adding heat “quickly” to that system is foolhardy, and exactly what we’re doing.
Because your water tap doesn’t care about climate change debate, just the snowpack, water managers, their eyes opened to “unthinkable” effects of major droughts following years and years of Rockies-fueled abundance, must absolutely consider the proportionate change that warming could provide. Purely as a precautionary scenario–though I think the trees are in fact telling us something–the study’s red flag is creditable. There are too many lives at stake in the best of times. As National Geographic summarizes:
Each spring, melted snow and ice from the Rocky Mountains recharge up to 80 percent of the Columbia, Missouri, and Colorado River Basins. Together, these basins form the primary water source for nearly 70 million people in an area plagued by droughts….
Our infrastructure has trouble handling the huge ranges that, over the longer-term, nature is capable of surprising us with. Ironically, as many have noted, this study has appeared during a peak snowpack season. But we can be overwhelmed there, too.
At the moment, reports the AP, our hydropower dams would generate more electricity than demand, but it’s not as easy as simply spilling excess melt-water. Too-high dissolved oxygen levels in spill water could hurt salmon. So the Bonneville Power Administration has been telling wind farms to shut down at night, and stop delivering unmarketable electricity. Wind farm owners, naturally, are upset–they need to sell all the electricity possible to recoup their investment. So we fluctuate, one year raising electricity rates because the snowpack is low; the next, curtailing production because there’s too much of it. The trees aren’t the only ones feeling the stress.
