NSF's Weather or Not!
NSF-funded William Anderegg explains his research suggesting that forests don't bounce back from extreme drought
National Science Foundation
Interviewer: Charlie Heck
Interviewee: William Anderegg
Charlie: In a perfect world--or should I say in a perfect virtual-climate-modeling-world--forests and other vegetation bounce right back from extreme drought, like flipping a light switch on and off. But new research, funded by the National Science Foundation, says that flip might not be so quick and drought has a longer-lasting impact than scientists previously thought.
William: It was always assumed that drought was a little bit like a light switch, that when you flipped on the drought, the trees were stressed, but as soon as the rains returned, you flipped off the light switch, and everything went back to being healthy and growing.
Charlie: That’s William Anderegg, an assistant professor at the University of Utah, and lead author of a new paper on the subject. I spoke with him about his research on forests--one of the Earth’s main carbon regulators--and the effects of extreme drought. First, Anderegg explains why the rebound time for forests is so important:
William: We looked at a data set of about 1300 forest sites across the northern hemisphere and a handful in the southern hemisphere and found actually, that light switch metaphor isn’t really how forests act. They tend to take a couple of years to actually have growth return after a severe drought. And this really matters because if the amount of time forests are healthy really influences how much carbon they take up from the atmosphere and all the other values and services forests provide to society. We find that they actually recover quite a bit more slowly than we expected, often taking two to four years to fully recover from severe droughts. In the coming decades, climate change is projected to bring more frequent drought and more severe drought, and this means that forests will spend more and more of their time recovering, consequently, again, taking up less carbon from the atmosphere.
Charlie: So if it’s not as simple as just switching the growth back on, what are some potential causes for this long rebound time? Anderegg and his team don’t know exactly why it takes so long but they have a few theories.
William: The first category is changes to the canopy and loss of leaves or loss of branches. That would have a tree growing slower the next year. The second kind of general set of theories involves things attacking trees. So during drought, tree defense are often weakened. And so beetles, or fungi, or little pests and pathogens can attack trees, and they would then grow more slowly the subsequent years, trying to fight off these attackers. The final category that we think is probably the most important and not captured in our current models of how forests work is that drought itself can damage trees' physiology. We know that drought can kill trees. A major concern is recent widespread forest die-offs, triggered by drought. And drought tends to damage the water transport systems of trees. These are millions of little pipes that move water from the soil to the leaves. And these pipes during drought, these water columns come under extreme tension, and eventually they start to break. It's kind of the equivalent of a tree heart attack -- small air bubbles shoot in and block these water columns. And that drought damage, we think, is probably one of the major things causing this slower recovery from growth. You can't repair that damage, if you're a tree, instantly. It takes quite a bit of time and quite a bit of energy.
Charlie: So how does a team of researchers study tree growth in a large array of ecosystems and over a decent length of time? Tree rings.
William: Most trees in the north hemisphere put down annual rings, and the width of that ring is kind of proportional to the environment, how wet and how pleasant of climate the tree was experiencing that year. And at 1300 forest sites across North America, and Europe, and some sites in Asia, and New Zealand as well, we looked at tree growth over the past 50 years, and really zoomed in on when severe droughts had occurred at those sites. And our question was simple: how long does it take trees to recover from a drought? And we found that, on average, it takes trees about two to four years to recover from a severe drought, and there was some really interesting patterns that emerged for which trees were slower, and which regions were slower and faster.
Charlie: And because you want to know and I REALLY wannna know, I asked Anderegg to share his wow, really moment in the study…
William: I was certainly surprised by how long it took trees to recover. The other surprising thing to me too was that pines tended to be the group of species that had the slowest recovery time, and what's surprising about that is pines tend to have a very cautious strategy during droughts. And so I was expecting them to have recovered fairly quickly and not sustain a lot of damage.
Charlie: So, forests clearly play a big role in carbon regulation. They remove massive amounts of carbon dioxide emissions and incorporate it back into their woody tissue. But, what you may not know is that forests can act as either carbon "sinks" or carbon "sources." A carbon sink absorbs more carbon than it gives off, while a carbon source emits more than it absorbs. So, with the results of this study, Anderegg says the goal is to factor in the impact of drought, improving the accuracy of climate models and hopefully pinpointing whether or not we can count on the Earth’s forests to take up more carbon than they emit
William: Currently, forests take up about 25 percent of human emissions of CO2 to the atmosphere, which is a major slowing effect on the speed of climate change. Our model that we use to look forward into the future really span the full range of forests continuing to be a large carbon sink, and taking up carbon all the way through the end of the century. In some models, and in others, they actually start to even cross over to become a carbon source, which means they would actually be accelerating climate change, rather than slowing it down. What we are doing now is working to build these lasting effects of drought, especially through the drought damage to tree water transport systems into one of these major ecosystem and climate model. We think that sort of improving the mathematical representations of these is going to help us both simulate what we find is in this large set of tree ring sites, that forests recover more slowly from drought, and also really improve our ability to predict when and where trees might die. In terms of forests taking up carbon, there's really two sides of a coin. One is growth and the uptake of carbon, and then the other is when trees die, because when a tree dies, it slowly decomposes and loses that carbon back into the atmosphere.
One of the key findings of our paper as well is that--and a large body of research as well, is that right now we have the opportunity to really influence the future and the fate of forests on our planet, that the decisions we make to slow and address climate change will greatly reduce the risks that forests will face from these droughts and these mortality events. So I'd like to highlight that we really have an opportunity right now that influences the fate of forests.
Charlie: That was William Anderegg, an assistant professor at the University of Utah and lead author of the paper “Pervasive drought legacies in forest ecosystems and their implications for carbon cycle models,” published in the journal Science. I’m Charlie Heck, co-editor of Science360’s news service and co-host of the Super Science News Show, at the National Science Foundation.
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