The Story of the Constant, Changing ForestPosted: January 3, 2013
If you’re a writer looking for a good symbol, consider the tree. The author of Genesis did, twice: he placed the tree of life and the tree of knowledge front and center in the Garden of Eden. Homer did, too: When his hero Odysseus returned home after twenty years of war and travel, needing to prove his identity to his skeptical wife, Penelope, he used a tree. “Move our bed into the hallway,” Penelope told her servant, laying a trap. (I’m paraphrasing here.) “It can’t be done,” Odysseus protested. “I carved a post of that bed from a living olive tree.” Only then did Penelope believe the strange man was really her husband, as steady as that post.
Trees have long impressed us with their steadfastness; in fact, some trees from Biblical times are still with us today. But a new story I read recently casts trees in a different role. I first came across a version of this story in a paper published in the journal BioScience in 2007. The authors looked at where trees live using a tool known as a “climate envelope,” which is a line drawn on a map around the entire range where a given species is able to survive. The scientists compared climate envelopes for 130 trees under 2007 conditions to those predicted for the end of the century, using the same computer models that the UN’s Intergovernmental Panel on Climate Change bases its forecasts on. On average, they found that trees’ envelopes moved 700 kilometers north, nearly the distance from Memphis to Chicago.
So does that mean our trees will be moving north as things get warmer? Traveling trees can make great stories: Shakespeare’s Macbeth was vanquished when Birnam Wood moved a few miles to his fortress at Dunsinane Hill. And it would be dramatic indeed if future northern woodsmen and women hunt deer among sprawling live oaks and big-leaf magnolias instead of spruce and pine trees. But the scientists who wrote the BioScience paper noted that actual trees are unlikely to track their climate envelopes’ northward migration in the coming years, at least if unassisted by humans. Trees can “move” up to a few miles in a generation, by setting their seeds aloft in the wind or encasing them in a shell so they can survive a trip in the gut of an animal. But tree generations are long, and most seeds land close to home. Sugar maples, for example, lead a chaste adolescence, and don’t start making seeds until the age of 22 or so. They then send out seeds attached to little helicopters, which spin and float at most the length of a football field before touching down. Scientists estimate trees’ maximum migration rate to be around 50 kilometers per century, with many traveling far slower—a tortoise’s pace in this race.
I sensed the possibility of a different story here—one about actual trees, not just climate envelopes. People depend on trees, they identify with trees; what would happen if the trees were no longer there? To find out, I asked some scientists, and found myself half hoping they would tell me that yes, our northern trees are going to overheat and die, leaving us with denuded land and no more maple syrup. Maybe I could finally shake people out of their stupor about climate change! But that’s not how scientists talk, to their credit. They’re far more likely to say things like what David Mladenoff, a forest ecologist at the University of Wisconsin-Madison told me: “One possibility is you have some species becoming less vigorous or dying, and so there’s a decline in the forest in the sense that the forest becomes more sparse, what we refer to as lower biomass, less live tree mass above ground.”
A reasonable statement, though “Scientist predicts less live tree mass above ground” doesn’t exactly scream headline material. But Mladenoff’s message is not necessarily aimed at the public so much as at land managers, who need to plant now for climates decades in the future. Although he has been building computer models of forests’ response to climate change since the late 1990s, (“way before we could get anybody to listen to us,” he says), Mladenoff realizes that people need a clear story in order to know how to act, and he is continuing to refine his models so that he can better tell that story. “We can say things are likely to change, but we can’t give the kind of certainty of what kind of change and how much, and that’s what managers need,” he says.
For another perspective, I spent an hour in the woods with Don Waller, a UW-Madison botanist who studies what makes forests change. Waller and his colleagues have compared the plants in the forest today to those meticulously recorded by a group of Wisconsin ecologists in 1940s and 50s, and found that differences between sites are decreasing. “It’s like the McDonaldization of nature,” he says. We talked about changes that might be related to climate: new pests and pathogens invading; tree-climbing vines to growing fat off elevated CO2 levels; tree-killing insects getting in two generations per year instead of one. But climate is just one of the drivers of change in the forest; deer browsing, plant invasions, and habitat fragmentation are equally important factors, at least for now. So, like Mladenoff, Waller is reluctant to make specific prognostications. “The world is becoming less predictable at the same time that it’s changing faster, so I’m always leery myself about people who try to make predictions about what will or won’t happen,” he says. “Interactions are complex and often nonlinear.”
I can appreciate this hesitation, having studied physics. Physicists can’t tell us exactly how three interacting bodies will behave, and a forest has millions of interacting bodies. To illustrate this another way, Waller points out that a person is required to get all kinds of training before flying an airplane, at least in the US, but there is no standard preparation for managing a forest, which is a far more complicated system with more moving parts.
But it’s not just a matter of understanding how the parts interact with each other; we often don’t have a clear picture of the parts themselves. “Even for a single species, like trembling aspen, I can’t tell you why it’s not in the forests of southern Illinois,” says Eric Kruger, a UW-Madison tree physiologist. Trembling aspen, with its ghostly white bark and thousands of flat leaves that flutter like tiny flags, has a tremendous range, stretching from far northern Canada as far south as Mexico, and encompassing all of Wisconsin and most of Ohio, Pennsylvania, and New England. There’s no obvious reason it can’t tolerate warmer climates, Kruger says, and yet climate envelope maps put out by the US Forest Service show aspen moving north out of Wisconsin altogether by the end of the century. “Why does it leave Wisconsin?” Kruger asks. “This is the fundamental question I’ve been asking. I look at these maps and I go, why the heck would that happen? I’ve tested hypotheses that I’m not even sure I believe in.”
Kruger’s favorite hypothesis right now is that aspen gets out-competed. He thinks aspen could grow in warmer places, in principle; it’s just that other trees do better there. Indeed, he’s planted aspen in southern Illinois alongside southern trees, and found that it does grow, just more slowly than its neighbors. But he admits he’s not convinced that competition alone could “deliver a knockout blow” to aspen in Wisconsin. “My suspicion is there are other, more important things; we just don’t know what those are.”
Peter Groffman is asking similar questions about New England’s sugar maples. Climate envelope maps show that tree, beloved for its fall color and its syrup, disappearing from the region altogether, but Groffman, an ecologist with the Cary Institute of Ecosystem Studies in New York, is skeptical. “You know, the trees don’t read the scientific literature,” he says. “So there’s an idea that climate envelopes might overstate some of the species change.”
Groffman and his colleagues have been studying the effects of climate change in the Hubbard Brook Experimental Forest in New Hampshire, and recently published a paper summarizing 50 years’ worth of observations. One of the team’s findings was that sugar maple is indeed facing challenges—early snow melt is exposing its roots to potentially damaging cold air, while increasingly abundant deer are feasting on its seedlings. However, Groffman isn’t ready to give up on the iconic tree; he thinks more research is necessary. “You can make predictions, and then we have to go out and see how they come to pass.”
So will trees be the mute witnesses of climate change, or its helpless victims? The answer, it seems, may be some of both. But one thing is certain: we need to know more. We need sites like Hubbard Brook, where generations of scientists can create continuous data sets showing change over decades. We also need studies like Waller’s, which take advantage of exceptional historical records. And we need models like Mladenoff’s that use modern computing power to crunch the massive number of variables it takes to describe a forest. Trees are old; the science of forestry is young. We’ve learned much, but we still have much to learn.
And yet, what the ancients knew about trees remains important. Trees are still constancy embodied. They aren’t given to chasing the latest trends. No matter what else is happening in March, they leaf out, just like they did the previous March, and the March before that. If it’s a bad year, they make a small growth ring and wait for the next. In many ways they serve as foils to us, fickle and flighty humans. We left Eden; the trees remained. But our fate is still tied to them as it was in ancient times. Trees still provide us food, fuel, and shelter; now they also soak up our carbon emissions and protect most of our planet’s remaining biodiversity. And that, to me, gives their story a particular urgency. I hope you’ll help me tell it.
And as an added bonus, you can listen to a radio story I produced about Wisconsin’s changing forests on my website.
(All images from wikimedia commons.)