Climate Change, Tree by Tree: How Would a Changing Climate Affect our Forests?

Climate Change, Tree by Tree: How Would a Changing Climate Affect our Forests?

Several climate-change models predict shorter snow seasons and more rain in winter. Most people who experienced the 1998 ice storm will wince when they hear the words "rain" and "winter" in the same sentence. Photo by Jim Block.

My third-floor office window overlooks a hillside a quarter-mile south that rises steeply into Montpelier’s Hubbard Park. The slope, steep enough to discourage buildings but not trees, supports a typical Vermont mixed forest almost a century old: sugar maple, some ash, a couple big red oaks. It’s a good view for a writer: middle distant, gently changing, and richly textured without being distracting.

One day this past August, however, the view grabbed my attention: despite the fact that it was still high summer, two of the maples were turning color. A few leaves were dingy orange, others were rimmed with ochre, and the trees displayed a generalized paling. The summer depth was gone, replaced by a hint of September coming through.

I did a double-take, confirmed it was August, thought, “Well damn – are the leaves turning early?” and returned to work. For two weeks, I didn’t think about it. Then I read in my local paper, the Barre Montpelier Times-Argus, that scientists at the University of Vermont (UVM) were asking, in more deliberate and organized fashion, the same question: would climate change affect the timing or intensity of the autumn color change?

It’ll be a while before the UVM scientists can venture a scientific answer. But that they’re asking the question is significant, for it reflects a curiosity, a need to know, that few of us felt a decade ago. Scientists, leaf peepers, economists, forest managers, birders, anglers, hunters, loggers, mill owners, and bed-and-breakfast hosts are all wondering, with increasing urgency, what climate change might do to the Northern Forest. The question takes many forms. Will we see different birds? Will there be maple to cut? Will we still make paper? Will the leaves still turn? Will bass rout the trout?

Perhaps the most fundamental form of this question, however, is how climate change will affect the mix of tree species. A forest, after all, is defined by its particular species mix – what ecologists call forest demography. We recognize this forest because it has a certain mash-up of hardwoods (dominated by maples, beech, and birch) and softwoods, with red and black spruce, balsam fir, hemlock, and white pine.

This mix we know as the Northern Forest. How it fares under climate change will influence whether these woods feel and behave like the place we know today. Any demographic change will have potentially huge effects on the forest’s economic, ecological, and aesthetic values: whether the leaves still change color, the sap still runs, the larches still amaze, the mills have anything good to cut.

How will climate affect the species mix? It’s the Northern Forest’s 26-million-acre question.

Unpleasantly Mild

The one thing certain is that the climate will change and, in fact, is already changing. According to a consensus report published in 2000 by a team of scientists assembled by the U.S Forest Service, the Northern Forest region’s average temperature warmed by about 1°F between 1895 and 1999. As a second, and more current, report from the Northeast Climate Impacts Assessment Synthesis Team (NECIA) relates, this warming is accelerating, with the Northeast overall warming at a rate of a 0.5°F per decade since 1970, with much of the increase coming since the NERA study of 2000. This applies to the Northeast as a whole, a region that reaches down to the mid-Atlantic states. It has so far warmed faster than New England and upstate New York, which makes these historical regional acceleration rates higher than NERA’s New England-specific rates. This points out an important aspect of climate change in the Northern Forest: though the Northern Forest region is warming and will continue to warm, other regions of the country – and the world – are expected to warm even more. We’re already experiencing less snow, longer growing seasons, and more days over 90°F.

The most robust and widely accepted climate models (most prominently the Canadian model, developed by the Canadian Centre for Climate Modeling and Analysis; the Hadley model, developed by the Hadley Centre in the United Kingdom; and the Community Climate System Model, or CCSM developed by a university consortium coordinated by the National Center for Atmospheric Research in Boulder, Colorado) all indicate it will warm up considerably more in the century to come. Most of these models offer two scenarios: a high-emissions scenario that assumes we’ll curb present rates of carbon-emissions growth only modestly and a low-emissions scenario that is optimistic both about how much we’ll curtail carbon emissions and how sensitively the climate will respond. Chances are strong that the reality will fall somewhere in between.

Climate change being controversial, the reliability of these models is often questioned. The central problem is that they contain uncertainty. That is, even the modelers admit that their models could be off and that the climate might warm up a bit less or – far more likely – more than the models call for. Doubters have seized upon this uncertainty to question the models’ credibility and even the existence of climate change.

There are two key things to understand here. One is that virtually all science, and all models, contain some uncertainty. The other is that the uncertainty of climate change models lies not in whether the earth will warm but in exactly how much it will warm. No credibly developed climate model predicts no warming. And all the main models pass the crucial test of producing accurate “backcasts”: that is, when they are fed climate and carbon-emissions data from, say, 1900 and asked to forecast climate change up to the present day, they “predict” climate changes and effects (like polar ice loss) that lie at the conservative end of what has actually happened. They all predict, for instance, that the years between 1995 and 2008 would contain the warmest decade ever recorded. They’ve also been right about why: when they manipulate the factors that have already occurred that might account for climate change so far, the only one they can remove and thereby create a no-cooling scenario is carbon emissions. The best reason to believe these models are right, in other words, is that they’ve been astonishingly right so far.

We can also have faith in the models because so many different ones developed by different people have identified the same relevant variables and ended up predicting similar backcasts and forecasts. It’s as if you asked five different major league scouts to evaluate a pitcher and they all agreed. These models now serve as checks on one another. And both separately and together, they enjoy almost universal support among scientists knowledgeable about climate and geophysics.

So how does this play out in the Northern Forest? The coming century, all the relevant reports agree, will see average temperatures in the Northern Forest region rise by anywhere from 3°F to 14°F.

The climate models predict moisture patterns, too, and in this, the region is unusual: along with the Pacific Northwest, we’re the only region of the country forecasted to become wetter. We would be warmer all year, with shorter snow seasons and more rain in winter; in summer, however, we’d get more hot dry periods and droughts. The extra wet, in short, would come mainly as winter rain and heavy summer thunderstorms. The snow season, meanwhile, would shrink to about half its present length and total snowfall. Summer droughts, however, according to these models, will expand, occupying one to three months every summer.

Overall, then, the worst-case scenario is that our climate will become warmer but not exactly “milder.” We’d have 2 to 5 times as many days over 90°F. Places as far north as Concord, New Hampshire, could see as many as 25 days over 100°F. We’d lose some bitter cold days, but we’d have more winter rain, less snow, more spring floods, more truly hot summer days, and longer droughts.

Potential vs Reality

What will these changes do to our forests? Here the main models, such as the U.S. Global Change Research Program’s National Assessment, or NERA, published in 2000, and the NECIA report Confronting Climate Change in the U.S. Northeast, published in 2007, offer a hedged but unsettling set of predictions.

First, a critical distinction. The scientists I spoke with for this article, and the various models themselves, take care to distinguish between changes in habitat suitability and changes in the species mix we might see in actuality. The warmer, wetter climate to come, all agree, will create a habitat increasingly hostile to our present forest and increasingly suitable to the great oak-hickory forest that stretches south and west from the Northern Forest’s southern edge.

Just as our maple-beech-birch forest does, the oak-hickory forest includes many other species; oak-hickory contains four different oaks, three hickories, and dogwood, laurel, and hawthorn. Oak and hickory, however, are most common and so win the title role, just as maple-birch-beech is shorthand for our northern hardwood forest that also includes ash, elm, hemlock, pine, spruce, fir, and, to make it interesting, a significant scattering of oak.

The line between the oak-hickory forest and the northern hardwood forest is a surprisingly narrow and well-defined border. Whether and how much this border might move or soften – in other words, whether changes in habitat suitability lead to actual turnover – is the heart of the forest demography question. And on this, the models are more guarded than on habitat suitability.

For one thing, actual species turnover is primarily driven by local variables, and regional models can’t account for those very well. Climate change may be a global phenomenon, but it, like the activist addressed on the bumper sticker, must ultimately act locally. Soil chemistry, interspecies interactions, disturbance regimes, and land use can all make a given patch of forest more or less susceptible to rapid species turnover. A stand of black spruce that is blown down in a windstorm, for instance, will offer a much different prospect for species turnover than will a healthy sugarbush growing on calcium-rich soil; a nearby but lower-elevation stand of less-sturdy maple mixed with oak and ash, meanwhile, may offer a softer target for turnover. The stands most resistant to the effects of climate change will be those with a mix of healthy northern hardwoods and little stress. Stands weakened by pests, weather damage, leached-out soil, and drought may turn over more quickly.

Disturbance is the one constant in this forest. But no one really knows how much climate change we’ll need to experience before these disturbances interact with varying (and still poorly understood) local dynamics to offer a landscape-wide advantage to the oak-hickory forest.

That’s why the major reports and experts tend to talk about a wide range in how much species turnover we might actually see. That said, the models and experts do roughly agree on what that range is. The following predictions pertain to how the forest will appear in 2100 – with a greater certainty about its appearance in 2200. No one expects deeply significant change by 2050.

The biggest and most certain population change will be the decimation of our spruce-fir or boreal forests. These boreal forests now grow mainly near the Canadian border and in the northern half of Maine. Because this is the southern edge of their range, they will almost certainly take a huge hit as it warms. The models predict that they will be all but driven from New York, Vermont, and New Hampshire and reduced to the upper quarter or third of their present range in Maine. One important caveat: if the lower-emissions scenarios prevail, the spruce-fir forests that do remain may grow faster because of the longer growing season, retaining some habitat for boreal species and providing a downsized paper industry with an ongoing softwood supply. (See related article “The Fallout”.)

Our northern hardwoods, meanwhile, would undergo less certain but still significant changes. Here the scenarios at either end of the scale differ more than with spruce and fir. The most dramatic predictions have the oak-hickory forest slowly but decisively replacing a fading maple-beech-birch forest, marching 200 to 500 miles north over the next two centuries. The less-dramatic scenario – one that emphasizes lower emissions and greater inertia in the existing forest (more on that shortly) – calls for the present hardwood forest to essentially mix with the incoming southern species to create a new, hybrid forest.

Hedge Factors

This is a wide range of prediction. On one hand, a rearrangement of the present demographics; on another, a whole new regime. No one really knows which prediction will prove accurate, but we can understand the factors that influence the predictions.

Here, it’s helpful to think of brakes and accelerator. Some factors will tend to slow species turnover and keep the forest more like it is today. Others will tend to speed up or encourage change. No one really knows how these factors will play out exactly. But knowing what they are can help us understand both the predictions and the root and range of the uncertainties within them.

Balancing the accelerator of the warming climate are three factors that serve as brakes, suggesting that the Northern Forest will in 200 years look much like our current one.

First, most trees live a long time. Maples, for instance, can live 400 years and typically live between 80 and 200.This longevity slows turnover.

Second, trees can’t walk. They move by seed dispersal. This imposes sharp limits on their ability to march northward. Oak and hickory both depend on animal dispersal of their nuts to spread; bears, blue jays, and squirrels move only so far.

Finally, history suggests that the boundary between these two forests – what ecologists call an ecotone – may move much more slowly than the temperature gradients will. As forest ecologist Charlie Cogbill points out, the average-temperature line that presently runs along the border between the oak-hickory forest and the northern hardwood ecotone has moved before without dragging the forest boundary with it. It has wandered north as far as 200 miles within the last 10,000 years without producing a corresponding move by the ecotone. “The temperature gradient moved 200 miles,” he says, “but the oak and hickory moved only about 20. It’s not as simple as the forest follows the thermometer. There’s something else that has kept this forest in place.”

Some ecologists call such resistance to movement and change the forest’s inertia. How fast the forest changes will depend not only on how fast the climate changes but also on how well this inertia resists the incursion of the southern forest. Some ecologists (Charlie Cogbill, for one) believe the inertia is large. Others believe it’s not.

Brian Beckage, a plant ecologist at the University of Vermont, recently found reason to believe the inertia is not as great as we might hope. He and some colleagues recently studied movement in the ecotone between hardwood and spruce-fir populations on Camel’s Hump in Vermont. (Such elevation-related ecotones can serve as rough proxies for north-south ecotones.) Specifically, they compared today’s ecotone with old data showing where it was in 1964. They concluded that the hardwood/spruce-fir eco-tone had moved 100 meters uphill in four decades.

“This is pretty much what you’d predict from the 2°F rise in temperature this mountain has experienced over that time,” he told me, referring to local data showing that much change; the 100-meter move upward is roughly analogous to a northward shift in the ecotone of 50 miles.

“The take-home from this,” says Beckage, “is that there’s less inertia in these systems than we might think. Here the forest is responding very quickly to climate changes.” Beckage grants that ecotones would move more slowly latitudinally – that is, northward over miles rather than upward over meters – because of limits in the speed of seed dispersal. “But this suggests pretty strongly that the forest may respond to regional warming more rapidly than we thought it might have.”

As to why, Beckage cites poor tree health not just among the spruce on Camel’s Hump, which has suffered from acid rain, but throughout the dominant species in the Northern Forest.

“This forest stands quite vulnerable to this sort of change,” he says. “You don’t have that many species making up the bulk of trees here, and most of them have health problems right now.” Spruce and fir are stressed by acid deposition, climate, and winter injury. Beech trees have beech-bark disease. Maples are suffering from “maple decline,” a poorly understood, generalized decline that many ecologists believe is caused by calcium depletion from acid rain. Paper birches are suffering a similarly pervasive but mysterious ailment. These vulnerabilities, Beckage believes, may well create a northward march of the oak-hickory forest that leans toward the faster end of the models’ predictions.

Beckage is quick to acknowledge that he could be wrong about this, and skepticism in science is a wise hedge. On the other hand, the overriding trend, over the last five years, has been that many effects attributed to climate change – from species changes to loss of ice near the poles – have proceeded more rapidly than most models predicted.

Any change in the forest cover, however, will be of the slow, gradual kind that’s easiest to adapt to, or, for that matter, ignore. Yet it will be quite real, and over the centuries it may create a different landscape and quite possibly a different culture. Vermont without syrup, New Hampshire without tundra, the Adirondacks without snow, Maine without spruce and fir.

Such a prospect can lead to both despair and a sort of complacency. How do you counter forces so large, slow, and mysterious? As the recent disasters in the world of finance suggest, some of the biggest problems are the ones that grow slowly, hidden in plain sight. The challenge of understanding and adjusting to a changing climate’s effect on the Northern Forest will be largely one of awareness and anticipation. It resembles, in many ways, the one we already face in trying to keep the Northern Forest a place where people connect in vital ways to a coherent, healthy landscape.

David Dobbs, the co-author with Richard Ober of The Northern Forest, writes on science, nature, and culture for The New York Times, The New York Times Magazine, Scientific American Mind, and Northern Woodlands.

 
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