As Tree Species Migrate, the Forests We Care about Transform
By Miranda Weiss
I REMEMBER IMPORTANT PLACES IN MY LIFE BY THE TREES THAT GROW THERE. In the yard of the first house in my memory, a 1925 brick colonial in suburban Maryland, a mimosa tree erupted in fantastical pink blossoms each summer. The fern-like leaves closed magically each night. As a kid, I had no idea that this tree was an invasive species in the region, only that a few weeks after my eighth or ninth birthday party, during which my friends and I ran relay races in the front yard carrying spoonfuls of beans, the shoots of bean plants appeared in the spare grass beneath that tree — as if called up by the mimosa itself — and I was thrilled.
In the decades since, with every new phase of life, with every new shift in my personal geography, trees have played a role. I grew attached to the tulip poplars with mitten-shaped leaves that towered behind the house we moved into when I was 11. And in New England, where I went to college, I grew to adore American beech trees and the way their smooth, grey bark made me think of the skin of the elephants I saw roaming the enclosure at the National Zoo. It was amongst the beech and hemlock forests of the region where I first observed aphid-herding ants — insects that farm — under rotting logs and caught dragonfly larvae that lurked in shady creeks. Those woods were wondrous places, propelling me to explore more.
My memory holds these landscapes and their trees as constants, unchangeable, but recent research shows that tree species are on the move, migrating in response to climate change and shaking up forests across the country.
Scientists call the phenomenon “range migration,” which means that tree species are shifting into landscapes in which they don’t typically grow. Individual trees, of course, can’t pick up and leave when conditions get tough. But seedlings in favorable habitat will survive, grow and cast off their own seeds, while harsh conditions, such as drought, flooding and temperature extremes, can kill off young and old trees alike and prevent the growth of new seedlings where they once grew.
Songlin Fei, a researcher at Purdue University, has uncovered clear evidence that eastern tree species are already moving in conjunction with changing temperature and precipitation patterns. Fei looked at 86 tree species over the past 30 years and found two distinct migration patterns: hardwoods are heading west and softwoods are moving north.
Fei and his team used U.S. Forest Service (USFS) Forest Inventory data, which is gathered by field crews who bushwack into woods across the country to census adult trees and saplings. Their study reports that over the last three decades, hardwoods — such as red maple, scarlet oak and sweetbay magnolia — ranged west at an average clip of one and a half kilometers each year, while softwoods — including red pine, short-leafed pine and bald cypress — shifted north an average of one kilometer per year.
Over the last century, average annual temperatures across the U.S. have increased — most notably in the north and at night, and precipitation patterns have changed. The West has seen record droughts, while the Northeast has seen increased precipitation. These radical, climate change- caused shifts in conditions open new habitat for tree species and make existing habitats unsuitable.
While increased precipitation in the Midwest appears to be luring many eastern hardwoods west, the most profound tree migrations are happening in the north. The highest latitude species in Fei’s study, including balsam fir and black spruce, shifted the greatest distances over the last 30 years. These shifts are part of the dramatic changes we’re seeing in subarctic, boreal forests worldwide.
In Alaska, tree range is limited by cold — to the north by the harsh Arctic environment, to the west by the chilly summer winds whipping off the Bering Sea, and up mountain slopes. As temperatures warm in this state twice as fast as in the rest of the nation, we’re seeing positive responders and negative responders, explained Glenn Juday, an emeritus forest ecologist at the University of Alaska Fair- banks. The results, Juday said, will be a migration of trees out of the relatively warm, dry interior of the state and into the colder, damper regions to the north and west. In Alaska’s far north, white spruce are already barging into tundra landscapes where only shin-high shrubs have been the tallest vegetation for perhaps thousands of years.
The Western U.S. is also seeing profound changes to its forests in conjunction with our changing climate. Low elevation forests are experiencing the most radical shifts, Solomon Dobrowski, a landscape ecologist at the University of Montana, explained.
Along the low elevation slopes in the Bitterroot Mountains near where Dobrowski lives and works, for example, he sees stands of mature ponderosa pines where conditions have become too hot and dry for ponderosa seedlings to grow. Seedlings are more sensitive to harsh environmental conditions like extreme heat and drought; they lack the kind of well-developed root structures and stored nutrients that adult trees rely on. In some of these forests, the summer temperature at the surface of the ground — the zone where seedlings are trying to gain a foothold — can be a scorching 120 degrees Fahrenheit. So, while a ponderosa forest might occupy a certain part of the landscape now, in the years and decades to come, it won’t be replaced.
The West’s basin and mountainous topography influences how tree ranges shift in this part of the country — and will shift in the future. Unlike the broad migration patterns we’re seeing in the East, here, tree range shift is more of a leap frogging type of business, proceeding in fits and starts where mountain ranges dictate dispersal. Dramatic topographical changes play a critical role: Tree species can be squeezed into an elevation band — constricted at the bottom by warming temperatures and drought and limited at the top by lack of soil.
Mountainous terrain creates harsh conditions for trees, but a tree species that might have to shift 550 feet up slope in order to reach a more suitable climate, would, in flat terrain, have to move 90 miles north to achieve the same result. Already, foresters are testing the limits of tree survival by planting trees far outside their natural range. One study is attempting to grow ponderosa in northern Minnesota, hundreds of miles from their natural geographic limit, where the experimental plot sees summer rain showers that are more frequent than in the Rockies, which might give ponderosa seedlings the leg-up they need.
One aspect of tree range shift that is both fascinating and troubling is that tree species in the same forests, even those that are deeply ecologically linked, are responding in different ways to climate change. American beech and hemlock, for example, are iconic species of old-growth forests east of the Mississippi. These trees colonized landscapes left bare 10,000 – 20,000 years ago as the Holocene ice sheets retreated with warming temperatures and have been linked for millennia.
Today, climate change is driving a wedge. Fei’s study shows that, over the last three decades in the eastern U.S., hemlock have moved 13 kilometers north while beeches have roved nearly 12 kilometers west.
“It’s sort of like an old family being broken apart,” Fei said. Changes to tree ranges affect everything in an ecosystem, from the hair-thin strands of fungi that help feed tree roots to top predators.
Disruptions like that — as well as from wildfire, changing land use and increased development — provide openings for invasive species and pathogens, such as Japanese barberry shrubs that are favored by Lyme disease-carrying ticks, the hemlock woolly adelgid, and the emerald ash borer. It’s a dismal feedback cycle in which ecosystem disruption begets disruption, which begets disruption.
“Whole ecosystems are getting jumbled up,” USFS scientist Louis Iverson said. Iverson relies on a handful of climate models to predict future tree ranges. Models show us that there will be some winners — southern oaks and pines will likely increase their ranges — and some losers — most notably northern species.
Forests are one of Earth’s most important carbon sinks. Across the U.S., forests sequester 10-20 percent of the nation’s annual carbon output each year.
So, what does range migration mean for the ability of our forests to sequester carbon from the atmosphere? “This is really a frontier that we’re trying to understand,” Fei said. This complex question involves many unknowns. While warming temperatures bring longer growing seasons and, presumably, increased carbon storage, climate change often causes drought, which restricts growth.
No one yet knows for sure what tree migration will mean for the ability of our forests to store the carbon we’re dumping into the atmosphere, or for the many other ecosystem services our forests provide. And, while there are many other unknowns when it comes to tree range shift, one thing is clear: the effects of greater carbon emissions stress trees more, forcing their ranges to shift greater distances and putting forests as we know them at greater risk.
“You can see the writing on the wall for a lot of these forests,” Iverson said.
Change and succession are, of course, natural processes in forests. And fossil evidence shows us that trees have migrated vast distances in the past. After the last glacial retreat, fossil pollen suggests that tree species migrated as rapidly as one kilometer per year. But those species moved across a landscape uninterrupted by human development. And today, as each new year seems to bring record high temperatures, it’s clear that tree species aren’t able to out-pace the impacts of climate change in all cases; one study showed that trees would need to migrate more than twice as fast as their current rate in order to reach suitable habitat.
And, unfortunately, tree range migration poses ecological, practical and economic challenges. But the other ramifications of tree range shift are impossible to quantify.
“Health, well-being, spiritual well-being…that’s all part of it,” Iverson explained. He likes to walk in a nearby oak-maple woods in his home state of Ohio. As climate change puts pressure on trees, the psychological and spiritual benefits of a forest that’s been a longtime friend are degraded.”
As we walk through the forests we love, as we remember the landscapes we hold dear in our minds, the impacts of climate change force us to ask: Will this tree survive? Will this forest?
Life has taken me from the East Coast’s glorious hardwood forests to where I live now, in Homer, Alaska, at the edge of the northernmost extent of temperate rainforest. There are only a handful of native tree species at this northern latitude. Each one plays an important role in the lives of people here.
In the fall, birches provide a burst of gold amidst our monotone spruce. In the spring, cottonwoods give off a pungent, herby perfume. Local folks collect the buds to make a healing balm. Alders can be weeds but can also provide needed privacy, and we burn spruce nine years old, and on summer days we adventure in our local woods. When it rains, we often seek a shady trail under a thick canopy of spruce. On sunny ones, we’ll head through a tunnel of alders on a trail down to a wild expanse of beach. Or sometimes, we visit a friend with a grassy yard that has a view of the mountains across our bay. There, my girls will fly skyward on a swing made out of a pink fishing buoy that hangs from the bough of a beautiful old birch.
These days, I think of the forests of my daughters’ lives. What trees will they remember? What woods? How will the places they come to know and love change throughout their lives?
Forests will continue to be fertile grounds for magic and memory, as they have been. But as tree species move and our forests transform, what we hold dear changes too.
Miranda Weiss is the author of Tide, Feather, Snow: A Life in Alaska and lives in Homer, Alaska.
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