June 11th
Trees on the Go.
Explore how, amidst the shifting climate, various types of trees are relocating and reshaping the forests that hold significance to us.
Written by Miranda Weiss
As Tree Species Move, the Forests We Value Change.
American beech and eastern hemlock have been ecological allies for thousands of years. Climate change is separating them.
Image Source: Nicholas A. Tonelli / FlickrCC.
I was thrilled to see the mimosa tree, known as the “up called” tree, beneath the spare grass. The tree appeared to have shoots of bean plants underneath it. I had no idea as a child that this tree was an invasive species in the region, only a few weeks after my ninth or eighth birthday party with my friends. Each night, the fern-like leaves of the tree would magically close. Every summer, the mimosa tree would burst into pink blossoms, creating a fantastical scene in the front yard of my first house, a colonial brick house in suburban Maryland in 1925. I remember important moments in my life that took place near those trees.
I was more fascinated by the enchanting woods that propelled me to explore further. In the shady creeks, I discovered dragonfly larvae and observed ants herding aphids on rotting logs. It was in the hemlock and beech forests of that region where I first experienced the diverse insect life on the farm. During my time in college, I developed a deep admiration for the smooth grey bark of American beech trees, which reminded me of the elephants’ skin I saw roaming in the enclosure at the National Zoo. When I moved to New England at the age of 11, I grew attached to the tulip poplars with their mitten-shaped leaves that towered behind our house. Trees have always played a significant role in shaping my personal geography, and with each new phase of life, they continue to inspire me. This has been the case for several decades.
Response to climate change, tree species are migrating across the country, shaking up forests and landscapes. Recent research shows that these memory-holding trees act as constants, unchangeable like trees, but in response to climate change, they are migrating.
The precipitation patterns in the central United States have become wetter, while the southeastern region has experienced drier conditions. Additionally, the eastern part of the United States has seen an increase in temperatures over the past 30 years.
Image Source: Songlin Fe
Scientists refer to the phenomenon of tree species’ “migration,” which means the shifting of landscapes into which they typically don’t leave and pick up individual trees. In harsh conditions such as extreme temperatures, flooding, and drought, new seedlings of growth prevent and similar trees, both old and young, can kill off. However, their own seeds are cast off and will grow, surviving in favorable habitat conditions.
Softwoods are shifting towards the north, while hardwoods are moving towards the west, according to researcher Songlin Fei from Purdue University. Fei discovered that eastern tree species are already adjusting their distribution in response to changing temperature and precipitation patterns. In his study spanning 30 years, Fei examined 86 tree species and identified two separate migration patterns.
The study reports that over the past three decades, the average number of trees per kilometer in the northern part of the country has shifted. This shift includes a decrease in the number of cypress, bald, and short-leafed pine trees, which are softwoods, and an increase in the number of hardwoods such as magnolia, sweetbay, scarlet oak, and red maple. The data for this study was gathered by field crews who ventured into the woods to conduct a census of both saplings and adult trees. The U.S. Forest Service’s Forest Inventory team, led by Fei, used this data.
The radical climate change has caused shifts in open habitat conditions, making the existing habitats unsuitable for certain tree species. The Northeast has experienced increased precipitation, while the West has witnessed record droughts. Over the last century, there have been noticeable changes in precipitation patterns, particularly at night and in the northern regions. Moreover, average annual temperatures have increased across the United States.
Over the past 30 years, the boreal forests, which are subarctic and located worldwide, have experienced dramatic changes. Among these shifts, the greatest distances have been covered by tree species such as black spruce, fir, and balsam, with the highest latitude species in Fei’s study showing the most profound migrations towards the north and west. In addition, increased precipitation in the Midwest seems to be attracting many eastern hardwoods.
In Alaska, the shrubs in the tundra, where the tallest vegetation is only shin-high, have already been barging into landscapes that were once dominated by spruce trees. According to Glenn Juday, an ecologist forest emeritus at the University of Alaska Fairbanks, this migration of trees from the relatively dry and warm interior of the state to the damper and colder regions in the west and north will be the result of warm temperatures occurring twice as fast in this state compared to the rest of the nation. The range of trees in Alaska is limited by the harsh Arctic environment, with cold winds whipping off the Bering Sea and the mountain slopes to the north.
Solomon Dobrowski, an ecologist at the University of Montana, explained that the low elevation forests are experiencing the most radical shifts. In conjunction with our changing climate, the forests in the Western United States are also witnessing profound alterations.
In the years and decades to come, it is unlikely to be replaced, a certain portion of the landscape might be occupied by a ponderosa forest at present. Therefore, while the summer temperature at the surface of the ground can reach a scorching 120 degrees Fahrenheit, which is the zone where seedlings are attempting to establish themselves, these forests lack the type of well-developed root systems and stored nutrients that mature trees depend on. Seedlings are more susceptible to harsh environmental conditions such as extreme heat and drought. For instance, Dobrowski observes mature stands of ponderosa pines in the Bitterroot Mountains, near where he lives and works, where the conditions have become too hot and dry for ponderosa seedlings to grow along the low elevation slopes.
Dramatic topographical changes can restrict the range of tree species, as the soil at the top becomes limited due to drought and warming temperatures, while the bottom becomes constricted. The critical role of tree species in this area is determined by their ability to disperse and adapt to the changing mountain ranges. Unlike the broad migration patterns we see in the East, where tree ranges shift in a more gradual manner, the future shift in this part of the country will involve a leapfrogging type of business, influenced by the mountainous topography and basin of the West.
In the experimental plot in northern Minnesota, hundreds of miles from their natural geographic limit, ponderosa seedlings are being grown in an attempt to give them the boost they need. These trees experience more frequent summer rain showers compared to the Rockies. Foresters are already pushing the boundaries of tree survival by planting them far beyond their usual range. Similarly, a tree species would have to move 90 miles north in flat terrain or shift 550 feet uphill in mountainous terrain to reach a more suitable climate. The mountainous terrain creates challenging conditions for trees.
One aspect of the shift in range of tree species is the ecological linkage and deep connection that exists between even those tree species in the same forests, which is both troubling and fascinating. It is concerning and captivating to observe how these tree species are responding in different ways to climate change. These landscapes, which were left bare and colonized by trees between 20,000 and 10,000 years ago, have been linked for millennia and have seen the retreat of ice sheets and warming temperatures during the Holocene. For example, the American beech and hemlock are iconic species in the old-growth forests of the eastern Mississippi. The shift in range of tree species is fascinating and troubling due to the deep ecological linkage that exists between even those tree species in the same forests, which are responding in different ways to climate change.
A recent study in the Eastern United States revealed that over the past three decades, the average yearly growth rate of softwoods shifted northward by one kilometer per year, while the average yearly growth rate of hardwoods ranged from half a kilometer to one kilometer per year in the west.
Image Source: Songlin Fe
Today, climate change is causing a division. According to Fei’s research, hemlock trees have migrated 13 kilometers north and beech trees have traveled almost 12 kilometers west in the eastern United States over the past three decades.
Fei stated, “It’s like a type of family that is old and broken apart.” The tree changes in an ecosystem, caused by the hair-thin strands of fungi that help feed predators from the top to the roots, affect everything.
The emerald ash borer, the hemlock woolly adelgid, and Japanese barberry shrubs that are preferred by ticks carrying Lyme disease, create opportunities for invasive species and pathogens, such as those from wildfires, alterations in land use, and enhanced development, as well as disturbances of a similar nature. Disturbance leads to further disturbance, which in turn leads to disruption in a gloomy cycle of feedback where ecosystem disturbance prevails.
USFS scientist Louis Iverson stated, “Entire ecosystems are becoming mixed up.” Iverson predicts future tree distributions based on a few climate models. The models indicate that southern oaks and pines are likely to expand their ranges, while northern species are expected to be the most affected.
Every year, woodlands absorb 10-20 percent of the country’s yearly carbon production throughout the United States. Forests represent one of the Earth’s vital carbon reservoirs.
American beech trees in the forests of the eastern United States are moving towards the west.
The increase in carbon storage presumably leads to changes in climate, with longer growing seasons causing temperatures to rise, while many complex questions remain unanswered. Fei said, “This is a frontier that we are really trying to understand,” referring to the ability of our forests to sequester carbon from the atmosphere and what it means for the migration range.
It is clear that when it comes to shifting tree ranges, there are many unknowns. However, one thing is clear: the greater risk of putting forests at stake and forcing trees to shift to greater distances is causing more stress on them. This, in turn, leads to greater effects of carbon emissions, as we are dumping carbon into the atmosphere instead of allowing forests to store it. Moreover, the migration of trees also raises questions about the impact on the ecosystem and the various other services that our forests provide. Despite all these uncertainties, one thing is certain: we do not yet know what the tree migration will mean for the ability of our forests to store carbon.
“You can anticipate the inevitable decline of many of these forests,” Iverson stated.
The study showed that in order to migrate, tree species would need to reach habitats that are at least twice as fast as their current rate. It is clear that these tree species are not able to outpace the impacts of climate change, as high record temperatures bring new challenges each year. Additionally, human development has disrupted the landscape, making it difficult for these species to move across uninterrupted habitats. Fossil pollen suggests that tree species have migrated rapidly, at a rate of one kilometer per year, after the retreat of the last glacial period. Fossil evidence also shows that trees have migrated vast distances in the past. Natural processes such as succession and change occur in forests.
The other consequences of tree range shift are difficult to measure. Unfortunately, the migration of tree ranges poses ecological, practical, and economic challenges.
Iverson elucidated, “that’s all a component of it,” spiritual welfare, welfare, health. In his place of residence in Ohio, he enjoys strolling through a neighboring forest with a mix of oak and maple trees. The mental and spiritual advantages of a woodland that has been a long-standing companion are diminished, as the impacts of climate change exert stress on the trees.
Climate change requires us to contemplate the woodlands of our children’s upcoming generations.
Photo Attribution: Miranda Weiss.
Will this woodland endure? Will this individual tree thrive? The consequences of climate change compel us to inquire about what we value in our thoughts, as we cherish the scenery we recall, and as we adore the woodlands we traverse.
In Alaska, specifically in Homer, where I currently reside, I am fortunate to experience the crucial role played by a select few native tree species in the northernmost region of the temperate rainforest. These magnificent hardwood forests have been brought to me from the glorious East Coast’s coastline.
My girls will fly skyward on a swing made out of a pink fishing buoy, hanging from the beautiful old bough of a birch tree. Sometimes, we visit a friend with a grassy yard that has a view of the mountains across our bay. Other times, we head down a trail through an alders’ tunnel to reach a wild expanse of beach. When it rains, we often seek the shelter of a shady trail under the thick canopy of spruce trees. During our summer days, we have adventures in our local woods. We also burn spruce to provide privacy, although alders can sometimes be a nuisance. Local folks collect the buds to make healing balm. In the spring, the cottonwoods give off a herby, pungent perfume, while the birches provide a burst of gold amidst the monotone of spruce.
I think about the forests of my daughters’ lives these days. How will they come to know and love the changing places they will live in? What will they remember about the woods? What will the trees mean to them?
What we hold dear is the ability to transform our forests and move tree species. But as time passes, these forests will continue to be fertile grounds for memory and magic.
Miranda Weiss resides in Homer, Alaska and is the writer of Tide, Feather, Snow: A Life in Alaska.
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