The future of forests

With a warming climate, trees face an onslaught of changes—heat, drought, fire, flood, pests, and disease. How will they respond?

Image of a river with thickly forested banks. Ducks are on the rocky shoreline.
Faced with an onslaught of changes—heat, drought, fire, flood, pests, and disease—forests are under stress.

A towering evergreen in the pine family, the Canadian hemlock has a range that extends from Alabama to the northern reaches of Quebec and Newfoundland. But not for long. The state tree of Pennsylvania, planted in several locations on campus from the BioPond to College Green, is becoming increasingly rare in the state.

Canadian hemlock, also called eastern hemlock (Tsuga canadensis), thrives in shaded, cool valleys overlooking streams. Threatened by invasive insects and warming temperatures, the tree—like many others—is slowly moving north.

Six thousand years ago, the Canadian hemlock almost disappeared from the historic pollen record due to unknown causes, says William Cullina, the F. Otto Haas Executive Director of the Morris Arboretum. While the hemlock eventually rebounded, other species—ash, oak trees, and beech—found their place in the forest during the hemlock’s wane, Cullina says, eventually diversifying the ecosystem. In the 21st century, it’s not just the hemlock but entire ecosystems that face the challenges of a changing climate in addition to invasive species, disease, and deforestation.

Upward view of a Canadian hemlock's canopy.
Canadian hemlock, the state tree of Pennsylvania, is becoming increasingly rare in the mid-Atlantic.

“Fast forward to now, and we are bombarding our native trees with an unprecedented number of organisms, attacking just about every species all at once.” The loss of one keystone species can make a significant impact on the ecosystem, he says. “If you lose six, what happens? Do we all go to grasslands?”

Forest ecosystems are stressed, Cullina says. They’re facing devastation from insects and disease along with climate change, which is causing increased heat and drought and, in some areas, fire and flooding. How will forests adapt and respond?

“Whether we like it or not, we have colonized and reengineered the planet,” says Richard Weller, the Martin and Margy Meyerson Chair of Urbanism and professor and chair of landscape architecture at the Stuart Weitzman School of Design and co-executive director of the McHarg Center. “Nature is no longer this big, beautiful backdrop to our activities” but has been fundamentally altered because of human actions. Climate change changes everything, he says.

Living in the Anthropocene

Some experts have deemed the current era the Anthropocene, referencing a geologic time period when humans have significantly impacted everything from ecosystems to climate. It’s an epoch in which the sixth mass extinction is unfolding, a decimation of biodiversity affecting every ecosystem.

Looking at the earth, “what we have is one dominant species that has colonized everything else and left it in a series of fragments,” says Weller. He’s referring, of course, to Homo sapiens. The rest of the species—and their habitats—have been “sliced and diced and left in fragments,” he says.

Weller, along with a team of students at the School of Design, is working on one possible solution. He calls it the World Park. The idea is to create three walkable, continent-traversing routes of protected spaces: one running longitudinally through North and South America from Alaska’s Aleutian Islands to the tip of Tierra del Fuego in Chile, the second spanning from Turkey to Namibia, and the third from coastal Morocco to the island of Tasmania.

A flattened image of the globe shows global protected areas, threatened eco-regions, and three proposed World Park trails.
Three proposed World Park trails would provide space for plants and animals to migrate as climate change alters their habitats, says Richard Weller. (Image: Madeleine Ghillany-Lehar)

Protected land covers nearly 17% of the earth’s terrestrial surface, but these patches “are disconnected, so the animals and plants that are protected are actually trapped,” Weller says. “When you see it like that, you think, Oh, shoot.”

The proposed land routes would connect 55 nations and 19 biodiversity hotspots through 163,000 kilometers of protected habitat, allowing species to migrate and mate. That way, if a species is experiencing heat stress in its current location, it’s able—like the hemlock—to move north.

“If you want a robust landscape, you need large patches of habitat and you need those patches to be connected,” Weller says.

To engineer solutions, “you need really big planetary scale initiatives, but you also need sensitivity to the very local, very specific, almost forensic attention to the detail of biodiversity and a diverse world of species and people and culture,” Weller says.

The World Park concept would bridge top-down resources and concepts with hyperlocal knowledge, he says. The idea is to create one overarching system that would galvanize people to pool their resources with one megaproject that would achieve biological representation and connectivity.

“Evolution is relentless,” says Weller. “But if it’s a world that we want to live in, with a certain diversity of species—many of which we’re currently killing—we need to restructure the landscape of human interests in order to create a different kind of mosaic. And we need to do it on a planetary scale.”

Assisted migration

“We are what’s considered the Northern Piedmont,” says Cullina, referring to the geology underfoot at the Arboretum. Stretching between the shadow of the Appalachian Mountains and the coastal plains, the Northern Piedmont is a transitional zone between the more humid, subtropical South and the colder climate of the Northeast.

Currently, the region is experiencing dieback among trees like the Canadian hemlocks and sugar maples, long-time staples of Northeastern forests and forest culture, says Nicholas Pevzner, assistant professor of landscape architecture. Those species are already beginning to move north.

Deciduous trees with dead limbs
Sugar maples, like these in the Chestnut Hill area of Philadelphia, Pennsylvania, are declining in the mid-Atlantic area.

“It’s a given that we will have range shifts and that species will move if they’re able to” he says. “Hopefully we can prepare enough space for these arrivals, making it possible for them to establish in our forests. And some species will need a bit more human assistance to move north quickly enough.”

“When we talk about looking for plants for the future, we’re thinking about things that grow in our eco-region, writ large, but are farther south,” Cullina says. These species have adapted to similar soil and co-exist with similar types of animal, insect and microbial life, but tolerate a longer growing season and warmer temperatures.

The Morris Arboretum is currently evaluating species native to Virginia and North Carolina which, one day, may be new native species in the Northern Piedmont. They’re looking at several oak species, including the iconic Virginia live oaks that grow with the epiphytic Spanish moss, and bald cypress, whose shallow root systems can tolerate flood conditions, projected to increase in the Northern Piedmont.

Sugar maples are projected to be essential gone from the Northern Piedmont by 2075, so the Arboretum is looking at Florida sugar maples and cloud forest sugar maples, the latter a species that was stranded in the mountains of Mexico after glaciers retreated and adapted to grow in that nation’s lower latitudes.

“We’re planting trees to see how they perform in the landscape,” Cullina says. “We have this opportunity to use our sprawling campus as an experiment to see what does well.”

Between the hotter microclimate of Penn’s urban campus to the greener environment of the Morris Arboretum in northwest Philadelphia to the cooler, rural setting of the School of Veterinary Medicine’s New Bolton Center campus, it’s a chance to see how these trees handle varying weather patterns, he says.

New threats: Invasive species and disease

In addition to climate change, native plants are often threatened by insects and diseases introduced through globalization. Chestnut blight killed most of the American chestnuts in the early 20th century. Dutch elm disease, a fungus introduced via the shipping industry, decimated American elms in the mid-20th century. More recently, the larvae of the brilliant green emerald ash borer beetle, native to Eurasia, systematically drill under the bark of elm trees, their consumption girdling and killing an adult tree within three years.

“It’s just like COVID, which spread around the world in months,” Cullina says. “If we’re the ones spreading this around, we have a responsibility to do something about it.”

A dirt road shaded by oak trees growing Spanish moss
Virginia live oaks, a common sight in the American South, may migrate north into the mid-Atlantic as the climate warms. (Image: Ryan Arnst on Unsplash)

Cullina considers a multi-pronged approach. One “tool in the toolbox” is genetic modification. While this should be used with caution, genetic modification has the potential to sustain keystone species, like the American chestnut and elm, that have all but been wiped out, he says.

When chestnut blight started killing the American chestnut tree, breeders decided to cross the American chestnut with the disease-resistant Chinese variety, Cullina says. That first generation had half of its genetic makeup from the Chinese chestnut and half from the American chestnut. Specimens that prove resistant to the chestnut blight are crossed back on to American chestnut and, little by little, the goal is to develop a variety that is very close to the native species while being resistant to chestnut blight. “This can be a slow process,” he says.

“Evolution is genetic modification,” Cullina says. “There’s a slow modification of the gene pool in response to the environment.” There’s a human role to play in assisting species survival, he says. “I don’t think we have time to wait because of this unprecedented onslaught that’s happening to the forest. Not only are you losing the species, you’re losing that genetic diversity within the species.”

Cullina tries to be optimistic about the future of forests, investing in facilities and research. “This is just the start, but for us, with a mission around trees, I don’t know how we cannot not try to do our part,” he says. “Just standing by, witnessing the extinction, is not an option.”

A row of deciduous trees in grass
Rows of Virginia live oak (Quercus virgniniana) planted as a trial at Morris Arboretum.


Fire management

“If we lived on a planet that was not fundamentally transformed by people, if we did not live in the Anthropocene, maybe we could simply leave forests alone and they’d be OK,” says Pevzner.

But even before industrialization, humans altered landscapes in fundamental ways, he says. Case in point: the cultural burning practiced by many indigenous tribes in North America, which created a high canopy and cleared understory.

With a century of fire management through suppression, “we’re getting these runaway blazes,” destructive and difficult to control. A blistering hot forest fire can damage the canopy, burn away the topsoil, and release a significant amount of carbon into the atmosphere, Pevzner says.

The “big picture with carbon emissions and wildfire is that forests are great carbon sinks,” he says. “But as the fire regime changes to have these more frequent and more intense wildfires, forests are changing, transitioning from carbon sinks into net sources of carbon emissions.”

Some forests don’t have the opportunity to regrow, especially those with iconic old-growth trees, like redwoods, which only reach their peak after centuries. And if the topsoil is burned away, the landscape is not able to return to any kind of forested condition, Pevzner says. In addition, the American West is experiencing a once-in-multiple-centuries drought event, “and that’s an indicator of where things are going,” he says.

Part of the fire risk is due to a hotter and drier climate, but the encroachment of urban development on wild environments also poses a threat, Pevzner says. These areas are now facing the same question that coastal communities have faced for decades: hold the line or retreat?

Trees and meadow in fall
Bald cypress planted in the wetlands of Morris Arboretum. 

“It’s definitely a land management problem and a policy problem,” Pevzner says. “Design can help with everything from the way in which we design these adjacent communities to the ways in which we start thinking about forest management as a design problem.”

It’s essential to find new ways of interacting with changing forests, from rethinking settlement patterns to pest management, but the “biggest thing we can do is focus on our carbon emissions in the near term,” Pevzner says. “Every fraction of a degree of global warming compounds and impacts the risk on people and ecosystems.”

Facing the future: ‘Nature’s a good guide’

This is not the first time that forests have faced threats, Pevzner says. But climate change is shifting multiple systems simultaneously and at a faster pace. “There’s a lot of uncertainty in where all of these variables are going to leave us once they run their course,” he says.

If the environment changes too much over a short period of time, says Cullina, “the trees will die; the plants will die.” What kind of response does climate change require, and can the human species still engineer their way out of the problems they created?

“Designing in the broadest sense means just the application of human intelligence to problems and foresight,” Weller says. “How do we envision the future and plan and design for that future? That’s, I think, what’s happening globally.”

“Nature’s a good guide,” Cullina says. The first Virginia live oaks planted on campus are now over 15 feet tall. Their leaves unfurl, stretching towards the sun. At Penn, these plants are a harbinger of what’s to come.