Trees can’t get up and walk away, but forests can

An army of treelike creatures called Ents marches to war in the second The Lord of the Rings movie, The Two Towers, walking for miles through dark forests. Once they arrive at the fortress of the evil wizard Saruman, the Ents hurl giant boulders, climb over walls and even rip open a dam to wipe out their enemy.

Mobile trees like the Ents are found throughout science fiction and fantasy worlds. The treelike alien Groot in Guardians of the Galaxy uses twiggy wings to fly. Trees called Evermean fight the main character Link in The Legend of Zelda: Tears of the Kingdom video game. And Harry Potter’s Whomping Willow — well, it whomps anyone who gets too close.

The trees in our neighborhoods may seem immobile compared to these fictional wanderers, but real trees and forests move too. They just do it really, really slowly.

All trees move as their seeds grow into saplings, stretching up toward the sun to convert sunlight into nutrients. But when they sprout in a shady place, they have to work harder. By slowly stretching their branches in a sunny direction, trees orient themselves to get the most sunlight possible, a phenomenon called phototropism.

Tree roots move too. When they sense moisture in the soil, trees push their roots toward the likely water source. While searching for water underground, roots may tap into wells and plumbing. “Sometimes they get into people’s toilets,” says Gerardo Avalos, a plant physiologist at the University of Costa Rica in San José.

While individual trees can’t cross rivers and climb mountains, entire forests can. And climate change is making their journeys treacherous.

“Trees have been migrating forever,” says Leslie Brandt, an ecologist formerly with the U.S. Forest Service in St. Paul, Minn. During the last ice age, when an ice sheet covered most of Canada and the northern United States, many tree species took refuge in warmer, southern climates. As northern habitats got colder, seeds thrived in the warmer south. More new trees grew on the southern edges of forests, while older trees up north died out. Slowly, forests migrated, moving around 100 to 500 meters a year, Brandt says.

But now, human-caused climate change is altering habitats faster than forests can move. Rising oceans are threatening coastal mangrove forests worldwide. Higher temperatures in Canada are making it difficult for white spruce to grow. And drier conditions in the American Southwest are harming pinyon pines.

“Trees just cannot keep up,” says Brandt. “So, humans are helping them out by moving them.”

Some scientists are planting seeds in areas with favorable conditions. Sometimes, scientists even replace species that are no longer equipped to handle a changed landscape with species better suited to the new conditions.

In Minnesota, Brandt has studied trees on the banks of the Mississippi River. The area is flooding more frequently and severely these days, and invasive beetles are destroying the forests. Floodplain trees like silver maples are dying and struggling to grow.

“We’re looking to replace the trees that are lost with those that are better adapted to the current climate,” Brandt says, like cottonwoods and willows.

For the Superior National Forest in northern Minnesota, Brandt and colleagues created a guidebook to help forest managers prepare for climate change. The team has been working with scientists and local Indigenous tribes to make sure the forest migration plan aligns with community needs.

“We don’t want to completely change the forest,” Brandt says, because people “rely on those trees.”