We're all familiar with migration: Wildebeests gallop across Africa, Monarch butterflies flit across the Americas ... but did you know that forests migrate, too?

In his new book The Journeys of Trees, science writer Zach St. George explores an agonizingly slow migration, as forests creep inch by inch to more hospitable places.

Individual trees, he writes, are rooted in one spot. But forests? Forests "are restless things." As old trees die and new ones sprouts up, the forest is — ever so slightly — moving.

"The migration of a forest is just many trees sprouting in the same direction," St. George writes. "Through the fossils that ancient forests left behind, scientists can track their movement over the eons. They shuffle back and forth across continents, sometimes following the same route more than once, like migrating birds or whales."

/ W. W. Norton & Company

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W. W. Norton & Company

A forest sends seeds just beyond its footprint in every direction, but the seeds that go to the north — assuming the north is the more hospitable direction — thrive a little more than the ones that fall to the south. Over time, this forest would march steadily northwards.

"The migration of a forest is communal, it's constant. It is accomplished over many generations ... " St. George explains. "It's a question of the species succeeding more in one part of its range, becoming more abundant in one part of its range, and less abundant in another part of its range."

This has happened over millennia, and climate change tends to be the driving force — pushing and pulling forests around the globe. Of course, today, climate change is speeding up, and trees can't keep pace.

Take California: it's getting hotter and dryer and scientists estimate that before too long, Joshua Tree National Park may not be able to sustain Joshua Trees.

Zach St. George describes a similar threat to Sequoia National Park, during California's epic drought a few years back.

"The scientists there had never seen anything like it," St. George says. "They worried that maybe Sequoia National Park would no longer be the place for giant sequoias. And I think at some point we will lose these ancient trees and we will have to think about what we do with the places, and do we plant new groves somewhere else?"

This is known as "assisted migration" — humans planting trees in other places where they're more likely to thrive.

But this process carries risks — people can accidentally introduce insects and diseases to new places, where they may wipe out entire native populations. So, St. George writes, there's a debate among conservationists and foresters today: Should humans help the trees escape?

"I think there are going to be instances where people are probably going to step in and help species move to places where they'll be more suitable in the future," St. George says. "So far, there are no huge movements of citizen groups moving trees north or upslope. But that's kind of one vision of the future that the people I interview sort of hope to see."

Writing a book about forests unable to outrun climate change may sound grim, but St. George says he still finds reasons for optimism.

"Climate change is going to have these really dramatic effects on forests," he says. "We're going to lose a lot of forests. We're going to see species rearranged. We'll probably see more fires and droughts and millions of dead trees. But, you know, the book is very much about trees, but it's also very much about people. And I met a lot of people in the process who sort of see these changes coming, and have mourned what has been lost and what will be lost — and are still continuing to try and do good and try and work towards a better future."

Gustavo Contreras, Gabe O'Connor and Christopher Intagliata produced and edited this interview for broadcast.

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