NewsOctober 5, 2011

As the Climate Warms, Magnolias Move North

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By Bruce Dorminey

Warmer temperatures and longer growing seasons, brought on by climate change, are causing the iconic Southern Magnolia to migrate north. Credit: Gottfried/flickr.

Scarlett O’Hara herself would likely be scandalized by what researchers found when scouring a plot of central North Carolina forest outside Chapel Hill. Jennifer Gruhn was looking for Southern magnolias, one of the most enduring symbols of the American South (besides Scarlett herself, of course), and the state flower of both Mississippi and Louisiana.

The scandal was that she found them — no fewer than 500 of the magnificent trees, with their dark green leaves and spectacularly fragrant blossoms — in an abundance unexpected for a location so far north. And as with so many changes in the natural world lately, Gruhn, a biology graduate student at Washington University in St. Louis, thinks climate change may be at least partly responsible.

Writing in the June issue of Southeastern Naturalist, Gruhn and her co-author Peter White, of the University of North Carolina, Chapel Hill, point to average temperatures some 2.7°F higher, and a growing season several weeks longer, than it was a few decades ago.

The garden-savvy reader will note that Southern magnolias can be found in the actual North as well, on the wrong side of the Mason-Dixon Line. Envious Yankees have been planting cold-tolerant hybrid magnolias for years (they’ve also been planting an entirely different type of magnolia, with lighter-green leaves; these have never minded colder weather). But the trees Gruhn found were growing wild, so it’s still somewhat surprising to find them thriving so far from home.

The garden-indifferent reader might wonder what difference it all makes — and for a single species, it might not. But lots of other tree species appear to be headed north as well, including American basswood, yellow birch, black ash, big tooth aspen, and sugar maple — the last of which may move entirely out of Vermont and into Canada in coming years, making “Vermont Maple Syrup” an archaic term.

When so many trees are all headed in the same direction, that’s a sign that species are on the move, which could yield ecosystem changes. Such changes might be disruptive to creatures that depend on such species for food, shelter and beauty — including us.

A word of caution: human-caused climate change may be the primary reason the magnolias are moving north, but it’s not an ironclad case yet. Ryan Boyles, director of the State Climate Office of North Carolina, notes that the Chapel Hill area was warmer in the past 20 years than it was in the 1960s and 1970s, but not as warm as in the 1930s and 1940s. To prove the human connection, he argues, you’d need good records of what the tree populations were like back then.

On the other hand, most of the trees Gruhn found were less than 30 years old, and magnolias can live a lot longer than that. The expansion and the advent of human-caused warming match up nicely.

“Their results support the broad conclusion that longer growing seasons and warmer winters have allowed seedlings to establish and survive” in new areas, said Scott Merkle, a forest biologist at the University of Georgia who was not involved in the study.

For the magnolias themselves, the northward push isn’t necessarily a big deal; long ago, these ancient trees thrived above the Arctic Circle, during a time when the world was much warmer and wetter than it is now.

But at that time, sea level was scores of feet higher. So even if the magnolias don’t have a problem with climate change, we just might.