California’s redwoods benefit from fog because the trees can open up their pores without risking a major loss of moisture. (istockphoto)

Last year, a team of climbers led by arborist Will Blozan measured the tallest tree in the eastern United States: a 192-foot tulip tree in the Great Smoky Mountains. Although the achievement was significant, it served to emphasize just how puny Eastern trees are compared with the giants along the Northern California coast.

The current height champion out West is Hyperion, a 379-foot coast redwood standing somewhere in California’s Redwood National Park. (Researchers have kept the precise location quiet to protect the world’s tallest tree.) That’s just a shade under double the size of the tallest Eastern tree. In fact, even the average coast redwood grows more than 100 feet taller than any tree in the East.

And the height disparity isn’t limited to redwoods. Douglas firs in the western United States and Canada might have grown close to 400 feet tall before logging eliminated the tallest representatives of the species. (There are historical accounts of equally tall mountain ash trees in Australia around a century ago, but those have suffered the same fate as the tallest Douglas firs and redwoods.)

There’s no denying it: Trees are simply taller out in the West. But why?

Temperature plays a major role. Both freezing temperatures and extreme heat can cause a phenomenon known as cavitation, which prevents trees from growing very tall. Here’s how it works:

Trees conduct water and nutrients from the ground to their leaves through tubelike passages known as xylem. When water in the xylem freezes and then thaws, air previously dissolved in the water forms a bubble. Those bubbles block the flow of fluid and nutrients to the leaves.

Since tall trees have to convey so much water and nutrients to their sprawling leaf system, their xylem passages are very broad, especially near the bottom of the trunk. Such large-bore tubes are particularly vulnerable to freeze-and-thaw cavitation. Trees in the East that experience these weather extremes, therefore, restrict their height to keep the xylem narrow, which prevents cavitation.

Heat can also cause cavitation. When the ambient air is warm and dry, it tends to pull moisture from the leaves. At extreme levels, the ambient air begins to pull very strongly on the water inside the tree.

“Heat can stretch the water column inside the xylem like a rubber band,” says George Koch, a plant scientist who has studied the biological limits of tree height at Northern Arizona University. “If the difference in pressure between the ambient air and the cells grows high enough, the water column shears apart, bubbles form, and water can’t flow.”

Not too hot, not too cold

To understand why Eastern trees are relative dwarfs, therefore, all you have to do is look at temperature ranges. In Redwood National Park, it rarely gets much above 70 degrees or much below 40 degrees. Few areas along the Eastern seaboard can boast such a narrow temperature range.

The advantages of moderate temperatures go beyond cavitation. Maples, oaks and many of the other trees that dominate the East manufacture sugar-alcohol compounds that act as an antifreeze. As a result, when temperatures drop very low, ice crystals form only on the outside of cells. Redwoods do not make these chemicals, which means a freeze can kill them.

Returning to the hotter end of the temperature spectrum, hot and dry air can stunt a tree’s growth. When warm air tries to pull moisture from leaves, it can close its pores, or stomata, to maintain its water supplies. But closing the pores also means stopping carbon dioxide from coming in, and carbon dioxide is what makes photosynthesis, a plant’s respiratory process, go. On scorching summer days, therefore, trees have to decide between growing tall and keeping their moisture.

Temperature isn’t the only issue. Soil is rich in the areas where the tallest trees grow. The ample supply of nutrients gives them the freedom to grow tall.

“The life of a plant is about balancing water, nutrients, carbon dioxide and light,” says Koch. “If a tree has plenty of all the others, life becomes a race to the light.”

Scientists don’t agree

Then there’s the fog. As anyone who has visited redwood country knows, it’s pretty foggy out there. Foggy air is wet air, and wet air means the trees can open their pores up without risking a major loss of moisture. And where the redwoods grow tallest, the fog rolls in in the afternoon and evening, after the sun has dried out the soil.

The degree to which redwoods rely on fog for their growth and health has some researchers very concerned. Environmental scientists James Johnstone and Todd Dawson of the University of California at Berkeley have discovered a startling trend in West Coast fog: Over the past half-century, the redwoods’ environment has become
33 percent less foggy, yet another result of global climate change. Dawson projects that, in 40 to 70 years, continued diminution of coastal fog would threaten the survival of the redwoods of coastal Northern California.

Not everyone is convinced that this change in climate spells doom for the redwoods, though.

“The loss of fog isn’t necessarily a bad thing for redwoods,” says Stephen Sillett, a renowned ecologist at Humboldt University, in the heart of California’s redwood country. “When you look at the tree rings, you see that the redwoods have grown at historically fast rates during the time Professor Dawson studied. When fog decreases, light increases. And more light means more growth.”

While the long-term effects of climate change on the world’s tallest trees aren’t yet clear, one thing is: The world’s tallest trees are not likely to get much taller. Koch’s research suggests that cavitation becomes so common in trees taller than 420 feet that they lose the ability to bring sufficient water and nutrients to their crowns. Any extra light they get from loss of fog won’t go toward height.

“Redwoods experience 90 to 95 percent of their height growth in the first 30 percent of their lifetimes” of around 400 years, Koch says. “They grow fast and hit a ceiling. Then they just grow wider, kind of like humans.”