July 2008

High Science - Making Discoveries in the World’s Tallest Trees

Hiking through a forest, it would be no surprise to find ferns, berry bushes, and the occasional salamander. But in California’s giant redwood forests, you can find them 300 feet above the forest floor, and it’s this unique ecosystem that Steve Sillett studies.
Sensors are installed on a 371-foot redwood to measure the flow of sap.
The broad platforms created by large limbs serve as important habitats in redwood forest canopies.
Sillett and his wife, Marie, in a large fire cave that occurs in the main trunk of this tree 161 feet above the ground.
A huckleberry bush growing from a fire cave.

Sillett is a redwood ecologist at California’s Humboldt State University, and he specializes in researching the complex web of organisms that live hundreds of feet above the ground––organisms that call the redwood canopy their home.

Climbing these giants, some more than 350 feet tall, Sillett confronts a variety of epiphytes, plants that grow on other plants. They are not considered parasitic, as they draw their nutrients from the air and rain. In the case of redwoods, epiphytes can be as large as hemlock trees thriving on the giant limbs of their hosts.

Additionally, when forest fires burn through a stand of redwoods, they can leave large hollowed out fire caves within the trunks, providing rotten, spongy pockets in which plants thrive. And it’s not unusual to find huckleberry bushes rooted in three feet of accumulated dirt within these fire caves. “We’re interested in quantifying how much water is stored in these rot pockets to allow the huckleberry bushes to thrive,” says Sillett. “Even in dry years, they seem to have plenty of water to make berries. In the wintertime, the huckleberries turn red, so this thing's like a beacon.” Even from a distance, he marvels, “you can tell it’s a huckleberry way the heck up here on this spire.”

“Redwood forest canopy is full of all kinds of creatures,” he says. “On the way up, encountered about 4 different flying squirrel nests. They live inside hollows created by bark that flakes away, and they strip the bark and make these soft little cozy dens. Also saw some really neat epiphytes… including one that I never noticed in the tree before, having surveyed this tree for many days, there is still something to be discovered. I found a sword fern growing on an old gnarly limb that I'd never noticed.”

Redwoods can survive for thousands of years, playing host to many generations of organisms. Gigantic sections of tree can die or rot out, and the tree itself can continue to thrive. “Centuries ago [the tree] was building this wood, and because it invested so well in the decay resistance of this wood, this top is lasting for hundreds of years after it's died,” Sillett explains. “And a lot of these trees have just enormous quantities of dead wood, like tens of cubic meters of just rotting wood. And then there's the main trunk, much of which is often dead. When their tops break out, the water seeps in over the centuries, and so they become these holding tanks for moisture. And all kinds of organisms can take advantage of that moisture, as long as they can survive in this well exposed and illuminated environment… quite a few species have evolved to deal with these situations… It's a pretty amazing ecosystem.”

So is there a limit to the growth of these ancient behemoths? Sillett recently measured the world’s tallest living tree, a 379-foot redwood, and found its growth to be negligible. “The thing's just eking along here, it's not doing much. Which isn't, I guess, too surprising. I expected a few centimeters of growth. But none is evident,” His team has run a battery of tests which suggest that the challenge of sucking groundwater 300-plus feet into a redwood’s crown has a way of restricting growth. The stress becomes too much, and photosynthesis, the tree’s “engine” so to speak, can shut down.

Links to related stories:
Science Diary: Redwoods – Tallest Tree
Science Diary: Redwoods – Too Tall
Science Diary: Redwoods - Clearcut
Science Diary: Redwoods - Replacements
Science Diary: Redwoods – Sap Flow
Science Diary: Redwoods – Battery
Science Diary: Redwoods - Cleanup
Science Diary: Redwoods – Ecosystem Aloft
Science Diary: Redwoods – Biodiversity
Science Diary: Redwoods - Straw
Science Diary: Redwoods - Climbing