iking through Redwood National and State Parks is a powerful experience. Trees as tall as 35-story skyscrapers stand in the same spot where they’ve stood for more than 2,000 years. What secrets do these ancient trees hold? And how do they get so tall?
Dr. Steve Sillett, a Humboldt State University professor researching redwood forest ecology, is working to answer these very questions. He spends his days scaling treetops in Redwood National and State Parks with a mere harness and rope, trying to figure out how trees work and how they grow. As simple as the concept sounds, it’s a relatively unexplored frontier.
“Despite all the years of research on forests, we still don’t really understand the functioning of an individual tree,” Sillett says. What stumps scientists is the connection between a tree’s “crown performance”—what’s happening in the treetop—and what’s going on below ground. Knowing how trees get tall is one small step toward answering that question.
Picture this: Water travels up from the ground through the tree’s roots, creating a water column that reaches the tops of the highest leaves, assisted by a phenomenon called capillary action, which literally allows water to climb walls. But the column of water faces many challenges along the way—fighting gravity as it travels higher up in the tree; overcoming friction as it flows against cells in the wood; and finally evaporating when tiny pores in the leaves, stomata, open up to breathe in carbon dioxide for photosynthesis. Between this friction, gravity, and transpiration, the water column stretches tighter and thinner, like a rubber band being pulled from the top. But the rubber band can withstand only so much tension before breaking—and when it breaks, air bubbles form. Since tension increases with height, the top of a tree is more vulnerable. If an air bubble forms at the top of the tree, whatever growth is above it gets cut off from the water column and dies back. It’s only when the water column is restored that new growth takes its place.
Redwoods overcome these obstacles better than most trees. But why? We know that the tallest individuals grow at low elevations along major streams on California’s coast, where soils are rich and moist year-round. It’s rare for these trees to become stressed over a lack of water—not only because of an ideal habitat, but because of a fascinating adaptation: Redwoods can actually absorb water through their leaves. “The vast majority of their water comes from the soil,” Sillett says, “but fog and rain can supplement that, which might be just enough to give them an edge.” Other adaptations, such as an impressive resistance to wood rot, fire, and shade contribute to its lofty heights.
“I’ve been measuring the tallest redwoods, eucalyptus, and Douglas fir, and they’re still growing, sometimes up to a foot a year,” Sillett says. “It’s clear that they’re capable of getting taller—so the question is what’s limiting them? Will they reach a height at which they can’t grow any further?”
Maybe. Redwoods face a challenge common to all trees whose leaves have evolved to close off their pores when water loss is a threat. Although this adapation preserves water at great heights, it shuts down carbon dioxide intake, forcing photosynthesis to come to a screeching halt. To what extent this tradeoff limits growth remains a mystery.
The answers, Sillett thinks, lie in the most ancient trees, whose complex crowns host entire ecosystems hundreds of feet off the ground. But only about 4 percent of original redwood forests remain today, scattered through-out 34 California redwood state parks, Muir Woods National Monu-ment, and Redwood National and State Parks. That’s bad news for species like the endangered marbled murrelet, an ocean bird that only nests in old growth forests; the fuzzy, mouse-like red tree vole; and the wandering salamander, who can spend its entire lifespan in the treetops.
Younger redwoods, which make up the majority, aren’t complex enough to support this kind of wildlife. And that doesn’t sit well with Sillett, who’s not ready to admit that we must wait 500 years to see the forests mature. If he can figure out the mysteries behind tree growth, he might be able to simulate ancient forests by manipulating tree structure in younger, second-growth stands.
“Tree-crown complexity appears to be a response to disturbances in the form of injuries, like wind damage and fire,” Sillett says. “We need to do experiments to find out if it’s feasible to carefully injure trees to promote the development of crown-level complexity.”
It’s an ambitious mission to restore diversity in forests with a “heavy-duty industrial past.” But success could foster a growth spurt of complex canopies, with all the charms and biological benefits of an old-growth forest.
Amy Leinbach Marquis is assistant editor for National Parks magazine.
Hiking through Redwood National and State Parks is a powerful experience. Trees as tall as 35-story skyscrapers stand in the same spot where they’ve stood for more than 2,000 years. What secrets do these ancient trees hold? And how do they get so tall?
