What keeps the tallest trees from growing even taller? A new study in statuesque redwoods finds that the trees stop growing when their highest leaves start dying of thirst.
Biologists have long thought that the height limit for trees comes down to a plumbing problem. Water rises through a simple process: As water evaporates from leaves, tension within the tree's pipes pulls water from the roots to replenish what's been lost. But after a certain height, the force of gravity becomes too much, and this flow peters out. As a result, photosynthesis grinds to a halt and the tree can't grow higher. At least that was the general idea--experimental data were scarce, and many of the details remained sketchy.
That's why George Koch of Northern Arizona University in Flagstaff and his colleagues strapped on their climbing gear and took to the tallest trees in the world, coastal redwoods (Sequoia sempervirens). At Humboldt Redwoods State Park in northern California, the researchers used a bow and arrow to get a rope over the lowest limbs. From there, they clambered up to the canopy and took cuttings at different heights on eight redwoods, including the tallest living tree on Earth, which towers 112.7 meters. They measured the water potential--a combination of water pressure and solute concentration that drives water transport--in the stems and leaves of their cuttings. Back in the lab, they measured the ratio of carbon isotopes, an indicator of photosynthesis.What they found confirms and adds new detail to what most biologists had envisioned. In the highest leaves, water potential was so low that scant moisture made its way into thirsty cells. This low flow prevented leaves from expanding, resulting in small scaly leaves up high compared to broad flat ones down below. To save what little water there was in the treetops, the leaves shut their pores to prevent evaporation. But this stingy strategy also kept out carbon dioxide, reducing photosynthesis in the uppermost leaves. All of these factors combine to put a ceiling on growth, the team reports in the 22 April issue of Nature. Based on their measurements, the team estimates that redwoods cannot surmount 122 to 130 meters--on target with the tallest redwood ever recorded.The study ties together a number of ideas that were floating around in the literature, says Michael Ryan, a research ecologist for the U.S. Department of Agriculture Forest Service. He points out that height limits for other trees are likely to be lower than those for redwoods, which have a unique advantage that helps them overcome water shortage in their highest branches. “They might get that tall because fog off the ocean helps counteract these problems.”
Humboldt State University Institute for Forest Canopy Research
Humboldt Redwoods State Park
Michael Ryan's site
University of California, Berkeley, Center for Forestry--Redwood Region Science Symposium last month