WILDLIFE | 11.22.16
The Making of a Giant
The ability of giant sequoias (Sequoiadendron giganteum
) and other tall trees to transport water and nutrients hundreds of feet from the tips of their roots to the tops of their crowns is an impressive example of nature’s engineering prowess. Using none of the tree’s own energy, this life-giving process, which enables these giants to survive and continue climbing, relies solely on the warmth of the sun—and the availability of a sufficient supply of water in the soil. See how they do it under normal conditions, and what happens when drought puts them to the test.
Giant sequoias take in carbon dioxide and release oxygen and water vapor through pores in their leaves, called stomata (plural for stoma). This gas exchange is critical to a tree’s ability to produce the carbohydrates it needs to survive and thrive. On sunny days, so-called “guard cells” hold the stomata open, allowing a steady flow of gases into and out of the leaf.
The trees can only afford to keep their stomata open when the potential for photosynthesis is highest, and when they have access to enough water to replace what they lose through evaporation.
At night, on cloudy days, and during periods of drought, the trees reduce water loss by keeping their stomata closed. This is normal and healthy for brief periods of time, but when droughts last for months or even years, the tree’s ability to photosynthesize and harness the sun’s energy for maintenance and growth becomes severely compromised.
Extended periods of drought can also impact the ability of the forest as a whole to remove carbon dioxide from the atmosphere. With the majority of trees in a forest prioritizing water conservation over photosynthesis, the forest can actually emit more carbon dioxide than it absorbs, potentially exacerbating the climatic conditions that led to the drought in the first place.
The giant sequoia transports water and nutrients along its great height through two separate plumbing systems. One of these systems, the xylem, moves water and dissolved minerals up from the roots to the leaves. The other, known as phloem, carries water and sugars back down toward the roots, and to cells throughout the tree that use this chemical energy to perform their basic functions.
While several forces are involved in pulling water hundreds of feet up through the tree, the main driver of this movement is the release of water vapor through the leaves’ open stomata, a process known as transpiration. As water is released, it creates a vacuum that pulls water upward, like the movement of soda through a straw when you take a sip.
Phloem transports water and sugars that are produced through photosynthesis back down from the leaves toward the roots and into other tissues, where the sugars are either used for tree’s basic maintenance and growth, or stored for periods when photosynthesis slows.
Sometimes the tension required to draw water up through the xylem to the leaves becomes too great. Hot, dry air and dry soils can cause the rate of release from the leaves to exceed the rate of uptake through the roots. When this happens, the water column that typically runs the length of the xylem can snap, disrupting the flow of water to the living tissues that need it.
The trunk of a giant sequoia is perfectly suited to support the mass and towering height of the world’s largest tree. Thick, fibrous bark protects the tree’s vital inner tissues from fire and insects. A layer known as vascular cambium produces new phloem and xylem cells as the tree grows. And as new layers of xylem form, the older layers become dense heartwood saturated with blood-red tannins that provides both structural integrity and resistance to internal pests and pathogens.
Giant sequoias harbor a tremendous variety of life forms. Forest ecologists describe the canopy branches and surfaces of sequoia trunks as ecosystems unto themselves. Among the creatures that call giant sequoias home are bats, frogs, salamanders, and bizarre microscopic creatures known as tardigrades, or “water bears.”
The giant sequoia’s network of endlessly branching roots is of course critical to keeping the largest trees in the world from toppling. But these extensive roots are also the means by which the tree acquires as much as 800 gallons of water from the soil each day.
Interaction/animations by James Davidson
ABOUT THE Illustrator
Jane Kim is an artist, science illustrator and founder of Ink Dwell, an art studio with the mission to inspire people to love and protect the Earth one work of art at a time. She developed her passion for art as a child by obsessively painting flowers and bears on her bedroom walls. She received more formal training at the Rhode Island School of Design and then Cal State Monterey Bay, where she received a Certificate in Science Illustration. She has created works for institutions including the Cornell Lab of Ornithology, National Aquarium, de Young Museum, Smithsonian, and Yosemite National Park. She still enjoys painting flowers and bears, though nowadays she doesn’t get in trouble for painting on the walls.
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