HAVE YOU EVER wondered how trees get water all the way to their tops? Or what limits the height of a tree? I mean, some western red cedars and Douglas firs get over 200 feet tall, but why don’t they get even taller? Given that our region is home to several of the tallest tree species on the planet, I thought I should investigate.
The coast redwood is generally considered the tallest tree species on Earth. The current record-holding individual is a specimen in Northern California, known as Hyperion, which tops out around 380 feet. Though we don’t have any redwoods, our native trees are still world-class giants. Washington state is home to Douglas fir, Sitka spruce, noble fir, western hemlock, ponderosa pine and grand fir — all of which rate in the top 30 tallest tree species in the world. In fact, two of the largest known specimens in the world live in our state: a noble fir growing in the Cascades and a grand fir in the Olympics.
So is there a limit to how tall a tree can grow? Researchers studying the coastal redwoods think so, and suggest the answer might lie around 400 to 430 feet. They believe the height of a tree is ultimately restricted at this height as the pull of gravity and the friction between water and the vessels it flows through make any further growth impossible. This is known as the hydraulic limitation hypothesis.
The hypothesis is based on the idea that, as trees get taller, it gets harder to pull water to their tops. Tall trees are generally slow-growers and have very narrow water vessels, called xylem. The fight with gravity and friction means that less water is available to the needles or leaves at the top of the tree. The lack of water limits the effectiveness of photosynthesis in this part of the plant. Eventually, the tree throws in the towel and says, “Shirley, it’s just not worth it!” It’s a waste of effort to grow any taller.
Even a tree growing in an environment with virtually unlimited water, such as the Olympic mountains, can’t justify carrying water up past a certain height. The branches and needles at the top of a full-grown grand fir are essentially in a constant drought. The drying effect of the wind up in these uncharted parts of the canopy doesn’t help the situation. Any further growth would use more energy than the tree could gain from the new branches.
As you might assume, the world’s tallest trees are also very old. Tall trees are generally slow-growing and very long-lived. Fast-growing trees are typically shorter and have shorter life spans. Fast growth requires wide xylem channels to move water quickly. Xylem are essentially very long straws inside the tree. These wide xylem vessels are less structurally sound and more likely to have failures, which can limit water movement and reduce the life span of the tree. Our tall conifers might grow so tall and live so long because of their narrower xylem. They are playing it safe, watching all those foolhardy local deciduous trees burn out in a blaze of glory.
Short- or long-lived, it’s impressive that any tree can grow a few hundred feet tall. So, how do they get water all the way up there? Turns out, nobody really knows. Just like we don’t know exactly how tall trees can get, we don’t understand the exact physics inside a tree’s xylem. There are a few things we do know. For one, a water molecule has a negative charge on one side and a positive charge on the other. This polar duality makes it easy for water molecules to attach to each other. This creates surface tension and helps plants draw water from their roots up to their leaves.
Trees are constantly losing water as their leaves transpire. As each droplet of water evaporates from the surface of a leaf, a constant chain of water molecules running all the way from the roots pulls up another molecule to replace it. This transpiration pull relies primarily on capillary action and the water’s surface tension to draw the water upward in the xylem. The xylem is like a never-ending production line in a factory. Every time a “Far Side” calendar drops off the end of the conveyor belt, another one instantly appears.
I hope you find these theoretical answers to the mysteries of our tallest trees somewhat satisfying. At a minimum, it can serve as a reminder to look up and appreciate the majestic feats accomplished by the towering trunks all around you.