To a large extent, the ultimate height of a tree is determined by its genes. Most of the remaining influence comes from the quality of the site in which the tree is growing. In other words, an oak tree may be genetically predisposed to reach about 60 feet in height, but the amount of sunlight and rain it receives determines if it will be a 40-foot-tall runt or an 80-foot-tall behemoth.
However, even trees with great genes and ample resources are still constrained by physical factors, such as gravity and the surface tension of water. It appears that these physical constraints create a cap on the ultimate growth of trees. Accordingly, even California’s tallest redwoods (Sequoia sempervirens), British Columbia’s tallest Douglas firs (Pseudotsuga menziesii) or Australia’s tallest eucalyptus trees (Eucalyptus regnans) are unable to grow much taller than they already do.
How Trees Drink
First, a review of some basic tree physiology:
- Water enters a tree via the roots, and then travels throughout the trunk, branches and twigs, via an assortment of vascular tissues, until if finally reaches the leaves.
- Water escapes the leaves (a process called transpiration) via small holes in the leaf surface, called stomata. The individual stoma open and close to alter the rate of this process, which varies over the course of the day.
- The leaves use a very small percentage of the water reaching them for basic cellular processes and photosynthesis, but most escapes into the atmosphere.
The movement of water through a tree is not an active process. Trees do not “suck” water from the ground, nor do they “pump” water up their trunks. Instead, trees rely on the surface tension of water and something called capillary action to draw water up through the tree. The process works because of both the attractive forces between individual water molecules and the attractive forces that occur between water molecules and other molecules.
Surface tension is produced by the cohesion of individual water molecules – this is why water forms droplets instead of spreading out. Capillary action is exhibited when liquid is placed in a narrow tube. The water molecules are attracted to the molecules in the tube (adhesion); but normally, gravity pulls harder on the water than the tube does. However, if the tube is narrow enough, the adhesive forces can overcome gravity.
Sponges illustrate this principle well: If you place a sponge (which is full of tiny capillaries) half-way into a glass of water, it will draw some of the water up into the sponge via the same mechanisms that trees use.
The water transpiring from the leaves creates a void, which cohesion and adhesion work to fill. This creates tension that further helps to draw the water up the tree. The rate at which water transpires from the tree influences the tension placed on the water column. The faster the water exits the tree the greater the tension on the water column.
Tree Height Limits
As you can imagine, it takes far more tension to raise water to the top of a 400-foot tall tree than it does a 40-foot-tall tree. Ultimately, some trees reach heights where the tension necessary to draw up the water becomes too great. When this happens, the column of water breaks down and bubbles may form in the capillaries – these bubbles break the surface tension of the water and leave voids in the system. The voids cause the capillaries to stop functioning properly, and become useless. This leads to a reduction in vigor, and prevents the tree from growing any taller.
In 2004, George Koch and three colleagues examined this mechanism in order to determine the theoretical maximum height of trees. By studying some of the tallest trees in the world (including California’s own “Hyperion,” which is the world’s tallest documented tree), the team concluded that the tallest possible trees may be able to reach about 425 feet, but not much more. Were trees to grow taller than this, the tension would simply be too great. (George W. Koch, 2004)
Problems with Climate Change
The method by which trees drink not only limits their ultimate height; it also limits their ability to survive climate change.
The tension on the water column is dependent on many factors aside from the height of the tree. These factors include air temperature, solar radiation, groundwater availability and wind speed. Generally speaking, when the air gets warmer, drier or windier, the water evaporates from the leaves more quickly. This increases the tension on the water column, and as we have seen, excessive tensions can cause permanent harm.
A 2012 study by Brendan Choat and 23 other researchers showed that the threshold at which these sorts of problems occur is remarkably close to the tensions trees normally produce. This held true for a wide range of tree species, across several different habitats. This means that if global temperatures rise relatively little, trees are likely to suffer greatly, as many will begin to transpire at rates that will cause them irreparable harm. (Brendan Choat, 2012)
Brendan Choat, e. a. (2012). Global convergence in the vulnerability of forests to drought. Nature.
George W. Koch, S. C. (2004). The limits to tree height. Nature.