Vertical hydraulic gradients and the cause of foliar variation in tall redwood trees (Sequoia sempervirens)
In the upper reaches of their crowns, coast redwoods struggle to lift water and nutrients into their leaves. This struggle begins a process that limits tree growth, according to a team of researchers studying redwoods in Prairie Creek and Humboldt Redwoods State Parks.
Trees transport water and nutrients from soil up to their leaves via a cluster of internal capillaries that work much like a straw. As water evaporates away from leaves, a negative pressure or tension draws the column of sap up to displace the exiting water. This phenomenon is commonly known as the cohesion-tension theory.
Above a certain height, however, the pressure driving the flow of water and nutrients can drop to the point that air bubbles form in the tree’s capillaries, creating a break in sap flow. These breaks in flow, called “embolisms,” severely lessen the amount of nutrients reaching high-up leaves and can even cause whole branches to die.
A tree naturally works to avoid this. It does so in two ways: 1) by producing smaller leaves in the crown that have fewer water-releasing pores, called stomata, and 2) by closing its stomata. This slows the flow of sap, reducing the risk of embolism.
Stomata-closing, however, is the beginning of the chain of events that limits tree growth. These microscopic pores admit carbon dioxide into the leaves that house them. Carbon dioxide is necessary for photosynthesis. Trees need to photosynthesize to build tissue. The result: trees slow or stop growth.
So how can redwoods grow to be so tall? Part of the reason lies in summer fog easing the stress on leaves to close their stomata. Less water escapes leaves that are surrounded by fog, allowing stomata to remain open and the photosynthetic factory to carry on. Dr. Todd Dawson of U.C. Berkeley has even shown that in some instances of heavy fog, a redwood tree will reverse its flow of sap and take in fog water directly through its leaves. Testing of leaf water content has shown that the morning after nights with especially soupy fog, as much as 6% of a redwood leaf’s water can be identified as fog water.
Taking into consideration the change in several biological characteristics with increasing height, including leaf structure, photosynthetic levels, and the pressure driving water and nutrient flow, the team estimates that the maximum height a redwood can grow is 130 meters, or 427 feet.
They note, however, that climate change might reduce this estimate. Smaller trees in the southern end of the redwood range are an example of how drier climates affect redwood growth. If climates dry out across the entire redwood region, they will likely cause crown branches to die and, on a larger scale, the forest to shrink in height.
–Jennings’ study was peer-reviewed by the biology faculty of Humboldt State University and accepted as a partial fulfillment of the requirements for an M.S. thesis. His thesis was the precursor to the article published in Nature under the title “The Limits to Tree Height.”
Grant Applicant Organization: Department of Biological Sciences (Humboldt State University)
Grantee: Stephen C. Sillett and Gregory M. Jennings
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