Most of California’s trees have evolved to live in our periodically parched state. Some species, such as wax myrtles (Myrica californica), produce glossy leaves which reduce water loses, while others, such as black oaks (Quercus kelloggii), send roots deep into the ground to access water few other plants can.
Yet some other trees have evolved a more unusual method for surviving in the arid west.
Creative Water Collection
The Pacific coast’s most iconic trees, the Coastal redwoods (Sequoia sempervirens), are somewhat drought resistant (at least mature trees – saplings are rather susceptible to drought stress), but large trees require large quantities of water. They do not produce especially waxy leaves or deep taproots, so how do they obtain their water?
They – and several of their ecological neighbors — obtain supplemental water in the form of fog. In fact, redwoods often satisfy up to 30 percent of their water needs annually from fog. Fog is a great source of water for the trees and plants of coastal forests, and, historically, its presence has been relatively consistent.
Further, because fog is typically more common during California’s summers than any other time of the year, the water is available at the time it is needed most. Our summers, as you know, are a touch on the dry side.
Much of the fog floating in from the Pacific Ocean condenses on leaves and other surfaces, rolls off onto the ground and soaks into the soil. Unfortunately, much of this water evaporates back into the warm summer air before the trees can absorb it with their roots.
Most trees do not significantly benefit from fog blowing across their boughs. However, having evolved in a place where fog was regular and rain was rare, coastal redwoods have evolved the ability to absorb water directly through their leaves.
By absorbing water in this fashion, the trees can help prevent water stress in their leaves. It also helps keep the surface of the leaf cooler, and reduce the rate of transpiration. But most importantly, this water penetrates photosynthesizing cells, which require water to function.
No Monopoly for the Method
Coastal redwoods are not the only species to exhibit this ability; tanoaks (Notholithocarpus densiflorus) and several other inhabitants of these fog-drenched forests are also capable of foliar absorption. In fact, according to a 2009 study by Emily Limm, Kevin Simonin, and Todd Dawson, sword ferns (Polystichum munitum), madrones (Arbutus menziesii) and Douglas firs (Pseudotsuga menziesii) all absorb more water than redwoods do.
This widespread reliance on fog as a water source helps illustrate its importance to the ecosystem. As we learn more and more about the forest, we may find other ways in which fog helps nourish the habitat.
Is Fog Becoming Rarer?
One unsettling factor in all of this is that some climate models predict that global warming may increase the amount of fog, which would likely be well-received by the species of the forest, while others predict that less fog will coat the coast in our warmer future. If the amount of fog decreases, the redwoods and their fellow flora are sure to suffer.
According to the work of University of Washington researcher Jim Johnstone, the amount of fog blanketing coastal California has decreased by about one-third over the last 100 years or so. If his conclusions are correct, it means that not only have the state’s redwoods been suffering from acute drought stress for about 4 years, they have been slowly suffering for nearly a century.
However, other researchers, analyzing different data sets have found the reverse to be true: More fog is coating the coast than ever before, and it is likely to increase in the future.
As is so often the case, only time and further research will reveal the truth.