Coastal redwoods (Sequoia sempervirens) are among the most famous trees in the world. Most of this fame precipitates from their incredible size, as they are among the tallest species on Earth.
However, coastal redwoods are more than just really tall trees – they also provide a number of examples of evolutionary adaptation, from their reliance on fog for moisture and their ability to maintain lush canopies, despite the harsh conditions found at these elevations.
Basic Description
Coastal redwoods are evergreen conifers with small, rounded canopies. Their branches usually droop slightly at the ends, and the tree’s red bark usually fades to gray over time.
Coastal redwoods bear different leaves on different portions of the tree. Flat, long leaves are found on young branches and shaded canopy branches, while scale-like leaves are found on the sun-drenched leaves of the canopy. Intermediate leaves adorn branches in transitional areas.
The scale-like leaves found at the tops of the trees help to slow evaporation. This is a crucial adaptation, as the high temperatures and low humidity found at these heights would quickly cause the trees to dehydrate, if they did not place some limits on the rate of evaporation.
Record Holders
Coastal redwoods are unquestionably the tallest trees in the world. The record holder, named “Hyperion”, measures almost 380 feet in height; unfortunately, woodpecker damage near the top of the tree will probably limit any future growth. However, many other redwoods are nearly as tall — more than 10 trees reach heights in excess of 367 feet.
Water Acquisition
Even though established redwood trees are somewhat resistant to drought stress, large trees require large quantities of water to remain healthy. Accordingly, redwoods have been forced to develop supplemental methods for collecting enough water.
In addition to drawing water up their roots, as most other trees do, redwoods utilize fog as a water source. As the fog condenses on their leaves and branches, it rolls off onto the ground below. This fog is so crucial to redwoods that their natural range stops about 50 miles from the Pacific Coast, which marks the maximum penetration of fog in the region.
However, much of this condensed water evaporates before it soaks into the ground. Therefore, while the ground has more moisture than it would have without the fog water, the precious resource is still at a premium. To help the trees further meet their moisture requirements, they rely on a dense mat of mycorrhizal roots to draw water more efficiently from the ground. The mycorrhizae increase the surface area of the roots drastically, which enables the immense trees to draw more water than they would otherwise be able.
Limits to Height
Though they reach heights that other species fail to attain, coastal redwoods face a barrier that prevents them from becoming taller: The method by which they hydrate themselves. Just like other trees, redwoods draw water from the ground and allow it to evaporate from their leaves; however, there are limits to how far water can be drawn in this manner. It appears that, were coastal redwoods to grow much taller, they would be unable to maintain a continuous column of water in their xylem cells, which would lead to air bubbles forming in the wood.
These air bubbles would stop the flow of water up the tree, and in many cases, prevent that section of the tree from carrying water in the future. Accordingly, redwoods approach the limits for tree height on our planet – no trees are thought to be able to grow much taller.
Trees try to prevent this problem by reducing the amount of evaporation escaping from leaves in the canopy. While this reduces the risks of air bubbles forming in the wood, it reduces the growth rate of the trees. Once again, fog plays a critical role in the lives of these trees, as it reduces the amount of water that evaporates from the leaves. This allows the tree to continue to draw water from the ground, and ultimately grow much higher than other trees.
Albinos
Some redwoods produce pale, white or off-white foliage. These specimens are “albinos”, who fail to produce chlorophyll. In some cases, entire trees are albinos, but, more commonly, albinos occur as portions of otherwise normal trees. Called chimeras, such trees possess two different genomes.
In both cases, these pale white leaves cannot produce food for the tree. In the case of chimeras, the tree’s other, healthy leaves produce enough food to make up for the handicapped leaves. In the case of trees that lack chlorophyll entirely, they survive by forming root grafts with nearby, healthy specimens. From within these grafts, the albino trees can draw enough energy to survive.
While it is possible that albino trees survive in spite of their condition, some researchers believe that albinism may be a response to stress. Given the lack of suitable rainfall over the last few years and the fact that most albinos occur on marginal sites, the idea is not without merit.