Photo by Cole Shatto licensed under CC BY-SA 3.0
Photo by George Bruder licensed under CC BY-SA 4.0
If you are a very lucky person hiking in the redwood forests of California you may just be able to see a ghost. Not a "real" ghost of course, but pretty darn close. Scattered about these ancient forests are rare and peculiar albino redwood trees! Seeing one is seeing something very special indeed.
Redwoods (Sequoia sempervirens) are some of the largest and oldest organisms on the planet. They are famous worldwide for their grandeur. Aside from their obvious charismatic physical traits, redwoods are quite interesting genetically. These giant gymnosperms are genetically hexaploid, meaning they have 6 copies of their genetic code. What this means for redwoods is the ability to experiment with a wider array of mutations than a diploid organism like you and I. A mutation in one set of chromosomes still leaves 5 other copies to maintain normal genetic function. Whereas this can translate into massive benefits in defenses against pathogens, it also means there is a lot of room for error as well.
The albino redwoods are an example of a seemingly dead end mutation. For a plant that relies of photosynthesis to survive, the loss of photosynthetic pigments should spell disaster. The question is why do albino redwoods exist at all? Well, the albinos become parasites on their photosynthetic parents. You see, albino redwoods are mutant offshoots of healthy trees. Something in a bud goes awry and the resultant shoot fails to develop chlorophyll. Sometimes chimeras arise which produce leaves that are half photosynthetic and half albino. Still, how do these mutations persist?
Researchers have found that the leaves of albino redwoods have twice the amount of stomata than do normal redwood leaves. This makes them quite susceptible to drought. During dry years, the trees quickly dehydrate and their host trees withdraw all support. The albinos will often die off but then re-sprout when conditions improve. This disappearing and reappearing act further lends to their mythos. However, this does not capture the full picture. The fact that photosynthetic redwoods tolerate the albinos on any level is quite curious. Even photosynthetic branches that don't produce enough energy are shed. What else could be going on?
Recent research might have found the answer. The albinos most frequently occur along the edge of the redwoods range where conditions just aren't that conducive. What's more, the soils around these albinos are often high in toxic metals such as nickle, cadmium, and copper. When researchers took a closer look at the chemical composition of the albinos, they found that they accumulate these toxic heavy metals at much higher rates.
In a healthy tree, these metals interfere with the photosynthetic machinery, making them quite toxic indeed. Because the albino redwoods are incapable of photosynthesis, this is not an issue. This has led to an interesting hypothesis. It could very well be that the photosynthetic redwoods tolerate their albino offshoots because the albinos accumulate the toxic heavy metals in their tissues and thus keep them away from healthy, photosynthetic tissues. This ideas is still in the hypothesis stage but work is being done to see if it plays out in the wild.
There doesn't seem to be a solid consensus on how many albinos exist in the wild. I have seen numbers as low as 25 and as high as 400. Either way, they are a rare element of the coastal redwood community. With thousands of acres still to be explored, it is likely that more will turn up. While some exist in protected parks, many are under threat with increasing fragmentation of these ancient forests. Very little of the coastal redwood forests are under protection and we may be losing more than we will ever know.
Photo Credit: Cole Shatto and George Bruder
Further Reading: [1] [2] [3]
