After 150 years of mystery, we may finally have an explanation for the ghostly white "albino" trees that speckle California's redwood forests.
Standing less than ten feet tall, the so-called "ghosts of the forest" are typically dwarfed by their giant, green-needled kin. These mutated plants can't produce their own energy and they live only a short time before dying off. It's a strange existence – one that has puzzled naturalists since these trees were first documented in 1866.
An "albino" plant isn't the same as an albino animal. The latter condition causes the absence of the pigment melanin, but in plants, it comes down to another missing pigment, the one that allows photosynthesis to happen, chlorophyll.
Without chlorophyll, these trees lack the ability to produce precious sugar, so they must sponge off the root systems of healthy redwoods to survive. Because of their reliance on a "parent" tree, albino redwoods have been called parasites over the years. But something about the interaction didn't add up for University of California, Davis PhD candidate Zane Moore: the healthy trees actually allow it to happen.
"Redwood trees can control where they send their sugars," explains Moore. At any given time, therefore, a healthy tree could easily stop feeding its pale freeloaders, killing them off in the process. "But that's not what we see in nature," he says. "Why would a tree want albino foliage to persist? Why promote a stressful tissue that does not provide for the rest of the plant?"
When a plant or animal willingly gives up its resources to another, there's almost always a reason. Whale lice keep the wounds of their gigantic hosts clean, so they're allowed to camp on their skin; bees fly from flower to flower gathering nectar, but pollinate in the process; the bacteria in our guts help us digest our food. So there had to be some benefit to keeping these arboreal apparitions around, Moore reasoned.
To find out what it was, he teamed up with local arborist Tom Stapleton. The pair focused their work on the rarest albino morph: a split-toned wonder known as a "chimera". While some mutants – known as "everwhites" – appear completely white or yellow, chimeric trees possess both green and ghostly branches. In fact, some chimeras even have green, white and split-coloured needles on the same branch. This is possible because the rare trees have two sets of DNA. Of the 400-odd known albino redwoods in California, just ten possess the rare trait.
"We looked at green and white leaves growing as closely together as possible," explains Moore. "I expected that the albino tissues would be different from green." But what he found buried within the trees' cells was unexpected: the white shoots weren't just sucking sugar from their normal neighbours – they were also absorbing harmful toxins. Essentially, the albinos were poisoning themselves.
"It gives new meaning to the term 'ghost tree'," notes Moore. "They're alive, but really, they should be dead."
Among the toxins were heavy metals like cadmium, copper and nickel. At high enough concentrations, these elements are lethal to plants – and white needles contained double (and in some cases triple) the amount found in green needles.
"The green tissues showed that they were at the threshold of toxicity for all of these metals, basically meaning that they were at the brink of irreversible poisoning," he adds. From the healthy plant's perspective, therefore, handing over some precious sugars to the albino (the risk) meant getting some relief from toxins in return (the benefit).
Albino plants are more efficient at absorbing heavy metals because they move water from root to leaf faster. Once absorbed, those toxins become trapped – no longer able to enter the redwood roots – and seemingly removed from the environment for good.
The team suspects that what started as a random mutation, may well have persisted through the centuries as an ecological lifeline. "Tom and I have noticed that albino redwoods occur with much more frequency close to areas of high human impact (places where redwoods do not naturally grow)," adds Moore. "The foliage grows quickly, accumulates toxins, and dies, and this happens repetitively."
Mine tailings, waste disposal, fertilisers, coal combustion residues, and leaded gasoline and paints are just some of the sources of these toxic metals. If Moore's hypothesis is correct, strategically grafting albino trees could help mitigate damage to the forest ecosystem.
The next step will be to conduct experiments in a controlled environment. The plan is to water trees with a solution containing high levels of nickel to see if redwoods with albino shoots grow better than those without a built-in clean up crew.
"If they do, this will really be remarkable," says Moore. "I think too often everyone takes plants for granted. I like to look at any plant and think, 'just how this plant becomes green takes hundreds of thousands of words to explain, and we still don't know all there is to know about it.' That helps even me to take the blinders off and really marvel at any plant I see."
Top header image: mollie c/Flickr