Wednesday, June 03, 2009

Nature Green in Tooth and Claw

I made a brief foray into the backyard today to once again try to tame the flora that is in danger of taking over everything. it kind of reminds me of a Brian Aldiss sci-fi novel I read a long time ago called Hothouse, in which plants evolved to take over just about every ecological niche on Earth, and a small remnant of humanity was banished into a large tree because if people got too close to the ground they’d be eaten by carnivorous plants. (Wouldn’t that make a great TV movie for the Home and Garden Network?) That’s pretty much my backyard. (I’ve decided to practice backyard Darwinism; whatever can survive me—or more precisely, my weed whacker—will live to spawn another day.) I guess I should mow the front yard at some point, before I have to yell at the lawn to stay off the kids.

It was in this context that I was reminded of poison ivy, as there is this colony of vines in the back that looks like it might be poison ivy. There is supposed to be a mnemonic phrase to remember how to tell if a plant is poison ivy. I think it goes something like “Leaves of three, sailors delight”? No, that can’t be right. Um, “Thirty days hath poison ivy, save for February, which has hairy vines...”? No... You know, I find it harder to remember mnemonics than the actual things.

Ah, here’s a picture. Box elder is on the left; poison ivy, with the serrated edges, is on the right. Kind of like a botanical police lineup.
It’s funny; growing up in the suburbs of New Hampshire, I don’t recall ever having had poison ivy, even though I had many friends who did. I find it hard to believe that I am not allergic to it. After all, it’s been estimated that 85% of people have allergic reactions to poison ivy (Toxicodendron radicans), and its brethren poison oak (T. diversilobum), and poison sumac (T. vernix). Heck, I even have allergic reactions to Yma Sumac (Soprano peruviana). Interestingly, the Toxicodendron genus is part of the family Anacardiaceae, which also includes the cashew, pistachio, and mango trees. All families have their ne’er-do-wells, I guess.

The culprit in poison ivy that causes all the problems is an oil called urushiol (the name comes from the Japanese urushi, which means “lacquer”). The urushiol touches the skin and chemically reacts with proteins on skin cells. The body fails to recognize these proteins as normal parts of the body (I think we’ve all had that experience, usually in fitting room mirrors) and sends in its army of T cells to destroy these invading pathogens—which are actually bits of the body’s own skin. The blistering and oozing are the results of the immune system response.

This urushiol is powerful stuff; as little as one billionth of a gram is enough to trigger an allergic response. Oh, and poison ivy can be spread, not by the ooze from the sores but rather from any stray urushiol that is transferred to clothing, pets, tools, toys, or just about any object. It can be transmitted from person to person if the one who made first contact still has the oil on his or her body.

Fortunately, poison ivy rashes are not usually very serious, just irritating, although they sometimes can lead to anaphylaxis, or a potentially life-threatening reaction.

People have tried to eradicate poison ivy but, like the weeds in my backyard, too easily recur. And I did come across an interesting study from 2006 that suggests that global warming may be a boon to poison ivy. Jacqueline E. Mohan, a postdoctoral scientist at the Woods Hole Marine Biology Lab’s Ecosystems Center, found that
[poison] ivy grown under high carbon dioxide grew twice as much over five years as the ivy exposed to today’s level of the greenhouse gas. Also, ivy grown in high carbon dioxide produced a more allergenic version of urushiol
Oh, good, more poison ivy and more toxic urushiol. Splendid. Furthermore:
Poison ivy is in a class of plants called woody vines. Around the world, woody vines are flourishing to the point where they can smother young trees, which are the “forests of the future,” said Mohan. Fifty to 100 years from now, if the level of carbon dioxide continues to rise and carbon dioxide promotes growth in woody vines, “what do we expect our forests to look like?” said Mohan.
Global warming doesn’t just affect carbon dioxide levels, but also the growing season (i.e., making it longer) as well as nitrogen resources. Mohan admits there is more to research before they can conclude what the effect of climate change on poison ivy will be. Still, it may very well be the case that, as the song says, you’re gonna need an ocean of calomine lotion.

Then I came across this news story this morning from the University of Delaware about another unpleasant plant: Phragmites australis, one of the most invasive plants in the United States. It invades by using a toxin which, as the UD researchers found out, gets even more potent in the presence of ultraviolet light (specifically, UV-B rays).

Phragmites secretes gallic acid, which it uses to kill off native plants and establish itself. However, the UD researchers found that UV light degrades gallic acid to produce a second toxin called mesoxalic acid, “effectively hitting susceptible plants and seedlings with a double-whammy.”

Nature adapts in its own ways to changes in the environment, and who knows what other changes in plant life climate change will bring? Maybe Brian Aldiss was exceedingly prescient, and plants will take over at some point. And then herbicide will be a crime. And where do they send people convicted of herbicide? Botany Bay, of course.

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