Signs of Summer 5: Leaves of Three

Photo by D. Sillman

Photo by D. Sillman

I take my dog Izzy for a short walk every morning right after she has her breakfast. We try to get out before the dog across the street (who is either Izzy’s arch nemesis or her boyfriend, I am still trying to figure out which) wakes up and before the general human activity up and down the block gets in motion. Izzy and I have a quiet and almost always very fruitful short walk and she can start her day with an empty bladder and colon. Always start your day with success!

Box elder (Photo by homerwardprice, Wikimedia Commons)

Box elder (Photo by homerwardprice, Wikimedia Commons)

Anyway, the other morning I was looking at the trees growing in the woodlot that borders my street as we walked along on the narrow grass strip. The names of the trees clicked along in my still un-caffeinated brain: choke cherry, sugar maple, yellow poplar, black cherry, gray birch, red maple, box elder.

And I stopped (Izzy kept on going). Box elder? I was not aware that any box elders grew in my wood lot. Box elder(Acer negundo) was a tree of my childhood. It was the tree that lined the cool, shady street that I grew up on in northern Ohio. It was the fast growing but weak wooded tree that got collectively obliterated when a thunderstorm rolled down that same street on a July 4 evening (a couple of  years after we had moved away) and turned the street from a leafy tree tunnel to a barren strip of asphalt. It is the only maple with compound leaves! How could I not have seen it here before?

Photo by D. Sillman

Photo by D. Sillman

By now Izzy was several houses away, and I ran after her and coaxed her to start back with me toward home. I stopped again at my “box elder.” Triple leaflets branched out high over my head in a dense mass of green. It was so tall! It had to have been here for years!
Something, though, wasn’t right. I looked more closely and saw the thick, fuzzy vines wrapped around the trunk of an old maple tree: leaves of three! My “box elder” was a tree sized mass of poison ivy!

Poison ivy (Toxicodendron radicans) grows abundantly in almost any habitat affected by humans. It thrives on disturbance and favors the edges and overlaps of fragmented ecosystems. It grows abundantly not only in my yard and in the ecotones between my mowed field and the surrounding woods and the woods and street but also almost anywhere in Western Pennsylvania. Deborah and I saw so much poison ivy when we hiked the Baker Trail back in 2010 that we wanted to make Baker Trail t-shirts with a poison ivy logo on them!

As I have previously noted about poison ivy (on our “Virtual Nature Trail” and also in this blog) many animals greatly benefit from poison ivy. Songbirds eat its white berries, and deer browse on its tender leaves. If fact, there is only one animal that has a problem with this plant: humans). In a sensitized human the delayed hypersensitivity reaction following exposure to the urushiol oil of poison ivy can range from an annoying itch to a life threatening anaphylactic shock.

Photo by D. Sillman

Photo by D. Sillman

A very logical question to ask about this impact of urishiol on humans is “why?”
There are scores of websites that talk about this property of poison ivy’s urishiol and relate it to a protective function for the plant. If it is a known cause of such toxic unpleasantness, then people will leave it alone. And, if people leave the plant alone, it will grow and thrive in its ecosystems.

That is very logical, but there is no long-term, evolutionary relationship between humans and poison ivy that could have led to the selection of this “protective” feature. Further, no other animals generate the allergic reaction to urishiol that humans do. As I said, birds avidly eat its berries (all of which contain urishiol), and deer and other browsers readily consume its urishiol-rich leaves. How could urishiol be protective when it has no impact on these potential consumers?

Photo by D. Sillman

Photo by D. Sillman

An Asian relative of these species may point us toward a possible answer about the function of urishiol in poison ivy. The Oriental Lacquer tree is a Toxicodendron species that grows in China and Japan. The sap of this tree has been harvested for many centuries to make a high quality varnish for wooden furniture. The urishiol in this sap is a major constituent of the lacquer and reacts with water and air to form a hard, protective encasing material. It is hypothesized that the urishiol in all Toxicodendrons carry out this function. Toxicondendron leaves are quite delicate in structure and are, thus, easily damaged by even light physical trauma. The urishiol in the damaged leaves could react on air exposure to form a resealing lacquer that helps to reestablish the structural and protective integrity of a damaged leaf. The plant benefits ecologically and evolutionarily via this urishiol repair system because it is able to put less of its total system energy into the construction of its leaves. It is thus able to make more leaves and grow more rapidly on a specific amount of available energy.

(And, by the way, the lacquers on these pieces of Chinese and Japanese furniture are capable of generating a “poison ivy” reaction in a sensitized individual. Be careful what you sit on!)

So the urishiol might be there to help repair damaged leaves. Why does it trigger such a powerful immune reaction in humans and, apparently, no reaction at all in all other organisms that come in contact with it? I have only one answer: I don’t know!

Izzy and I went back to the house. I filled up the bird feeders with sunflower seeds and dumped a scoop or shelled corn and a handful of peanuts on the ground beneath them. The crows were hungry and were cawing and calling from spruces next to the yard. Two squirrels waiting in the arbor vitae for me to go back into the house, and a blue jay watched from the red maple. They would all dive for the peanuts as soon as I went back into the house.

Coffee was more important right now that either poison ivy or box edlers.

This entry was posted in Bill's Notes. Bookmark the permalink.

One Response to Signs of Summer 5: Leaves of Three

  1. marilou mcnavage says:

    This was an interesting article for all of us who have “experienced” poison ivy first hand. Don’t know how you keep turning out these informative blogs week after week.

Leave a Reply

Your email address will not be published. Required fields are marked *