Is Dog Pee Killing City Trees?

By Michael Mountain, August 30, 2012

You’ve probably never thought about it as you lead Fido from tree to tree on his neighborhood walk, or as he mingles with other dogs for the latest water-cooler gossip around the trees of the dog park. But there’s mounting evidence that Fido and his pals are killing those trees.

On the Atlantic Cities blog, John  Metcalfe explores the research that’s being done to understand how much of the damage to city trees can be attributed to dogs:

Whether pee hurts trees is a question that’s attracted virtually no research attention since its earliest mention in the academic literature (earliest that I can find, anyway), “Why shade trees die along city streets,” a presentation given in 1959 by Pascal Pirone at the International Shade Tree Conference. A plant pathologist at the New York Botanical Garden, Pirone was the first person to sound the klaxons on what he called “dog canker.”

It makes sense. The ingredients in dog pee include ammonium (toxic in anything but small doses), salts that create a crust on the oil to make it virtually impenetrable to water, and other chemicals that reach deep into the roots, even if the tree is protected by a fence.

Under its scaly armor [the bark], a tree has a layer of tissue called the cambium that makes it grow in diameter. The chemicals in urine can soak through exterior bark and damage this vital substance, explains [Nina] Bassuk [of Cornell University’s Urban Horticulture Institute], either destroying the tree or impairing its growth. “It’s like cutting off part of the circulatory system,” she says. And a tree with a dysfunctional bark is easy prey to burrowing insects and oozing diseases like “bacterial wetwood.”

Plus there’s the problem of thousands of paws tamping down the soil around a tree and starving the upper roots of oxygen. And when dozens, even hundreds of dogs are hitting the same tree every day, it’s disastrous.

Sam Bishop, education director for Trees New York, says that on a dense city block where everybody’s dog ends up using the same tree, the dirt can become highly polluted with salt. His organization has planted thousands of trees around New York City and now feels forced to defend them with signs reading, “Curb Your Dog.”

Bishop says it is especially frustrating when he runs into New Yorkers who believe that their furchildren are actually doing the tree a favor. “I’ve heard people jokingly refer to it as ‘watering the tree,’ and some people think it’s good for the plant and is fertilizing it,” he says. “But no, no, it is not good for the plant.”

There’s an interesting section on why dogs  do what they do. It’s not so much, as was frequently assumed, to do with marking territory; it’s more to do with all kinds of communication – from who’s who to who’s hot to who’s where on the pecking order.

Last year, Wisconsin researchers who were monitoring dogs at a park called Muttland Meadows, and also at an experimental “urine course,” concluded that the peeing had much to do about status. How’d they figure that? Well, they noted which dogs enjoyed the most popular regard by measuring the angle of their tails – the higher the angle, the more important the dog – and then correlated that with how often they “overmarked,” or peed over another dog’s urine. High-status dogs turned out to be the ones that overmarked the most.

What to about it if your dog is one of the culprits?

Believe it or not, there is a way to stop it. That’s not to try to biohack your dog to alter its urine chemistry, despite all the folksy prescriptions for tomato-juice or baking-soda cocktails. It’s to curb the animal, which means train it to unload in the gutter. That’s where you’re supposed to do it anyway, according to laws in many large cities like New York.

The trouble is many dog guardians are in complete denial. But take a moment to consider the trees. Especially the trees that are crammed into small spaces on city streets. They need some love, too.

The full article is here.

And check out this earlier article about how to help the trees in your neighborhood.