Trees are more valuable to cities than people realize. They produce shade and convert carbon dioxide into oxygen, plus add to property values, but they also perform an important civic function: trees absorb storm water that would otherwise overwhelm sewer systems.

Some trees soak up 1,100 gallons of storm run-off per year – the more trees, the less sewage running into the bay.

So with all these benefits in mind, who wouldn’t love trees?

This week’s column looks at a face-off over trees in San Francisco and tells the story of two articulate, smart devoted public servants who were forced to take opposing sides in a lengthy fight over the future of one tree. There was even a public tree trial (which you can watch here on video, agenda item #5).

It’s a case worth noting because the same battle could potentially take place thousands of additional times over the next few years in the city.

At issue is the tree Metrosideros excelsus, better known as the New Zealand Christmas Tree. Nearly 5,000 have been planted in San Francisco since the 1980s, which turned out to be a terrible idea.

The species grows so fast and has such thick roots that it’s known around the world as a tree that destroys sewers and sidewalks. Ironically, as seen in this document, Metrosideros is on the list of trees that the water department in Auckland, New Zealand tells people not to plant. (Note, in New Zealand the New Zealand Christmas Tree is called Pohutukawa – reminiscent of how the French don’t use the expression French Fries.)

I interviewed half a dozen experts for this story, and each one said it was a mistake to plant this species of tree in most parts of San Francisco. Now that the trees have matured, they are causing widespread damage. Under city law, property owners are responsible for the destruction.

But the city has decided to vigorously fight anyone who wants to remove and replace the trees with a more benign species. Is this environmental extremism defying common sense? After reading the story, and learning how and why all this happened, you be the judge. We welcome your comments.

It’s worth noting that many species of trees can eventually wreck sidewalks and sewers, if the space allotted for the tree is too small or the dirt is too densely packed – the New Zealand Christmas Tree simply does damage more quickly and aggressively than other species. But that doesn’t mean it’s a bad tree for every area of San Francisco. It’s so hardy that experts say it is one of only two types of trees that can survive in the Outer Sunset’s salty air.

Just don’t plant it near any sewers or sidewalks.