If that tree could talk

A canopy of trees over a driveway helps to reduce stormwater runoff

Everyone assumes that trees are stoic. Standing there, year after year, through raging blizzards, torrential rains and broiling summer days, they accept their lot in life without a peep. Then again, a tree doesn’t have a larynx, so if it could talk, maybe it would grumble and moan all day long. Lord knows they certainly have plenty to complain about. As if the weather isn’t enough to deal with, there are a whole host of pests and diseases, ranging from oak wilt to emerald ash borer, not to mention the daily strain of filtering toxins out of our air.

As a pregnant lady passing my six month mark, I can empathize with the trees plight as daily nuisances begin to mount. I experienced my first breathless moment this week while giving a presentation to Washington County Public Works staff, and I ended up sounding like Stevie from Malcom in the Middle. You might not hear the trees gasping for breath, but they are out there respiring, just like us. Every school kid learns that trees take up carbon dioxide and give us oxygen. In fact, David Nowak from the US Forest Service estimates that trees offset about two-thirds of our oxygen consumption. If every tree in the world disappeared tomorrow, however, he assures us that we would still have plenty of oxygen due to all the other types of plants that also produce oxygen. Trees are perhaps more critical to humans for the role they play in cleaning our air. Stomata in their leaves absorb nitrogen and sulfur, as well as particulates and heavy metals from sources such as cars, factories and residential chimneys. In fact, in some communities where atmospheric pollutants tend to rain down from larger cities nearby, a canopy of trees is like an umbrella, protecting the people down below from all those toxins. There’s a limit though, and too much pollution will kill a tree just like it would a person.

Have you ever been caught out in the rain and rushed to stand under a tree to stay dry? Maybe you wouldn’t want to stand there as long if you could hear the tree listing all its trials and tribulations, “Oh great, here comes the rain and wouldn’t you know it, I just got my leaves permed?” Luckily for us, though, we can’t hear them whining, and they really do a good job of keeping us dry down below. According to Janna Kiefer from Barr Engineering, a full leaf canopy can intercept between 0.09 and 0.17 inches of rain. That might not sound like a lot, but when you look at the course of a full year, a stand of conifers catches 20-40% of the precipitation, while a stand of deciduous trees captures 10-20%. This ability has a lot of people excited about the potential for trees to help us reduce stormwater runoff.

Stormwater pollution is created when rain and melting snow rain of off impervious surfaces where water can’t soak in like rooftops, parking lots and streets. The water that runs off picks up nutrients, pollutants and other waste and transports them to nearby lakes and rivers, causing big problems. Trees can help by not only intercepting some of the water that would otherwise runoff, but also by improving the soil’s ability to absorb water. There are limitations, however, the most obvious being that a big tree will catch a lot of water, but a small tree will hardly catch any. In other words, trees planted today won’t start helping us for another ten years or so. The second is that trees only reduce stormwater pollution when they intercept water that would have otherwise run off the land. A tree in the middle of the yard might catch the same amount of rain as one beside the street or over the roof, but most of the water it catches would probably soak into the lawn anyway. By US Forest Service estimates, a city would need to increase its tree canopy by 10-15 % to counteract the extra stormwater runoff created by an increase in impervious surface of only 1%. In other words, trees help, but they aren’t a substitute for more effective solutions like limiting grading during construction (ie. not cutting down an acre of woods to build one house), building narrower streets and smaller parking lots, and using best management practices like pervious pavement and raingardens.

I can already hear the tree in my backyard nagging me about all the things I forgot to mention. Yes, they also help to reduce home energy costs by providing shade in the summer and blocking wind in the winter. They also provide food and habitat for wildlife, increase residential property values, and add beauty to our landscapes. Maybe that tree in your yard would be happier if it had a few friends to keep it company, or at least someone to hear it whining. If so, take some advice from University of Minnesota professor Gary Johnson. Autumn is the best time of year to plant trees in Minnesota.