What's Killing Roadside Trees?

By Raquel Maria Dillon on Tuesday, April 1, 2003.

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Many New Hampshire residents have noticed something strange this spring. Some evergreen trees are no longer green – they’re brown… especially on the sides of roads and highways. NHPR’s Raquel Maria Dillon went to the experts for an explanation.

You don’t need to get close up to see the dead branches and twigs on pines and firs. The burnt-looking trees line roads and highways around the state.

The tree experts at the University of New Hampshire’s Cooperative Extension say they’re getting dozens of calls from people wondering why…

Those calls go to Forestry Specialist Karen Bennett.

BENNETT :10 this is an eastern hemlock, Tsuga canadensis, you notice that the foliage is really orange brown. These needles are dead.

SOUND stomping thru snow...

Here, next to a quiet road not far from I-89, this tall hemlock shows signs of the rough winter. Bennett says there are two reasons for the foliage browning.

BENNETT :16 when we have these extremely cold, cold nights, combined with warm sunny days. The tree sorta gets faked out and it thinks it’s spring and it starts to move water. Then we get the cold nights and then we get cell freezing.

And, Bennett says, road salt exacerbates the problem.

BENNETT :16 without being technical about it, the salt mixes up the water balance in the tree roots and needles. When you have these warm fluctuations it’s very vulnerable to winter injury.

So a combination of cold temperatures and road salt has taken a toll on roadside trees. The symptoms are worse closer to highways and in swampy areas where the salt collects. White pines and hemlocks are especially susceptible.

Over the course of this past winter, there was a lot of salt put on roads statewide. New Hampshire Department of Transportation Maintenance Engineer Steven Gray says there are alternatives to road salt.

GRAY :16 but unfortunately in our climate with the low temperatures, those materials are very expensive. Same amount of material to do what salt would do would cost roughly 30 time what it would cost as an alternative.

So instead, engineers like Gray keep up with the latest research and try to use as little sodium chloride as possible, while still maintaining safe roads.

UNH Natural Resources Professor Barry Rock says when he sees brown trees, he thinks of two more important factors.

ROCK :13 The combination of road salt, dry summer, cold winter and ozone.. it was really a 1-2-3-4 punch.

So, add ozone and drought to the list. With rivers around New England overflowing with snow melt, it’s hard to remember last summer’s drought and haze.

ROCK :18 summer of 2002 was a very bad one for air quality and ozone exposure. Ground level smog ozone. Not stratospheric, Down here where you and I live.

Ground level ozone contributes to smog – the atmospheric haze that’s the product of car exhaust and sunlight.

Rock says New England has a pretty significant smog problem, and the trees are showing the signs of last summer’s elevated ozone levels.

ROCK :12 but it’s not until we have the really cold winter drying effect that browning becomes dramatic enough that we can see it from a car at 60 mph.

As Springtime temperatures rise, buds and new needles will emerge on trees that survived the cold and the salt and the drought and the air pollution. And even though dead trees on the roadside aren’t pretty, they’ll make good homes for insects, birds, and animals.

For NHPR News, I’m RMD.