The street tree gets a smart makeover

The condition of the urban forest goes beyond pure aesthetics. A leafy city is a cooler, cleaner city; it's simply a nicer place to live, and it makes us healthier in mind and body. The tree, it turns out, is the one hugging us.

Today, the nation's capital has an active program of replacing dead trees and uses interactive maps to encourage residents to get involved in the care of newly planted trees. Moreover, there is a collective sense that in an age of climate change and more extreme weather, the need for a healthy urban forest has never been greater.

There is another aspect of this revival that I find compelling. Today, we have a mix of street tree species and varieties that seemed unimaginable just a few years ago. This eclecticism is not some passing fancy but reflects a fundamental shift in what constitutes a suitable city tree in the 21st century. This change was led in part by the late Frank Santamour, a research geneticist at the U.S. National Arboretum who invented a formula for urban forest biodiversity.

Washington is a leader in this tree diversification movement, but it is not alone. "The same concept is going on all around the country," said Keith Warren, a prominent tree breeder, nurseryman and author. "It's a cumulative awareness of the dangers of too narrow a selection of genetics in the urban forest."

Washington's urban forest - which years ago saw an alarming decline in the canopy of the urban forest due to neglect and development - is lush and vital. I am finding trees along city streets that I had previously thought of as choice ornamental plants. These include the serviceberry, sweetbay and cucumber magnolias, Japanese apricot, American hornbeam, Persian parrotia and red buckeye. A few weeks ago, I was stopped in my tracks by a deciduous yellow blooming magnolia hybrid named Butterflies that previously might have had gardeners salivating at a rare plant sale.

One obvious reason for the contemporary assortment of trees that mature at half or a third the size of maple trees is they will fit on the side of the street where the electricity lines are strung up on poles. This quaint old way of delivering power to the people seems unlikely to change, in my lifetime anyway, so the trees must shrink to avoid the gross mutilation you find when big trees clash with utility cables.

There is another reason the urban avenue has changed. The old model was of identical trees lining both sides of the street, to provide symmetric uniformity that produced marvelous effects majestic American elms coming together 60 feet above the pavement to form pointed arches. But such plantings are risky, because the arrival of a new pest or disease could devastate them.

This is precisely what happened with the elm, ruined by the beetle that brought Dutch elm disease. In the upper Midwest, where green and white ash trees defined whole neighborhoods, the emerald ash borer arrived 20 years ago to wipe them out. (Some cities had half their canopy given over to ash.) On the West Coast, the disease sudden oak death is a problem, and, in the South, the ambrosia beetle is a serious pest. In Washington, we had come to rely too heavily on red maples and northern red oaks, with new diseases beginning to threaten the latter, at least.

The District Department of Transportation's Urban Forestry Division plants more than 8,000 trees annually (removing some 3,000 that are dead or dying), but now relies on more than 125 species and varieties.

This diversity flows from the division's strategic policy of "right tree, right place," said associate director Earl Eutsler. This means that small and medium trees are used to minimize conflict with utility lines, but, equally important, trees of tall stature are purposefully chosen where they have the space.

This mix has led to some really enlightening choices. One is a native Prunus named chokecherry - Eutsler and his arborists use a cultivar named Canada Red, with deep maroon leaves. Most purple-leafed trees in the

Mid-Atlantic have strong color in the spring but then wash out in the heat of summer. The chokecherry does the opposite, starting green and coloring up when it turns warm. Like the chokecherry, the American hornbeam matures to about 20 feet but, with its sinewy silver bark, has the presence of a much larger tree, Eutsler said.

Another great choice is the Japanese flowering cherry hybrid Snow Goose, nicely upright to keep those branches out of the way of cars and pedestrians.

Warren, the nurseryman and author, said that a generation ago, the black gum or tupelo was a native tree little grown in urban landscapes. Today, its use as a street tree has generated varieties that correct natural traits for street use — a dominant trunk and branches that don't droop. Warren introduced Afterburner and a seedless form, Firestarter. Eutsler uses one named Wildfire.

In Portland, Ore., Warren said, the paperbark maple - a plant once known only by connoisseurs - is used as a street tree. One of the craziest adaptations to street use is the bald cypress, a deciduous conifer associated with Southern wetlands, now being used in Washington. Not only can it take inundation but, once established, drought, too - key traits for a street tree.

Another remarkable development has been the expansion in the choice of oak species planted. Moving away from red oak, Eutsler and his team of arborists and tree crews now routinely plant the overcup oak, the swamp white oak, the nuttall oak and the shingle oak.

All establish well in the stressful environment of the tree box and grow quickly. The overcup oak in particular has become a firm favorite in recent years, lush and vigorous in the toughest conditions while keeping its branches out of the way. It makes for a superb street tree, says Warren, co-author of "The Tree Book: Superior Selections for Landscapes, Streetscapes, and Gardens."

Eutsler said some of his choices turned out to be poor performers — flowering dogwoods, sourwoods, for example — and some, such as the Japanese apricot, are experimental. Some require more attention to formative pruning to direct future growth. But it seems the strategy is working - a walk through the neighborhood provides the delight of an unlikely arboretum.