Street toughs

Salt is dumped into their soil during winter storms. Their branches are clipped near power lines. and if they survive all these indignities, humans still wreak damage.

“People sometimes take out their frustrations on a tree,’’ said Eugenie Johnston, a board member of Trees for Watertown. “They just grab a branch and rip.’’

So this year, Johnston’s nonprofit group, which seeks to protect the town’s leafy and needled vegetation, decided to experiment with a species of tree it hopes will be tough enough to survive on Watertown’s streets. The group bought eight knee-high Osage orange saplings in the spring.

The trees, which have been growing bigger and stronger in the nursery at Mount Auburn Cemetery, are a new variety of the species called White Shield, a more street-friendly version without thorns. Trees for Watertown bought only male trees, which won’t bear fruit that will drop onto sidewalks and streets.

Towns and cities have traditionally planted familiar trees along roads: sycamore, oaks, honey locusts, Norway maples. But those trees have often succumbed to urban and suburban travails; and some, like sycamores, have become vulnerable to pests and disease. Tree experts believe towns are better off diversifying as they plant new trees.

“The thinking about street trees in an urban environment is changing a bit,’’ said Dennis Collins, horticulture curator at Mount Auburn Cemetery. “As the mortality rate shows, some things just don’t work. A lot of towns are trying new things.’’

Lexington has been experimenting with the Japanese tree lilac, which has small flowers, and the sourwood, more common in the Appalachians. Brookline is trying to plant more species of trees, including the Korean mountain ash and a type of sweetgum that grows in a columnar shape and can fit into small spaces.

Lexington decided to save money by starting its own tree nursery in 2007. The town plants about 100 saplings there each year, and lets them grow a few years until they are hardy enough to plant on the street.

The town’s Tree Committee volunteers work in the nursery, weeding and doing other jobs to help town employees, said Karen Longeteig, a committee member.

When Brookline surveyed its 11,000 street trees in 1994, officials discovered that Norway maples made up 29 percent of the total, the largest percentage of any species, said Hugh Mattison, chairman of the town’s Tree Planting Committee. As a result, the town has stopped planting Norway maples, and the percentage is now about 21 percent.

The town would like to keep its stock of trees diversified enough that no single variety makes up more than 10 percent of the total population, Mattison said.

“The whole idea is to have a great diversity, so if there were a disease it wouldn’t decimate all our trees, as Dutch elm disease did,’’ he said.

Many of Watertown’s street trees are Norway maples, which worries tree warden Christopher Hayward. The Asian longhorned beetle, which has destroyed trees in Worcester and nearby communities including Shrewsbury, favors hardwood trees, including the Norway maple.

Planting street trees in Watertown can be particularly difficult because its roadsides often do not offer much space. Mt. Auburn Street is tricky because it is lined with overhead wires for the electric buses, Hayward said.

“We don’t want the trees growing up into the wires,’’ he said.

Johnston, with Trees for Watertown, had long known about Osage orange trees, but became interested in them while looking for tough trees for Watertown’s streets. She learned that the Osage orange trees were planted close together on the American prairies in the 19th century to fence in cattle before the use of barbed-wire fences.

“I thought a tree that will endure that kind of abuse will endure the kind of abuse it will get on our streets,’’ she said.

Mount Auburn Cemetery, which sits primarily in Watertown although its entrance gate is in Cambridge, agreed to let the young Osage orange trees grow in its nursery until they are strong enough to be transplanted to their permanent home alongside Watertown streets.

“Your basic street tree has about a 35-year . . . life expectancy in today’s urban forest,’’ said Peter Wild, owner of Boston Tree Preservation in Woburn. “That might mean that some of them might live to be 70. Some of them might live to be 15.’’

One tree that many towns and cities hoped would be hardy, the Bradford pear, has proven disappointing, Wild said. Although the trees grow densely, ideal for urban and suburban streets, the angles of their branches, and their tendency to keep their leaves into late fall, make them susceptible to damage during winter storms, he said.

“That’s an example of a tree that was heavily overplanted two decades ago and even recently,’’ Wild said, “and it hasn’t proven to be the tree that we thought it was.’’