MCA VIGNETTE: The Urban Forest of the Future is Diverse

The challenges for growing trees in Missoula began 200,000 years ago. Glacial Lake Missoula, which formed the valley, alternated between draining and filling over time, depending on the solidity of the ice dams on its western edge. With each draining, nutrient rich soil washed off the valley floor, and by the time the lake disappeared for the last time, the Missoula valley was a treeless plain, with just a thin layer of topsoil left on top of a gravelly bed.


It took settlers to bring trees to the valley. The first records of them are from 1874, when Missoula’s founders imported fruit trees and Norway Maples. In 1909, the young city began raising public funds to pay for neighborhood trees, charging $3,300 tax (in today’s dollars) for each 3,900 square foot lot. Missoula’s urban forest was born. Today, those early efforts define a Missoula nicknamed “the garden city,” where trees shade streets and houses. When viewed from the overlooks of Mounts Sentinel or Jumbo, Missoula is a green square surrounded by brown treeless hills.


On an early-August morning, 142 years after those first trees, city arborist Chris Gray stands in Bonner Park surrounded by shade-throwing Norway Maples and says the urban forest is in “a rapid state of decline.”


It’s not hot out yet, but it will be soon. Long and hot summers are proving devastating to trees already reaching old age, and Norway Maples, imported from the east, aren’t well adapted to Montana’s arid climate. While the city’s past residents got the trees to where they are, today many don’t get the water they need.


The combination of these three things; old age, hotter summers and poor watering practices have Missoula’s forest at a point where trees are dying faster than they can be replaced. While this creates enormous challenges and expense, it also gives Gray and others an opportunity to contemplate the city’s urban forest in a future with an uncertain climate.


Missoula hired its first urban forester in 1991, and Gray joined the department five years later. He had no forestry background, and was working for the parks department. But living outside town, he’d learned how to run a chainsaw cutting his firewood. He got hired as an arborist and learned his now considerable forestry knowledge on the job.


“For a long time I climbed and pruned,” he says, spending time up in the canopy of the maples and other shade trees throughout the city. “You get to have a fairly intimate knowledge of individual trees.”

Now, Gray is the city’s lead arborist, and much of his job is removing those trees—now dead or so close to dead that their branches are dangerously easy to break—which he once got to know. He’s seen other changes over his time as well, particularly in the climate and seasons. Norway Maples need around 20 inches of precipitation a year. Last year Gray says they got 10. As warmer temperatures take up more of the year, city trees are budding a week or two earlier than they used to, and the fall color change is coming a week or two later. That means a longer period each year that trees are spending energy and water to maintain their leaves. If there is a drought, that would mean a longer stretch of stress on the tree.


Today, Gray is following a street repair truck through the University District. The workers in the truck are chip sealing—laying new asphalt over cracks in the streets—and the truck has a propensity to knock branches off overhanging trees.


Occurring over the first half of the 1900s, the paving of Missoula’s streets was not a boon to the city’s trees. When the streets were dirt roads, the city was divided into sprinkling districts. By watering streets to keep down the dust, the city also inadvertently watered the trees that lined them.


By the early 1960s however, that process had stopped, and watering trees was left to private citizenry. According to the arborists, that’s when the urban forest began its decline.


Today, arborists estimate the city is losing 3 to 400 trees a year. They’re replacing them as they can, but that takes time and money the forestry department often doesn't have. In the last ten years alone, the departments’s pruning cycle—the frequency with which they can revisit individual trees to remove dead branches—has doubled from around 20 years to 50. The department seems stretched to its breaking point. Clean up after big storms, like a monster wind storm in the summer of 2015, can set the department back months.


Driving down Stephens Avenue, a main thoroughfare branching out from the historic university district, Gray points to dead and brown leaves and notes that there are two months of hot and dry weather left. “By the end of those two months they're going to be just hammered,” he says.


He stops the truck to look at two dead saplings—five foot tall withered skeletons bereft of green leaves—that front a residence. They’re centered in a block of sun-drenched grass, also brown and either dead or dying. It’s a direct contrast to the green grass growing under the benevolent shade of two Norway Maples in the lawn next door.


“We watered them for two years,” he says of the dead trees, and they lasted one summer without watering after that. The first years after a tree is planted will make or break it. The tree recently transplanted from the nursery needs water to establish a root system in new ground, which will define its survival. But the city can only provide each new tree with two years of water, and after that it’s up to residents.


Gray describes homeowners as inconsistent at best when it comes to watering city trees. A lot of houses are rental properties with absentee owners, and much of the city just isn't aware of the need to water, Gray says. Sometimes that’s because they don’t realize city government isn’t doing the watering, and sometimes it’s because they're from the eastern half of the country, where shade trees receive more precipitation and don't need human care.


It’s a question that will only grow more prominent in the future if longer summers continue. “I can’t say how much water we’re going to have or how much it will cost or who will own it,” Gray says. Already, he runs into elderly people on a fixed income who can’t afford to water.[1]  While there is no certain indication that drought will become more common in Montana, there is evidence for longer and hotter summers going forward, and trees could require more water to get through them.


A recent study showed Missoula’s tree canopy is divided by socioeconomic lines. Wealth and high home prices tend to concentrate in the well-shaded historic districts, while emerging neighborhoods lacking an urban forest are those of lower income households. The need to pay higher water bills in hot summers could become a further financial strain, or one foregone by those with less financial resources.

Trees carry an economic benefit for both individual residents and the city. Through improvements in aesthetics (which affects property values), air quality (affecting health costs) and in slowing storm water, the City of Missoula’s Urban Forestry Division estimated the economic benefits of the city’s trees to be just under $2.5 million in 2014. For individual households, one of the most prominent benefits is through energy savings—by shading households in the summer, trees lower air conditioning costs. Already, it’s the poorer households that would benefit most from the shade of trees that are left without them.  


Karen Sippy’s organization Trees for Missoula works to support the forestry department by gendering public support and awareness. She believes that the key to the forest of the future will be creating community involvement and finding ways to water efficiently. “Watering and watering the right way, that’s how you sustain an urban forest in Missoula, Montana,” she says. [2] 


One of Trees for Missoula’s goals is to take up slack for the forestry department by creating a civilian corps of volunteers to help prune and replant in their neighborhoods. Another is to raise public awareness of the need to water trees.


Given everything she knows about local climate, and the Norway Maples, Ash and other species selected early on in the city’s development, Sippy says, “I’m astounded at how well these trees are doing.”


Gray and Sippy both say that the city is now at a turning point. An increased need to replace dead and dying trees is a challenge, but it’s also an opportunity to diversify the urban forest. Gray says incorporating more native trees like ponderosa pine, which are far more drought tolerant, could significantly reduce the water needed in the forest.


But there’s a political angle to that as well.


“People love their Norway Maples, which is fine, but not thousands and thousands of them,” Sippy says. Just as uniform stands of lodgepole or Douglas fir leave Western Montana’s forests vulnerable to high mortality from pine beetle or wildfire, so too is a uniform forest in Missoula vulnerable to pest outbreaks or death in a particularly hot year.

Still, getting people to accept a new variety of trees is a difficult task of public education. Some resident’s concerns are well founded, Gray admits. For example, while conifer species like Ponderosa are better adapted to dry Montana summers, they don’t lose their needles in the winter. This blocks homes from the sun that could warm them on cold days. But oftentimes it is just a reluctance of residents to change, accustomed as they are to the maple lined streets of the past.


The compromise is probably somewhere in the middle, and Sippy and the arborists hope to convince residents to accept some trees that don’t resemble the street trees of the past. “Diversity keeps your urban forest intact,” Gray says.


Montana’s urban forestry problems are not isolated to Missoula, but the choices and problems vary from city to city, depending on climate and the decisions of those who first planted the urban forest.


“We try to talk to talk from region to region to see if there’s any trends,” regarding tree death, says Jamie Kirby, the Urban and Community Forester for the state of Montana. “Is it a similar species, is it a diversity issue, is it because it’s so old?”


In cities on the eastern side of the Rocky Mountain divide, ash trees are prominent. Dillon’s urban forest is 68 percent ash, and Great Falls is similar. In Great Falls, Kirby says a lot of the forest has been dying due to climactic stresses. A big, early-season frost in 2008 damaged the ash trees and many went into decline.


“If it was perfect conditions from that point on, the trees probably would've recovered,” Kirby says. But with the stress of long hot summers they never did.


Meanwhile, in Dillon and other ash-dominated cities, foresters dread the arrival of the emerald ash bore—an insect that has been killing ash trees in the eastern half of the U.S. and is moving west as humans inadvertently transport it.


““They could just get hammered if that little bug comes here,” Kirby says. “And it’s a matter of not if but when.”


Problems differ, but climate is an obstacle all over the state. To lower the impacts of die-off, Kirby says the state goal moving forward is to plant no more than 10 percent of any single species in urban forests. Diversity is again key.


There’s no perfect street tree that can withstand all the ranges of weather Montana gets, Kirby points out. “It all comes down to picking the tree that will perform best in the location you desire.”


If Gray had his way, Missoula’s climate-change resistant urban forest would like something like this: Ponderosas and larch trees would be spaced 75 to 100 feet apart along the boulevards. In between them would be burr oaks and elms—drought tolerant shade species. And between those, small flowering ornamental trees. “Just because I think they’re pretty,” he says.


With this variety, a particularly hot and dry summer, or the introduction of a new pest, would cost you individuals but never the whole forest.


Gray and his department could probably do it too, if they can get residents behind them, and if they can get more funding from the city to pay for it. Cities go through political cycles, he says, where minds change over how they feel money should be spent.


To succeed in revamping the forest, Gray envisions a buy-in similar to the $3,000 a year in property taxes Missoula’s first homeowners used to pay for tree planting.


"I'm sure people thought that was a lot back then too, but they paid it and they paid it for a long time." The city invested in the trees in its youth, and for a long time now Missoula has reaped the benefits.