THIS IS a time of year when trees remind us how worthy they are of appreciation. Look out across New England’s rolling hills, and the annual foliage show is well underway: the crowns are brushed with scarlet, gold, and burnt orange. And now is when the region’s many varieties of apple trees, from Akane to Zestar, are offering up their fruits.
But unusual new research suggests that trees are hard at work in cities, too. A single tree, one study found, can help deter crime in its immediate vicinity. Babies born to mothers with a tree close to their house are less likely to be underweight. And trees raise the property value of nearby homes by nearly $13,000, according to one analysis. Trees should be counted among those urban dwellers who make important contributions to the life of the city.
“Trees have a public health benefit,” says Geoffrey H. Donovan, a research forester with the Forest Service who has been in the forefront of this new work.
The benefits of trees are often discussed in grand terms. Forests take carbon out of the atmosphere, helping to moderate the climate-warming effects of manmade pollution. They pump out oxygen on a massive scale, part of the green machine that makes other life on the planet possible.
Yet researchers have also been uncovering the more local contributions of our arboreal neighbors. In cities, for example, trees bring down the temperature by reflecting some of the sunlight that would otherwise be absorbed by all the blacktop. Trees lower cooling bills, especially in the late afternoon, when utility demand is high. Trees moderate storm water surges. They scrub pollutants from the air.
More recently, there has been a move to quantify these benefits.
Donovan, working in Portland, Ore., brought sophisticated statistical techniques to bear on the question: What, exactly, does a tree contribute to property values? Last year, he published an analysis finding that, on average, a “street tree” – one situated between a sidewalk and street – added $7,130 to the sales price of the adjacent house. A tree, he found, also adds value to nearby homes, bringing its total impact to $12,828.
The extra property tax receipts a city receives because of trees, he reported, more than covered the city’s cost of maintaining them.
Donovan’s research inspired him to start looking more closely at the social impact of trees. Using aerial photography and mapping software, he sought links between trees and crime in Portland.
The results – which control for home value and many other variables – suggested he is on to something. In general, street trees, no matter their size, appeared to deter crime. In a yard, the story was more complicated: Large trees were associated with less crime, yet small trees with more.
What could account for this? In most cases, trees send a message: This is a place where the residents care about, and pay attention to, their surroundings.
“A tree is the opposite of a broken window,” says Donovan.
This is outweighed only when trees obstruct lines of sight between the sidewalk and the first floor windows, providing potential cover for crime. When trees get large enough, their crown clears the first floor, and the cover is gone.
Donovan has also shown that the presence of trees near the home of a pregnant woman decreases the odds that the baby will be born undersize, a risk factor for later health problems. (The mechanism is unclear, but Donovan believes it might have something to do with stress reduction.) An independent team working in Barcelona, using very different methods, recently arrived at a similar result.
Taken together, the evidence suggests that it is time to think of urban trees in a more expansive way. An oak or maple is not just a physical object, providing shade and a touch of elegance to our thoroughfares, but also a social presence. Trees affect how urban dwellers feel and behave. They make excellent neighbors.