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Air Quality and Cancer Aside, How Planting a Tree Makes You Safer

Baltimore doesn’t get enough credit as a green city, speaking as someone that lives here and frequently looks for good reasons to continue living here. We’re ringed by big parks full of swimming holes and secret places, are cut by several wooded river...

Baltimore doesn’t get enough credit as a green city, speaking as someone that lives here and frequently looks for good reasons to continue living here. We’re ringed by big parks full of swimming holes and secret places, are cut by several wooded river/stream valleys, and we’re fortunate enough to have had early leaders with the foresight to set large amounts of urban land aside for interior parkland, which you will find referenced about once in a certain very popular HBO show as a body dump, unfortuntely connecting Baltimore’s murders with its trees in millions of brains across the world. It’s sad.

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Like most urban places, those trees start to go away when you get into poorer neighborhoods, where housing becomes even more dense, and when you head deeper into the center city. It’s actually a remarkable gradiant of greenness that you find traversing Baltimore, starting with treeless urban canyons downtown and gradually filling in with trees as you move outward to uptown fox country (foxes, for real) and further into the world of standalone houses, firepit backyards, and eventually the outer city’s ex-suburbs-cum-rich neighborhoods. Baltimore is also violent, making for a perfect locale to study the potential impacts of trees on violence, as the University of Vermont’s Austin Troy et al did in this month’s issue of Landscape and Urban Planning.

Studying the possible effects of vegetation on crime is nothing new, but it’s hardly a settled issue. In fact, a lot of research (and intuition) points to vegetation, particularly low, dense vegetation, as being precursors to crime, for exactly the reason mentioned in The Wire: it gives criminals a place to hide (or hide things). For example, a 1983 study found that dense vegetation is a common characteristic of rape sites1. But this is also a very specific sort of green: most actual urban greening comes from more run-of-the-mill street trees and grassy parks with high tree canopies. Troy’s paper discusses a couple of reasons this sort of maintained vegetation has a positive effect on crime rates: trees make a neighborhood appear more cared for and attended to (a “cue-to-care,” in the jargon), and they attract people to congregate outside (the best security system there is).

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In any case, Baltimore is perfect because it has two things: availability of high-resolution imaging to analyze tree canopy, and a ton of geo-data on crime. Crucially, the study goes beyond the city limits, through the suburbs, and into rural agricultural areas. (This is a nice thing about Baltimore — from the center city to farmland takes less than 30 minutes driving.) So, Troy et al were able to look at the widest possible variation in canopy cover possible, besting all other current research.

The results? Unsurprising. “The results of this study indicate that crime has a strong negative association with tree cover, even after controlling for socioeconomic
variables such as income, housing age, ruralness, race, housing type, housing tenure, population density, and amount of protected or agricultural land.” Using the most conservative data model, the researchers found that a 10 percent increase in tree cover correlates to an 11.8 percent reduction in crime rate (after controlling for all that other stuff). There was, however, one notable and quite large exception to this relationship taking place in neighborhoods south of the city around the Curtis Bay industrial and port facilities.

The paper explains:

With major harbor facilities nearby, the neighborhoods in question have a significant interface zone between industrial and residential land that could be considered a "no man's land." It is quite possible that the small patches of unmanaged trees that are often found adjacent to warehouses, truck yards, factories, etc., provide an excellent hiding place for criminals looking to prey upon residents going to and from nearby homes. A preliminary photographic analysis of the area shows that there are a considerable number of such low, overgrown patches.

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The paper, going even deeper, finds also that trees make more of a difference when they’re on public land. Which makes a whole lot of sense, particularly in regards to the earlier suggestion that trees give an incentive for residents to be outside of their homes, along with their eyes and ears. It’s a quirk in the numbers that should be an even bigger push to urban planners and policy-makers to do one of the cheapest civic improvements out there that isn’t routinely pushed for. No kidding: plant a tree, save a life.

One might immediately point out that, of course, areas with more trees have less crime; those are going to be wealthier neighborhoods. The study accounts for this, still finding a correlation. “By including variables for those things, (e.g. median household income by Census block group) in our regression model, it essentially means that we’re able to look at the relationship between trees and crime while holding all those other things constant,” Troy explains in an e-mail. “In other words, by including those variables, we can say that for two neighborhoods with the exact same socio-economic status and housing type, if one of those has more trees than the other, it will have lower crime rates.”

Hundreds of years before Troy et al‘s study, Baltimore’s planners were thinking and understanding why we need trees in a city.

But there’s more good to trees, of course. The Natural Resources Defense Council Staff Blog adds a few things:

The new research adds to a plethora of findings and wisdom about trees. In a fascinating, extended essay in The Wilson Quarterly, Jill Jonnes examines the history of city trees and research on their benefits, noting that a breakthrough study was conducted in Chicago in 1994. The researchers found that the city's trees could improve air quality by as much as 15 percent, removing measurable amounts of carbon monoxide, sulfur dioxide, nitrogen dioxide, ozone, and particulate matter. Moreover, Chicago's trees sequestered about 155,000 tons of carbon per year and could absorb far more over time with more planting. Including the benefits of shade in reducing energy needs for air conditioning, each Chicago tree was found to be worth $402 over its life span, over twice the average cost.

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As editor-at-large at Motherboard, sometimes I get a bit carried away with the “at-large,” which basically means I do this from wherever I want with an internet connection and an alarm clock. So I’ve become very good at itemizing the reasons I like Baltimore and why I stay here. It’s an “easy living” city for one — cheap, easy to get around, laid-back — but it’s also a green city, sometimes feeling even like a kind of refuge. In some places, you’re more likely to find a good park in walking distances than you are to find a grocery store, which sucks, but is also indicative. Hundreds of years before Troy et al‘s study, Baltimore’s planners were thinking and understanding why we need trees in a city. They are not there to fool us; they’re there to keep us human.

1Stoks, F. G. (1983). Assessing urban public space environments for danger of violent crime – Especially rape. In D. Joiner, G. Brimikombe, J. Daish, J. Gray, &
D. Kernohan (Eds.), Conference on people and physical environment research (pp.331–343). Wellington, NZ: Ministry of Works and Development.

Reach this writer at michaelb@motherboard.tv.

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