We Should Be Planting More Trees, Purely For Selfish Human Reasons

We Should Be Planting More Trees, Purely For Selfish Human Reasons

Yesterday we learned that humans have killed half of the Earth’s trees, which today number in the trillions. At the same time, there’s been a boom in scientific studies showing that humans are healthier and happier when exposed to our leafy friends.

At least three news studies this winter focus on the mental and physical benefits of having access to trees and nature. They’re part of a growing canon of research that suggests the natural world plays a very, very important role in how healthy we are — way beyond the fact that they benefit our air quality. Here’s a brief run-down on some of the latest.

Trees Make Humans Feel Healthier and Younger

The latest study, published in Nature, comes from a group of researchers in Chicago and Toronto who decided to find out whether the number of trees on streets actually correlated with how humans feel about their health. To do this, they looked at Toronto (which of course, crucially, has universal healthcare) and compared the number of trees on each street to how citizens self-reported their health status, including conditions like diabetes, hypertension, heart disease, obesity and more.

What they found was incredible: Even when they controlled for socio-economic factors, people on streets with more trees reported they were far healthier than their tree-less peers. Having ten extra trees on a block improved how people reported their health to the equivalent of being 7 years younger — or making $US10,000 more a year.

As the authors point out, that kind of improvement, even if it’s coloured by how participants self-report, could have a huge impact on a city with relatively little investment:

According to our findings improving health perception and decreasing cardio-metabolic conditions by planting 10 more trees per city block is equivalent to increasing the income of every household in that city block by more than $US10,000, which is more costly than planting the additional 10 trees.

Trees and Nature Can Improve Your Mental Health

Meanwhile, a Stanford study published in July showed how walking through leafy areas benefit our brain health. The study asked half of participants to go on a 90-minute walk through a park or nature area, while the other half walked around the city for an hour and a half.

The people who walked in nature had different brain activity than the city-goers. The part of the brain that’s linked to risk for mental illness, the subgenual prefrontal cortex, had less neural activity than those of the city walkers. The participants also reported less rumination, or “repetitive thought focused on negative aspects of the self.”

So exposure to nature has clear neurological and emotional benefits. But another interesting aspect of their findings is that they tell us more about why rates of mental illness are higher in cities. “These findings are important because they are consistent with, but do not yet prove, a causal link between increasing urbanisation and increased rates of mental illness,” as one author, James Gross, said in a statement on Stanford’s news site at the time.

And Increase Our Mood and Memory

Some of the same authors behind that paper also published a related study in Landscape and Urban Planning in June. This study looked at the effect of 50-minute walks on participants’ brains — but also dealt with memory and affect, which it defines as “decreased anxiety, rumination, and negative affect, and preservation of positive affect.” It found that a walk in nature actually improved cognition on some (not all) tasks, including increasing verbal working memory. They attribute that result to Attention Restoration Theory, or ART:

According to ART, natural environments invoke a different sort of attention from people — a sense of “fascination,” “being away,” “extent,” and “compatibility” — that may result in the replenishment of directed attention because they are less heavily taxed in these alternative environments. This, in turn, may lead to improved performance on tests that measure memory and attention.

In Fact, the Death of Trees Is Associated With Human Mortality

A seminal example of this kind of research was published in the American Journal of Preventative Medicine in 2013. The study was made possible by the tragic natural death of more than 100 million trees — thanks to an invasive bug called the emerald ash borer.

Dead ash tree in Des Moines, Iowa. AP Photo/Charlie Neibergall.

Using this “natural” dataset, a team from the US Forest Service looked at counties affected by the borer and the rates of death amongst citizens by cardiovascular and respiratory disease. In areas where the pest had killed the most trees, a marked increase in deaths was seen:

The magnitude of this effect was greater as infestation progressed and in counties with above-average median household income. Across the 15 states in the study area, the borer was associated with an additional 6113 deaths related to illness of the lower respiratory system, and 15,080 cardiovascular-related deaths.

There are many more studies looking at other elements of this idea, and we’re likely to see many more over the coming years. The question is whether it’s changing how cities spend their money on public projects. In New York at least, it seems to be: The city launched a campaign called Million Trees that aims to plant a million new trees over the next ten years. Major planning and design firms like Arup are also arguing for more (and larger) trees in cities, too.

But a fourth new tree study published in April in PLOS ONE gives us a vivid glimpse into just how unequal our cities are, even when it comes to trees. By analysing the tree canopy cover in seven major US cities and how that cover corresponded to income. “Our findings show that high-income neighbourhoods in our selected cities are more likely than low-income neighbourhoods to have high tree canopy cover,” the authors concluded. “Money may not grow on trees, but this study suggests that in a way, trees grow on money.”

So our increasingly complex understand of how health, cities, and nature are related continues to evolve.

Image: Robert Crum