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Surprising Science

Scientists Now Know How Many Trees You Need to See to Relax

How many trees do you need to see before you feel relaxed? Less than you think. 

You already know that going for a walk and looking at trees are good tools for reducing stress. Ideally, you should be walking underneath those trees to get rid of the most stress. New research out of University of Illinois at Urbana–Champaign and University of Hong Kong confirms that advice – and they didn’t even take people outside.

Why does that matter? Because until now there was no correlation between how much greenery someone looks at and how much less stress they felt because of it. That correlation is what this most recent study published in Environment and Behavior wanted to nail down. The research team, led by Dr. Bin Jiang, wanted – and was able – to identify the definitive relationship between trees and stress. In order to do that, the researchers created an experiment that combined the best practices of all the previous research. They asked 160 participants (roughly 50/50 male and female) to assess their day-to-day stress levels before the experiment, after being stressed out during the experiment, and after watching one of 10 videos at the end of the experiment.

Once the participants wrote a baseline assessment of their everyday stress levels, the researchers stressed them out in one of 2 ways: preparing and giving a job interview speech in 8 minutes, or subtracting numbers in front of the research team without any tools. Both tasks were chosen because they were incredibly stressful for a short period of time (and would certainly have stressed me out!). The participants then immediately assessed their post-task stress levels. Lastly, participants were shown special 3-dimensional videos of tree-lined streets with varying amounts of greenery. Like these:

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Credit: Bin Jiang, et al./Environment and Behavior

After watching the video, participants were placed in a quiet room for 15 minutes and wrote a narrative about their experience.

The results were overwhelmingly positive, even when accounting for any errors incurred by self-assessment. After crunching the numbers, the research team found that “at the lowest level of tree density [2%], 41% of participants reported a calming effect. As tree cover density reached 36%, more than 90% of participants reported a stress recovery experience.” That means that the greater the amount of trees, the greater the amount of stress will be reduced.  That result confirms all previous research indicating that the more trees people see, the greater their experience of stress reduction is. Or, as Jiang puts it, “stress reduction has a significant cubic relationship with tree cover density. Anxiety reduction has a marginally significant cubic relationship with tree cover density.”

tree scatterplot.jpg

results chart.jpg

Credit: Bin Jiang, et al./Environment and Behavior

More importantly, these findings also show that it takes almost no trees to produce stress-busting benefits — which is fantastic news for people living in urban areas. While walking in “forest landscapes,” like a wooded area or a nature preserve will “offer the greatest benefits,” but even looking at a single tree or green lawn will help. Again, the key finding is that “exposure to settings that contain nature helps people recover more quickly from the psychological symptoms of stress.”

The team also discovered that both men and women experienced the same degree of stress relief from tree coverage, and that “younger participants experienced greater anxiety reduction” than older ones.

All that said, the results aren’t flawless. The “participants with greater baseline stress levels experienced less stress reduction,” which is unsurprising but somewhat worrisome as the study doesn’t specify whether those participants need to see more trees or spend longer periods of time looking at those trees to reap better benefits. Also, the idea of stress is subjective and therefore difficult to measure. While this research team measured reductions in three physiological factors – anxiety, tension and avoidance – previous research measured more physiological factors like uneasiness and jitteriness. Why those additional factors were excluded is unclear, as is their possible effect on the three that were included.

Still researchers are hoping to use their results to provide concrete solutions for communities all over the world:

Lack of this [correlation] knowledge prevents health care providers and public health officials from recommending exposure to urban forests as part of preventive health care or clinical treatment programs… [and] also costs landscape planners and city managers the opportunity to make science-based management decisions regarding the allocation of resources that might enhance the urban forest and thereby improve the health and longevity of the people they serve.

There’s a lot of truth to that statement. Geobiologist Dr. Hope Jahren agrees with it – and explains why here:

Feature image credit: Pexels


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