Trees have healing powers.

At least, that is the conclusion of landscape architecture researcher Bin Jiang. He, along with fellow researchers — Chun-Yen Chang, researcher at the Laboratory of Healthy Landscape Healthy People of the National Taiwan University, and William Sullivan, professor of landscape architecture — compared the effects of tree canopies with human cortisol levels, which are indicative of stress levels.

To begin the study, Jiang and researchers focused on finding healthy participants for their study, specifically people who were from the Champaign-Urbana area and had lived in the U.S. for at least 18 years.

Once participants were recruited, the researchers measured reactions to stress by placing their 158 participants in stressful situations, including having them do computational math problems without paper or calculators, and giving improvised speeches on the spot — known as the Trier Social Stress Test.

Participants were next split into 10 groups and watched six-minute videos of tree canopies with anywhere from two to 52 percent of foliage. The researchers measured the reactions of participants’ stress to the video by measuring skin conductance, salivary response, perspiration and people’s vocal responses to the videos.

They found that men and women differed greatly when it came to how each gender reacted to the foliage they saw in the video. For women, there was “no relationship between tree cover density and stress reduction,” according to Jiang and the researchers’ paper “A dose of nature: Tree cover, stress reduction, and gender differences.” In other words, women were not at all affected by the videos.

In contrast, the men reacted strongly to the tree foliage they saw.

According to their paper, “For men, the dose-response curve was an inverted-U shape … As the percent tree cover increases from barren to greener scenes, there is a rapid increase in stress reduction until the density of the tree cover reaches 35 percent.” However, once the percentage of tree foliage increased from 35 to 52 percent, the efficacy of the videos declined, with “a decrease in stress reduction.”

Thus, the results were found most successful in the male participants who actually displayed a reduction in their stress, specifically those who watched videos of trees containing two to 35 percent foliage. Sullivan and researchers are curious about the findings, and specifically why women did not experience a change in stress reduction.

“We’re not exactly sure why it is,” Sullivan said.

However, he notes that these findings do not tell the entire picture of how each gender responds to stressful experiences.

In a similar study, the researchers followed similar procedures, but in addition to measuring participants’ biological reactions to the video, they asked the female participants in a self-report before and after watching how stressed they were. The self-report consisted of ten questions and for each answer, the female participants marked their stress levels on a 10-centimeter line, with the zero line being very little and the far right end being a lot.

The researchers then measured the number of centimeters over from the zero line for each question and found a linear relationship between watching the videos and the stress level reduction the female participants felt.

“In both measures you see that low levels of vegetation are associated with less stress recovery and medium levels of vegetation are associated with better stress recovery,” Sullivan said. “I give a lot of credence to their self-report in spite of the fact that you’ve got these objective measures, too. I think you got to take it as a whole.”

The results of this study raise major implications for how this could benefit neighborhoods and communities in general. For Jiang, this implies putting more emphasis on the greener aspect of cities.

“We should spend more money in improving every street (through) improving nature, not hotels,” Jiang said. “Trees have magnificent effects on healing.”

For Sullivan, it’s a matter of living a healthy life.

“It does have important implications because we live in stressful times,” he said. “People are under a lot of pressure these days … If you live in a neighborhood or work in an office building or go to a school that’s green, you’re going to be able to recover faster than you would otherwise in a more barren place.”

In addition, Jiang and Sullivan said that they hope that this study will help architects think more seriously about how they might shape classrooms and offices.

Things people can do to help recover from stressful experiences can be as simple as putting up pictures of trees and green landscaping or placing plants in offices and classrooms, Sullivan said.

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