Trees for Tranquility

Admiring nature may confer stress relief, as outdoors enthusiasts, painters, and writers have observed.  Bin Jiang, from the University of Illinois (Illinois, USA), and colleagues enrolled 160 study subjects, assessed via a standardized stress assessment scale, in a study in which they were assigned to watch 1 of 10 three-dimensional videos of street scenes that varied in the density of tree cover (from 2% to 62%).  Multiple measures were used to assess participants’ stress levels and recovery during the experiment, including skin conductance, salivary cortisol levels and self-reports. Among the male participants, the researchers found a bell-shaped dose-response curve. As the tree canopy increased from 2 to 24%, the men’s biomarkers of stress recovery improved proportionally. The men experienced the most stress recovery benefits when they viewed tree canopy in the 24 to 34% range, and stress recovery declined when the percentage of tree cover surpassed 34%. While women did not show the same physiological responses in salivary cortisol and skin conductance levels as the men, the researchers’ analyses of the self-reports suggested that the women also experienced stress reduction benefits that increased proportionally with the percentage of tree canopy viewed. Forty-one percent of all the subjects who watched videos with minimal tree canopy described calming effects in their self reports. Interestingly, when the percentage of tree canopy increased to 36%, more than 90% of viewers reported feeling calm or relaxed while watching the videos. The study authors conclude that: “These findings suggest that viewing tree canopy in communities can significantly aid stress recovery and that every tree matters.”