The complex effect of vegetation on thermal stress - real or imaginary? Measuring the psychological effect of vegetation

Studies show beneficial effects of vegetation for humans, such as improvement of air quality, heat moderation and stress reduction. It is also assumed, so far with no robust evidence, that vegetation has a measurable psychological effect that enhances thermal comfort and reduces thermal stress, and that this effect is manifest in addition to its well-established physiological effect. Individuals in a warm environment feel cooler in the presence of extensive vegetation than in a similar setting with an identical Physiological Equivalent Temperature (PET) but less vegetation. We provide, for the first time, empirical evidence that quantifies the psychological effect of the Vegetation View Index (VVI) on thermal comfort and thermal stress. An outdoor study with 322 participants showed that when average PET exceeds ∼32 °C, the presence of vegetation in a person's field of view (FOV) has a positive effect on thermal sensation, thermal comfort and thermal preference. Thermal comfort and preference votes improved as VVI increased, and thermal sensation votes were progressively lower, as VVI values increased up to a median of ∼9 %. These subjective responses were accompanied by a measurable and significant difference in participants' physiological stress levels, as assessed by skin conductance level, for a Physiological Equivalent Temperature of more than ∼31 °C. The beneficial effect of vegetation increased with an increase in the Vegetation View Index, particularly from a median coverage of ∼11 %, but the effect was nonlinear. These results demonstrate psychological and physiological contributions of vegetation to thermal stress reduction, as recorded simultaneously under similar hot meteorological conditions.