Scent of trees: Investigating the short-term effects of two tree essential oils on mood, psychological stress, and cognition

The sensory experience of nature involves all senses, yet the olfactory pathway has been largely overlooked. Research shows that exposure to natural environments benefits mood, stress, and cognitive functioning. To explore this further, we conducted two studies to assess whether short-term exposure to essential oils from two trees, the Douglas fir ( Pseudotsuga menziesii) and the Hinoki cypress ( Chamaecyparis obtusa ), influences these outcomes. In both studies, participants (first study n =68, second study n =34) completed one mood and psychological stress assessment along with seven cognitive tasks on two separate days. During testing, an ultrasonic diffuser containing either essential oil from one of the trees or clear water was activated. Participants were randomised into one odorant group (Douglas fir vs. Hinoki) and experienced both exposure conditions in randomised order (odorant vs. placebo). The initial study showed marginal evidence of an effect of the Douglas fir scent on vigilance, prompting a follow-up study to boost statistical power. However, combined analysis revealed no significant effects of either odorant group on mood, psychological stress, or cognitive performance. Interestingly, most participants could not reliably identify the odour, suggesting limited conscious awareness and/or semantic associations. Our findings suggest that short-term exposure to these two tree essential oils does not produce immediate effects on the tested outcomes. Further research is needed to explore the role of conscious perception, semantic associations, individual odour preferences, and long-term exposure to forest scents in modulating emotional states and cognition. • Two studies investigated Douglas fir and Hinoki essential oil effects on mood, psychological stress, and cognition. • Douglas fir exposure minimally affected vigilance, prompting a second study for more power. • Analysis of both studies show no effect on mood, psychological stress, and cognitive performance. • Most participants misidentified scents and many participants did not detect the scent at all. • Results highlight olfactory complexity, suggesting future research on odour-cognition associations.