Unraveling urban plant diversity: The independent effects of plant species richness and greenness on health

Urban green spaces are essential for the well-being of both wildlife and humans, with numerous studies reporting positive associations between plant diversity and health outcomes. The specific impact of plant species richness on health, however, is frequently confounded by overall vegetation cover. Additionally, highly vegetated spaces can sometimes be linked to negative health outcomes. This study examined the independent effects of plant species richness and green view levels on human health, including mental fatigue and stress, and the extent to which these effects persist with green view levels adjusted. We also assessed the extent to which individual characteristics moderate these relationships. Using an experimental design, we displayed simulated scenes of varying levels of plant species richness and green views via videos to assess changes in acute attentional capacity, psychological stress (e.g., emotions), and physiological stress (e.g., heart rate, heart rate variability, and skin conductance) before and after the exposure. With the data from 154 participants, our results indicated that while green views had a positive effect on measures of relaxation, plant species richness variations had no impact. Moreover, individual characteristics, including long-term attentional functioning, stress, and nature relatedness, showed moderation effects in the relationships between plant diversity and health outcomes. Future research should address the intensity of the intervention and consider the appropriateness of research tools for various population groups to discern any causal relationship between urban plant species richness and human health.