In a study that compared psychophysiological stress recovery and directed attention restoration, people recovered from stressful situations faster when they sat in a room with a window view of trees compared to when they sat in a viewless room. Walking in a nature reserve fostered greater stress reduction than walking in an urban setting. Likewise, both performance on attention tests and mood improved after walking through a nature reserve; both declined after walking in an urban environment.
Hartig, Terry, Gary W. Evans, Larry D. Jamner, Deborah S. Davis, and Tommy Gärling. (2003). Tracking restoration in natural and urban field settings. Journal of Environmental Psychology, 23(2), 109-123.