This article by Jonah Lehrer over at Wired is from last year so I am a bit late about finding it, but it's really interesting and has some great links.
"......the typical city park is little more than an expansive lawn, punctuated by a few trees and playing fields. I’ve got nothing against grass and Little League, but it’s probably worth pointing out that, if you want to maximize the psychological perks of greenspace, this is probably the wrong approach. In a 2007 paper, Richard Fuller, an ecologist at the University of Queensland, demonstrated that the mental benefits of green space are closely linked to the diversity of its plant life. When a city park has a larger variety of trees, subjects that spend time in the park score higher on various measures of psychological well-being, at least when compared with less biodiverse parks."