Urban middle school students in Canada were more physically active when their neighborhoods contained undeveloped natural areas with trees but not meadows. For every 5% increase in tree-covered land, there was a corresponding 5% increase in free-time physical activity.
Janssen, Ian, and Andrei Rosu. (2015). Undeveloped green space and free-time physical activity in 11 to 13-year-old children. International Journal of Behavioral Nutrition and Physical Activity, 12(1), 26.