The ecotone is a transition space between two ecosystems: there meet different species original from adjacent areas, and other species peculiar to the transition area itself. The permeability (wood-priaries, sweet water -salty water), allows high level of biodiversity. The concept is to provide an ecotone that connects two environments supposedly incompatible: the forest park and the built area. A park has different dynamics that the ones of men and city. Construction times of a building are less than the ones to let a forest grow. Who visits this park is intended to follow its development, getting attached to it in a symbolic and cherished manner. The network of environment sustainability make the Valdebebas Park completely self sufficient from an energetic point of view, increasing its "green lung" quality for the city of Madrid.