This paper addresses both the conceptual complexity and the pragmatic aspect of the ecosystem services concept. By providing a concrete example of assessing ecosystem services, the purpose of this paper is to demonstrate that this complexity is not an obstacle to the application of the concept. In this context buttom-up approach has been developed. Through the characterization of urban vegetation, this ecological assessment studied the functioning of urban trees and quantified ecosystem services using “i-Tree Eco model " in Strasbourg city, France.