This paper presents a methodology to estimate the environmental service value provided by trees in small farms in Chiapas, México, through the use of a Functional Value Index based on the functional traits and ecological characteristics of trees. You can access the paper here.

Full citation:  
Marinidou E, Finegan B, Jiménez-Ferrer G, Delgado D, Casanoves F (2013) Concepts and a methodology for evaluating environmental services from trees of small farms in Chiapas, México. J Environ Manage 114:115-124.