Flores Velásquez, P., Martínez de Anguita, P., Romero Calcerrada, R., Novillo, C. J. and Ruiz, M. A. (2008) “Payment for environmental services between the adding and the subsidization: application to the valuation of the landscape in the San Juan dam, Madrid, Spain”, Forest Systems, 17(1), pp. 39-53. doi: 10.5424/srf/2008171-01022.