Development of a site form equation for predicting and mapping site quality. A case study of unmanaged beech forests in the Cantabrian range (NW Spain)

•The first site form curves have been developed for Cantabrian beech forests.•Site form is a reliable measure of site quality in the beech stands under study.•Dominant height and diameter can be accurately estimated using ALS metrics.•An unbiased site index raster model with a resolution of 25x25 m/pixel was developed.•The proposed model allows site quality estimation at inventory plot and forest levels.