Developing a site index model for P. Pinaster stands in NW Spain by combining bi-temporal ALS data and environmental data

•Spanish National Forest Inventory and bi-temporal ALS data, can be used with other sources of freely available environmental data to develop regional SI models for P. pinaster stands in Galicia.•Site productivity was modelled by an age-independent difference equation method based on ALS metrics and environmental variables.•A two-equation system comprising an equation from two ALS flights and another equation including environmental variables as predictors is proposed.•The proposed models can be used to map SI over large areas of forest to determine forest productivity at a much finer spatial resolution than usually possible.