Involved in the biosynthesis of the labdane-type bicyclic diterpene labda-7,13(16),14-triene. Catalyzes the conversion of geranylgeranyl diphosphate (GGDP) into labda-7,13(E)-dienyl diphosphate. (2E,6E,10E)-geranylgeranyl diphosphate = (13E)-labda-7,13-dien-15-yl diphosphate The Asp-Xaa-Asp-Asp-Thr-Ala (DXDDTA) and Arg-Xaa-Xaa-Asp-Gly-Ser-Trp (RXXDGSW) motifs are expected to bind to Mg(2+). Belongs to the terpene synthase family.