Involved in the biosynthesis of the labdane-type bicyclic diterpene labda-7,13(16),14-triene. Catalyzes the conversion of labda-7,13(E)-dienyl diphosphate to yield labda-7,13(16),14-triene. (13E)-labda-7,13-dien-15-yl diphosphate = diphosphate + labda-7,13(16),14-triene Binds 3 Mg(2+) ions per subunit. The Asp-Asp-Xaa-Xaa-Xaa-Glu (DDXXXE) and Asn-Xaa-Xaa-Xaa-Ser-Xaa-Xaa-Xaa-Glu (NSE) motifs are important for the catalytic activity, presumably through binding to Mg(2+). Belongs to the terpene synthase family.