Involved in the biosynthesis of the labdane-type bicyclic diterpene labda-8(17),12(E),14-triene. Catalyzes the conversion of (+)-copalyl diphosphate to yield labda-8(17),12(E),14-triene. (+)-copalyl diphosphate = (12E)-labda-8(17),12,14-triene + diphosphate 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. Not expressed under laboratory conditions.