Terpene synthase (TPS) involved in the biosynthesis of sesquiterpene natural products included in conifer oleoresin secretions and volatile emissions; these compounds contribute to biotic and abiotic stress defense against herbivores and pathogens (PubMed:21385377). Catalyzes the conversion of (2E,6E)-farnesyl diphosphate (FPP) to alpha-longipinene (PubMed:21385377). (2E,6E)-farnesyl diphosphate = alpha-longipinene + diphosphate Binds 3 Mg(2+) or Mn(2+) ions per subunit. Sesquiterpene biosynthesis. Terpene metabolism; oleoresin biosynthesis. The Asp-Asp-Xaa-Xaa-Asp/Glu (DDXXD/E) motif is important for the catalytic activity, presumably through binding to Mg(2+). Belongs to the terpene synthase family. Tpsd subfamily.