Monoterpene synthase (TPS) involved in the biosynthesis of monoterpene natural products included in conifer oleoresin secretions and volatile emissions; these compounds contribute to biotic and abiotic stress defense against herbivores and pathogens (PubMed:23679205). Catalyzes the conversion of (2E)-geranyl diphosphate (GPP) to (+)-car-3-ene and, to a lower extent, to terpinolene (PubMed:23679205). (2E)-geranyl diphosphate = (+)-car-3-ene + diphosphate (2E)-geranyl diphosphate = diphosphate + terpinolene Binds 3 Mg(2+) or Mn(2+) ions per subunit. Terpene metabolism; oleoresin biosynthesis. Secondary metabolite biosynthesis; terpenoid 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.