Involved in defensive oleoresin formation in conifers in response to insect attack or other injury. Involved in monoterpene (C10) olefins biosynthesis. Produces mainly (+)-alpha-pinene (97%) with a small amount of (-)-alpha-pinene (3%). (2E)-geranyl diphosphate = (1R,5R)-alpha-pinene + diphosphate Binds 3 Mg(2+) or Mn(2+) ions per subunit. Optimum pH is 7.5. 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.