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 (79%) and lesser amounts of (-)-beta-pinene (4.2%), nearly racemic mixtures of camphene (2.8% (+)/2.2% (-)) and limonene (2.4% (+)/3.7% (-)), as well as small amounts of (+)-alpha-pinene (3.3%) and (+)-beta-pinene (2.4%). (2E)-geranyl diphosphate = (1S,5S)-alpha-pinene + diphosphate Binds 3 Mg(2+) or Mn(2+) ions per subunit. Optimum pH is 7.0. 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.