Involved in defensive oleoresin formation in conifers in response to insect attack or other injury. Involved in monoterpene (C10) olefins biosynthesis. Produces 57.3% alpha-terpineol (15.1% (+)/84.9% (-)), 27.6% limonene (25.2% (+)/74.8% (-)), 8% terpinolene, 4.7% beta-pinene (14.8% (+)/85.2% (-)), 1.3% alpha-pinene (100% (+)) and 1.1% myrcene. (2E)-geranyl diphosphate + H2O = (S)-alpha-terpineol + diphosphate Binds 3 Mg(2+) or Mn(2+) ions per subunit. 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.