Involved in defensive oleoresin formation in conifers in response to insect attack or other injury. Involved in monoterpene (C10) olefins biosynthesis. A mixture of alpha- and beta-pinene (35:10) is produced by this enzyme. (2E)-geranyl diphosphate = (1S,5S)-alpha-pinene + diphosphate (2E)-geranyl diphosphate = (1S,5S)-beta-pinene + diphosphate Binds 3 Mg(2+) or Mn(2+) ions per subunit. Terpene metabolism; oleoresin biosynthesis. By wounding and insect attack. 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.