• Larix gmelinii;
  • oxygen isotope;
  • permafrost;
  • sap water;
  • taiga

Stable oxygen isotope ratios of plant water (sap water) were observed at Spasskaya Pad experimental forest near Yakutsk, Russia in 1997–1999. The δ18O of sap water in larch trees (Larix gmelinii) decreased soon after leaf unfolding every year, indicating that snowmelt water was used in the beginning of summer. During mid to late summer, a clear difference in the water source used by plants was observed between wet summers and severe drought summers. The δ18O values of water in larch trees were high (−17.8 to −16.1‰) in August 1999 (wet summer), but low (−20.4 to −19.7‰) in August 1998 (drought summer). These results indicated that plants used rainwater during a wet summer, but meltwater from permafrost was used by plants during a drought summer. One important role of permafrost is to provide a direct source of water for plants in a severe drought summer; another role is to keep surplus water in the soil until the next summer. If this permafrost system is disturbed by future global warming, unique monotypic stands of deciduous larch trees in east Siberia might be seriously damaged in a severe drought summer.