A fossil pollen record from the Costa Rican Cordillera de Talamanca presents the first AMS radiocarbon-dated evidence of a temperature decrease during the Younger Dryas Chron. This cooling event is named La Chonta stadial after the bog at 2310 m altitude, located at the actual lower to upper montane forest boundary. High-resolution pollen analysis revealed that between 11,070 +/- 130 (AMS C-14 age) and 10,400 yr B.P. (interpolated age) vegetation comparable to present-day subalpine forest occurred about 300-400 m lower than in the previous warmer interval, and 600 to 700 m lower than at present time. The radiocarbon dates ale close to the earlier estimated ages of 11,080-10,500 yr B.P. for this interval, based on interpolation and pollen concentration rates (Hooghiemstra et al., 1992). The downslope shift of the upper forest line indicates an estimated temperature drop of 2-3 degrees C during the La Chonta stadial. The local vegetation development is indicative of a drier climate. From 10,400 to 9800 +/- 120 yr B.P. (AMS C-14 age) subalpine rain forest was replaced by upper montane forest, a transitional period to Holocene environmental conditions.
