20th century seasonal moisture balance in Southeast Asian montane forests from tree cellulose δ18O

The seasonally varying moisture balance in a montane forest of Southeast Asia is reconstructed for the 20th century from the oxygen isotopic composition (delta O-18) of subannual tree cellulose samples of Pinus kesiya growing at 1,500 m elevation on Doi Chiang Dao in northern Thailand. The cellulose delta O-18 values exhibit a distinctive annual cycle with amplitude of up to 12 parts per thousand, which we interpret to represent primarily the seasonal cycle of precipitation delta O-18. The annual mean delta O-18 values correlate significantly with the amount of summer monsoon precipitation, and suggest a temporal weakening relationship between the South Asian monsoon and El Nio-Southern Oscillation over the late 20th century. The cellulose delta O-18 annual maxima values, which reflect the dry season moisture status, have declined progressively over the 20th century by about 3.5 parts per thousand. We interpret this to indicate a change in the contribution of the isotopically distinct fog water to the dry season soil moisture in response to rising temperature as well as deforestation.