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

Abstract

The seasonally varying moisture balance in a montane forest of Southeast Asia is reconstructed for the 20th century from the oxygen isotopic composition (δ18O) of subannual tree cellulose samples of Pinus kesiya growing at 1,500 m elevation on Doi Chiang Dao in northern Thailand. The cellulose δ18O values exhibit a distinctive annual cycle with amplitude of up to 12 ‰, which we interpret to represent primarily the seasonal cycle of precipitation δ18O. The annual mean δ18O values correlate significantly with the amount of summer monsoon precipitation, and suggest a temporal weakening relationship between the South Asian monsoon and El Nino-Southern Oscillation over the late 20th century. The cellulose δ18O annual maxima values, which reflect the dry season moisture status, have declined progressively over the 20th century by about 3.5 ‰. 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.