The δD records of n-alkane and n-alkanoic acid of tropical trees reflect δD of precipitation during the early stages of the leaf growth

The hydrogen isotopic compositions of the leaf-wax n -alkanes (δD) and n -alkanoic acids (δD) are known to reflect ambient climatic conditions (including precipitation δD values, δD). However, the climatic conditions of exactly which period (i.e. early or entire period of the leaf’s lifespan) these biomarkers represent, i.e. the seasonality in δD and δD records, is still evolving. The seasonality studies on the δD and δD values, done only in extra-tropical regions, mostly indicate the δD values are biased towards the early growing season whereas δD values are not biased towards any season. To decipher the seasonality in the δD and δD records from the tropics, we conducted a long-duration experiment wherein deciduous and evergreen species were grown using normal water (δD = −2‰) during the early stages of the leaf’s growth and later using isotopically-labeled water (δD = 1000‰). Our experiment revealed (i) in deciduous and evergreen species, δD and δD values reflect δD during the early stages of the leaf’s growth, (ii) synchronous synthesis of n -alkanes and n -alkanoic acids, and (iii) in deciduous species, minor incorporation of the previous year’s photosynthates in the leaf wax pool of the current year’s mature leaves. Our study suggests that the δD and δD records in the tropics are biased towards the climatic conditions prevailing during the early stages of the leaf’s growth. This bias should be considered while comparing the δD values generated from the leaf wax proxy records and isotope-enabled atmospheric circulation models.