Tropical Leaf Wax n‐Alkane and n‐Alkanoic Acid Reflect δD of Precipitation During Early Stages of Leaf Growth: Insights From an Isotope Labeling Experiment

Abstract The hydrogen isotopic compositions of leaf‐wax n ‐alkanes (δD alk ) and n ‐alkanoic acids (δD acid ) reflect ambient climatic conditions (including precipitation δD values, δD precip ). However, the understanding of climatic conditions of exactly which period (i.e., early or entire period of the leaf's lifespan) these biomarkers preserve is still evolving. Previous studies on the δD alk and δD acid values, done only in extra‐tropical regions, mostly indicate that δD alk values are biased toward the early growing season whereas δD acid values are not biased toward any season. To decipher the seasonal bias in δD alk and δD acid records from tropics, we conducted a long‐duration experiment wherein deciduous and evergreen species were grown using normal water (δD = −2‰) during early stages of the leaf growth and later using isotopically labeled water (δD = 1,000‰). Our experiment revealed (a) in deciduous and evergreen species, δD alk and δD acid values reflect δD precip during early stages of the leaf growth, (b) synchronous synthesis of n ‐alkanes and n ‐alkanoic acids, and (c) in deciduous species, minor incorporation of the previous year's photosynthates in leaf wax pool of the current year's mature leaves. Our study suggests that δD alk and δD acid records in the tropics are biased toward the climatic conditions prevailing during early stages of leaf growth. This bias should be considered while comparing the δD precip values generated from leaf wax proxy records and isotope‐enabled atmospheric circulation models.