The diel variation of leaf water 18O enrichment in Amazonian forests and pastures

We measured oxygen isotope ratios (d¹⁸O) of leaf and stem water from a suite of major functional types in Amazonian forests and pastures to investigate diel patterns of isotopic leaf water enrichment (D_o) in March and September 2004 as part of the Large-Scale Biosphere Atmosphere Project (LBA), an international effort to better understand ecosystem processes at regional scales in the Amazon Basin. Two experiments were conducted separately: one in the wet season (March) and the other in the dry season (September). During each field experiment, leaf and stem samples were collected on two-hour intervals at night and hourly during the day for a total of 50 hours from six species in the forest and two species in a pasture, representing four major functional types in the Amazonia, including overstory trees and lianas, lower canopy trees and lianas in the forest and C₄ grasses and C₃ shrubs in the pasture. Measured D_o showed similar diel variations between the two species in the pasture while considerable variations were observed among the six species in the forest. Physiological and gas exchange properties, including leaf water content and stomatal conductance, also were measured at times representing the two seasons in order to predict isotopic leaf water enrichment using three different models that considered either steady state or non-steady state description. Species-specific and environmental factors that influence predictions of D_o with respect to these models are discussed.