Bagging: a cheaper, faster, non-destructive transpiration water sampling method for tracer studies

Abstract

Stable isotope tracer experiments provide a powerful tool for understanding plant root distributions, resource uptake, niche partitioning and water cycling. Plant water is typically collected from pre-transpiring tissues to avoid the effects of evaporative isotope enrichment at the leaf surface, but extracting water from these plant samples is difficult and expensive. The purpose of this study was to test a simple transpiration bagging approach for measuring hydrologic tracer uptake. Sampling was performed as part of a depth-specific tracer experiment in which 2H2O was injected to target depths (5, 15, 30, 60, or 150 cm) in different replicated plots. One day following injections, leaves from three species were sealed in bags for 16 h and transpired water was collected. Water from pre-transpiring stem tissue was then collected in a separate set of samples and extracted using cryogenic distillation. Deuterium concentrations from the two techniques were correlated (R2 = 0.84) and both approaches produced similar descriptions of vertical root distributions for three dominant plant species. 18O concentrations from the two techniques were not correlated. Bagging transpired water produced similar estimates of 2H tracer uptake as the standard sampling technique. Bagging requires no destructive sampling, specialized laboratory equipment, training or consumables and is expected to halve sampling costs. While effective in this tracer experiment, bagging may be not be effective in natural abundance experiments, or tracer experiments with very small plants or small transpiration rates (i.e., early-season or arid sites).