Plant source water apportionment using stable isotopes: A comparison of simple linear, two-compartment mixing model approaches

What is it about?

Plants via their roots take up water from different parts of the ground. Stable isotopes serve as tracers of root water uptake. The utility of these tracers follows the 'rule of thumb' that root water uptake is an isotopically conservative process. That is, the 'identity' of the tracer from its source in the ground to its fate in the plant does not change. However, there are exceptions to this rule. And where such exceptions exist, traditional approaches in quantifying plant source water proportions may or may not be valid. Here we leverage new isotope data from a diverse set of tree species in New Zealand. We present a critical evaluation of some of the main model approaches used for source water apportionment. Our results suggest that plant source water quantification methods should take into consideration cases where such 'rule of thumb' may be violated.

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Why is it important?

Water stable isotopes provide direct means for tracking and quantifying plant water sources. Much of what we do with water stable isotopes, however, is based on the assumption that root water uptake is an isotopically conservative process. Our finding that some environmental settings may pose an exception to this rule suggests that some source water apportionment approaches may be more appropriate than others, thereby, calling into question the validity of model approaches that fail to account for these exceptions.