Zhao (1) considers our examination (2) of the relationship between deuterium offset (hereafter “Δ”) and stem gravimetric water content (hereafter “W”) across all the study species to be misleading, and instead suggest that data should be analyzed by individual habitat type based on a vague argument that “different habitats/species will behave differently.” Such a suggestion is unjustified as it ignores the fact that the results of our experiment have already ruled out root uptake-related fractionation (which, if present, would justify the use of a habitat type-based approach) as a mechanism underpinning Δ. As a matter of fact, given our demonstration that extraction-associated deuterium depletion is an unavoidable consequence of plant stem dry matter comprising a high fraction of exchangeable organic hydrogen (3, 4)—unless there is evidence for a role of habitat type in influencing this “fraction”—we have no reason to expect that plants from different habitat types will display different Δ–W relationships. It is interesting to note that the habitat-based reanalysis of our data as performed by Zhao is where “misleading” truly lies given that each habitat type subjected to the analysis contained only three species: From a statistical point of view, such a small sample size would be quite unlikely to yield unbiased estimate of the true Δ–W relationship for each habitat type.
In our reexamination of the Evaristo et al. (5) dataset, we made correction of potential deuterium artifact by applying the experimentally determined, species-averaged Δ value (−8.1‰) to a metadataset containing many species spanning various biomes. As acknowledged in our paper, this certainly is not the most rigorous way of performing deuterium correction for this dataset. Hence, notwithstanding the fact that values of Δ as determined in our experiment broadly agree with those reported previously in various plant types under a diversity of settings (6–10), we agree with Zhao that measurements made across a wider range of species and environmental conditions would be necessary for better constraining the mean and variation of Δ. We therefore have been cautious in data interpretation and explicitly state in our paper that the conducted analysis “was intended to illustrate the need for measurement artifact correction” for studies aiming to test the concept of ecohydrological separation, rather than to draw a definitive conclusion about the validity of this concept. When Zhao stated that “the species-averaged value cannot be extrapolated to correct deuterium offsets,” and also that our correction method “does not directly impact ecohydrological separation,” he apparently failed to recognize the illustrative nature intended for this data reanalysis that we performed.
Last, Zhao infer from our figure 4 that large differences existed in Δ determined from our transpiration and rehydration experiments. This claim is in contrast to our result description made in the legend of figure 4 that no statistically significant difference was detected in the data obtained from the two different experiments. Thus, we have trouble appreciating the significance of a hydration–dehydration effect Zhao claims to find.
Footnotes
The authors declare no competing interest.
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