This paper tests at the ecosystem and eddy flux scale interesting observations made at the chamber scale of decoupling photosynthesis and evapotranspiration (latent heat flux) during heat waves, which makes it appropriate for publication in BG.

Two expert Reviewers recommend major revisions and reconsideration and I concur.
In their response it seems the authors accept many of the Reviewers’ comments and address others. They are therefore invited to implement these revisions including the following points.

While the authors seem to accept that issues associated with estimating GPP and lack of energy closure effects on LE should be highlighted and discussed, it is also appropriate, even recommended, to cite relevant papers that advance new ideas, such as Kowalski’s paper on possible non-diffusive effects at high temperature, or Vesala et al 2015 on the Kelvin effects on WUE. Only such exposure may lead to their consideration and testing (BTW, with respect to energy closure at high temperatures, studies at our site like Eder et al 2015 and subsequent papers might be of some interest).

Likewise, a referee’s efforts in checking units and consistencies should be taken advantage off and carefully considered. Including the inclusion of some key equations that would be great help in conveying key points and saving the troubles “background checks” (as it seems the author propose in response to Ref 2 comments).

Finally, a more traditional organization of the paper, including clear conclusions (concrete or otherwise) will also greatly help, as the authors seems to accept.

As stated at the outset, the paper deals with an interesting and important issue of decoupling A vs T in response to extreme events, specifically during heat waves, and should be appropriate for publication in BG. The paper was revised and improved, mainly in reorganizing and adding a useful Discussion section (although the Results section is still “results and discussion”). And I do agree with the notion that inconclusive results in a good study can still be valuable. But with some controversy and with some partial responses to comments, the paper was sent out for review and check by additional expert reviewer. Indeed, the reviewer noted that he was “somewhat disappointed with some of the replies to the reviewer’s comments…” I therefore try below to highlight a few of the points I noted (I did not extensively review the paper), and recommend additional revisions to further improve the paper before final publication.

1. The title could be improved as suggested to better reflect what is repeatedly stated as the topic—"trees decouple photosynthesis and gs...”; and “we analysed the photosynthetic decoupling between photosynthesis and transpiration..." (the current title seems to point to the VPD driven response which the paper wants to extend).
2. On the same token, one Ref noted the importance of considering in more detail conductance. It seems that A vs E decoupling would indeed require g response, and it is somewhat odd that while the paper has no problems making assumptions regarding LE and GPP, it would not attempt to estimate or discuss any form of G that is often obtained at the canopy scale, at least to some extent without extensive modeling (e.g. from VPD*/LE when LE is argued to reliably reflect T and temperature records are discussed in detail).
3. Similarly, substituting LE for T, and indirectly estimated GPP are now better noted with references, but the implications to the current study, such as to the slope vs temp are not discussed. (e.g. understory and LAI issue are commented by two Refs).
4. As suggested the authors added more explanation of the theory/background leading to their analysis, but it’s hidden in the Methods while it fits better up front in the Intro.
5. There were also related arguments on the physical basis of some of the complicated ‘units’ and in addition to noting these "units" it will help readers to better clarify that indeed, there are some none-physical parameters that derived from ‘relationships’ or ‘correlation’ and as such may not require formal units.
6. In the context of discussing the theoretical basis of a core issue such as unexpected conductance or transpiration response, and considering that Kowalski happened to be a reviewer (fortuitously), I do think it’s appropriate to cite his less conventional study, especially as the authors note it adds another perspective rather than a contradiction to the results. The doubts regarding leaf internal vapor pressure are at least as intriguing as the decoupling issue. The Kowalski paper may in fact be relevant together with another, contrasting, paper recently out (Cernusak et al 2018) on reduced e inside leaves at high VPD. Maybe less of concern are the other suggested references of Vesala et al., and of Eder et al., although it’s surprising that the later one is rejected because temperature at the site was not found (there are dozens of papers on that particular site, including at least two specifically on heat-wave response in pine trees; Tatarinov et al., 2015; Wohlfahrt et al., 2018).
7. A Ref point regarding the data time scales is partially addressed in the Response and not at all in the text. Seems valid to ask why daily sum GPP could not be coupled, for example, with daily mean temperatures, or max temp related to midday GPP?
8. Finally, in the context of the discussion, it might be relevant to discuss the broader point that the expectation of decoupling that involves enhanced T, is relevant to conditions of very high temp associated with non-limiting water supply, which may not be the “norm”...
9. More technically, Figure 1, and probably Fig 2, are ineffective in showing slopes or trends, as the scale in panels a to f seem to be dictated by that of the unusually high values of panel g. Without adjusting the scale, it seems not much point in displaying them. In the captions, the term slope is used and it would help to more clearly indicate what slope.
10. The new Discussion and conclusion is a good addition, but the separation of the sub-sections of “Route forward” and “Conclusions” seems excessive.

The authors have considerably improved the response to the Reviewers comments and the paper should be published at this stage.