|I’m not a modeler, but it seems like the model works really well/represents a broadly applicable advance. My remaining comments are generally focused on the setup, presentation, and interpretation of results. |
1) The first three paragraphs of my previous review (general comments) were not specifically addressed. In my experience, this is not typical for the reviewer response process, and is likely to be unacceptable to some reviewers and/or editors going forward.
In particular, I would still appreciate the authors responding to the third paragraph of my previous review, repeated here:
Relatedly, Figures 4-9 all show similar long-term timeseries data with scatterplots that lend themselves to similar interpretations in terms of R2 of MAE. These are useful, but perhaps they could be condensed and/or supplemented with other figure types that were more conducive to process-based interpretation. For example, I found Figures 11e and 11f fascinating insofar as they highlighted seasonal differences between vegetation types, but little explanation was provided to “unpack” these results (grasslands and shrublands not even mentioned). Likewise, Figures 12a and 12b present a rich opportunity to speak to differences between the biophysical controls on ET at the SNOTEL and East River sites. Some of the specific factors I’m left wondering about are differences in snow accumulation and melt between sites, evaporation versus transpiration, and heterotrophic versus autotrophic respiration. I understand that you don’t have all these measurements, but you’ve generated a lot of suggestive data that could be leveraged to push this field of research.
2) Some of the results and discussion require more nuanced and/or focused interpretation (see detailed comments below). At the same time, the manuscript is long and could be shortened/tightened in many places to more accurately present/highlight key results (details below).
L10: The decision to focus on Reco could be set up better. In other words, why Reco instead of NEE or GPP and/or all three? I don’t necessarily have a problem with your decision to focus on Reco, but it must be clearly justified.
L17 “sites within sites”?
L21: Suggest adding “USA” here for the global audience.
L27ff: Please specify “air, soil, snow”, etc. whenever “temperature” is invoked. Lots of room for confusion here because most would expect ET to vary more with air temperature versus Reco that is more sensitive to soil temperature.
L34-35: Same comment as L10.
L129-131: Recent work by Chu et al. 2021 on the representativeness of statistical tower measurement footprints to surrounding areas may be relevant here.
L191-192: “Same” is not the equivalent of “too close to distinguish on the map”.
L483: Is it that earlier snowmelt triggers the onset of vegetation activity or that higher air temperatures trigger both snowmelt and the onset of vegetation activity?
L485-486: Can you speak to the synoptic meteorological conditions in 2012 versus 2015? Why choose these two years for comparison? Similarly, the comparison of March, April, and May between years is interesting, but what about the rest of the year? I’d be very interested in a similar post-monsoon analysis, potentially between years with strong and weak monsoons.
L492-497: Please edit this section to remove/acknowledge differences in NDVI that would be expected due to deciduous versus evergreen physiology. Some of this basic information currently comes across as results. I appreciate the attempt to relate these results back to processes, but this section needs refinement.
L517ff: What does the “1” syntax correspond to?
L525-526: Please be specific about the meaning of “drought” in this context. Is it simply meant to connote some limitation to ET and/or Reco? If so, can you justify the underlying expectation that these variables would be affected at the same moisture threshold? I’d also argue that “usually” is the wrong word here. Earlier snowmelt certainly “can” trigger summer drought, but this scenario is subject to modification by monsoon precipitation and other factors as the authors acknowledge in this sentence. See recent work by Knowles et al. 2020, Xu et al. 2020, and many references therein.
L583-596: I support this opportunity to discuss physiological differences between evergreen and deciduous vegetation, but simply citing Baldocchi et al. 2010 is insufficient. More thorough and nuanced discussion that incorporates foundational research on this topic is required.
L600-601: See comment on L525-526.
L604-605: This implies that growing season length determines snow water storage when in fact, it’s closer to the opposite i.e., air temperature and/or snow accumulation determine the onset of the growing season. See Lian et al. 2020 and Zhang et al. 2020 for examples of more recent work on this topic. Combining the Sloat et al. 2015, Wainwright et al., 2020, and Hu et al., 2010 references here also raises an important distinction. Whereas the Sloat and Wainwright references invoke fore-summer i.e., pre-monsoon drought, the Hu reference pertains to late summer drought i.e., after snowmelt water inputs have subsided. This distinction reflects the typical relative importance of snowmelt vs. monsoon precipitation at a given site e.g., snow-dominated sites may be susceptible to moisture limitation after the snowmelt pulse (late summer; Hu et al. 2010), whereas monsoon-dominated sites may be susceptible to moisture limitation before the onset of monsoon rains (early/fore-summer; Sloat et al 2015; Wainwright et al. 2020). Please establish the typical relative importance of snow versus monsoon precipitation at the East River site and how your results may be expected to change at sites where moisture availability is typically more or less affected by snowmelt versus monsoon precipitation.
L612: Hard to follow, I think “whereas” may be the wrong word here.
L629: “Microclimate” is misspelled.
Chu, H., Luo, X., Ouyang, Z., Chan, W. S., Dengel, S., Biraud, S. C., Torn, M. S., Metzger, S., Kumar, J., Arain, M. A., Arkebauer, T. J., Baldocchi, D., …, and Zona, D.: Representativeness of Eddy-Covariance flux footprints for areas surrounding AmeriFlux sites, Agricultural and Forest Meteorology, 301–302, 108350, https://doi.org/10.1016/j.agrformet.2021.108350, 2021.
Knowles, J. F., Scott, R. L., Biederman, J. A., Blanken, P. D., Burns, S. P., Dore, S., Kolb, T. E., Litvak, M. E., and Barron‐Gafford, G. A.: Montane forest productivity across a semiarid climatic gradient, Global Change Biology, 26, 6945–6958, https://doi.org/10.1111/gcb.15335, 2020.
Lian, X., Piao, S., Li, L. Z. X., Li, Y., Huntingford, C., Ciais, P., Cescatti, A., Janssens, I. A., Peñuelas, J., Buermann, W., Chen, A., Li, X., Myneni, R. B., Wang, X., Wang, Y., Yang, Y., Zeng, Z., Zhang, Y., and McVicar, T. R.: Summer soil drying exacerbated by earlier spring greening of northern vegetation, Science Advances, 6, eaax0255, https://doi.org/10.1126/sciadv.aax0255, 2020.
Xu, B., Arain, M. A., Black, T. A., Law, B. E., Pastorello, G. Z., and Chu, H.: Seasonal variability of forest sensitivity to heat and drought stresses: A synthesis based on carbon fluxes from North American forest ecosystems, Global Change Biology, 26, https://doi.org/10.1111/gcb.14843, 2020.
Zhang, Y., Parazoo, N. C., Williams, A. P., Zhou, S., and Gentine, P.: Large and projected strengthening moisture limitation on end-of-season photosynthesis, Proc Natl Acad Sci USA, 201914436, https://doi.org/10.1073/pnas.1914436117, 2020.