|This review deals with how trees shape the structure and function of the critical zone (CZ). More specifically, it examines the role of tree roots in ‘building and plumbing’ the CZ, through several biophysical processes associated with tree functioning: mechanical stability and water and nutrient acquisition. The paper starts with a conceptual summary of plant-fungi interactions, tree root form and function and CZ architectural dynamics. But the core of the manuscript is the 9 hypotheses on how trees build and plumb the CZ. Overall, the authors have managed to link existing interdisciplinary knowledge on the CZ to highlight knowledge gaps and set the foundations for future CZ research. |
Although I agree with comments from previous reviews pointing to the lack of more explicit guidelines for future work, I am aware that the paper is already lengthy and that the field is too interdisciplinary to provide more specific ways forward for the 9 hypotheses posed here. This manuscript provides a first roadmap to develop more specific approaches, that may need to be dealt with in papers more individually focused on ‘plumbing’ or ‘building’ processes, for example.
Nevertheless, I do think that sections 4 and 5 could still be improved. Section 4, on synthesis and challenges is short and does not build on the hypotheses presented earlier in the manuscript. For example, could the authors prepare a graphic summary of the hypotheses in a figure that could be used to structure this last section? Maybe showing areas of overlap or interaction between ‘building’ and ‘plumbing’? Illustrating whether fungi play a role or not?
The abstract ends quite abruptly after listing the 9 hypotheses, without any closing sentence. This also relates to the insufficient treatment of the synthesis and future work sections in the paper.
Figure 1. I wonder whether this figure is necessary to support the text, because some of the information may seem quite obvious (e.g. forest-atmosphere exchange of water and energy) or somehow already shown in Fig. 2 (e.g. erosion).
P. 2, L. 17. I think there is a major leap here between CZ research in broad terms and the focus on trees. To me, the major biotic influence on CZ is through vegetation, and this includes not only forests, but also shrublands and grassland, for example. Are there any papers on the broader role of vegetation on CZ that could be cited here? I am not asking for a long sentence, but, being a review paper, and before moving on to trees, maybe the authors should acknowledge any previous work on the vegetation-CZ relationship.
P. 2, L. 23. The transpiration estimates from isotopic measurements in the Jasechko paper may be biased too high according to several papers published thereafter. The number presented by the authors could be adjusted to reflect more realistic estimates gathered from various methods: Schlesinger, W.H. & Jasechko, S. (2014) Transpiration in the global water cycle. Agricultural and Forest Meteorology, 189–190, 115–117.
P. 14, L. 15. When you refer to ‘some depth’ could you be more precise?
P. 16, L. 4. Instead of ‘tree age and size’, I would say that that tree density is affected more by competition, in line with the preceding statement on self-thinning.
P. 23, L. 19. The reference by Nippert and Knapp is on grassland species, and in fact the paragraph seems to be focused on ‘plants’ in general, not on trees. This can be a bit confusing, given the focus of the paper on trees only. I suggest that you provide a more clear link to tree functioning in this paragraph.
P. 23, L. 25. I think that the authors should express more clearly that the two water worlds hypothesis is a controversial one, obscured by several methodological issues. I suggest that they look at a couple of recently published papers which deal with the TWW hypothesis from the ecophysiological and hydrological perspectives:
Berry, Z.C., Evaristo, J., Moore, G., Poca, M., Steppe, K., Verrot, L., Asbjornsen, H., Borma, L.S., Bretfeld, M., Hervé-Fernández, P., Seyfried, M., Schwendenmann, L., Sinacore, K., De Wispelaere, L. & McDonnell, J. (2017) The two water worlds hypothesis: Addressing multiple working hypotheses and proposing a way forward. Ecohydrology .10.1002/eco.1843
Sprenger, M., Leistert, H., Gimbel, K. & Weiler, M. (2016) Illuminating hydrological processes at the soil-vegetation-atmosphere interface with water stable isotopes. Reviews of Geophysics, 54, 2015RG000515.
P. 25, L. 8-9. There is another study, published before Meinzer et al. 2016, which provides a more comprehensive compilation of predawn water potentials globallIy. In fact, this study could be cited earlier (P. 24,L. 20-21) to support the statement that the concept of a universal ‘wilting point’ might not be appropriate.
Martínez-Vilalta, J., Poyatos, R., Aguadé, D., Retana, J. & Mencuccini, M. (2014) A new look at water transport regulation in plants. New Phytologist, 204, 105–115.
P. 30, L. 4-6. What about other global databases and networks? Could they be used to advance CZ research?
For instance: fine root database http://roots.ornl.gov/, soil moisture networks, https://ismn.geo.tuwien.ac.at/, sap flow database, http://sapfluxnet.creaf.cat