the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 15 Oct 2024
Investigating Plant Responses to Water Stress via Plant Hydraulics Pathway
Abstract. Drought-induced plant hydraulic failure is one of the main factors for large-scale plant mortality. Understanding the response of the plants to water stress is of paramount importance to elucidate the dynamics of water, energy and carbon fluxes under drought conditions. In this study, we implemented the plant hydraulics pathway in STEMMUS-SCOPE (hereafter as STEMMUS-SCOPE-PHS) by considering xylem vulnerability, and validated the model at a karst site in Chongqing, China. Plant water potentials of root, stem and leaf are calculated in STEMMUS-SCOPE-PHS. A leaf water potential-based plant water stress factor (PHWSF) replaces the original soil moisture-based water stress factor to represent the effect of water stress on plant growth. Results show that the PHWSF captures the diurnal dynamics of water stress. The STEMMUS-SCOPE-PHS improves the simulation of diurnal dynamics of latent heat flux, net ecosystem exchange and gross primary production compared to STEMMUS-SCOPE with the value of Kling-Gupta efficiency (KGE) increasing from 0.74 to 0.83, 0.57 to 0.76, and 0.57 to 0.80, respectively. This research delineates the plant hydraulic responses to water stress and highlights the importance of leaf water potential in reflecting the plant water stress.
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RC1: 'Comment on egusphere-2024-2940', Anonymous Referee #1, 23 Oct 2024
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Dear authors,
Dear editor,Thank you for letting me referee this very interesting manuscript. After careful consieration, I must unfortunately recommend rejection of the manuscript.
Summary
This manuscript by Song and colleagues presents an extension of the ecohydrological model STEMMUS-SCOPE that incorporates an impoved representation of plant hydraulics into the model. Similar work has been presented in Simeone et al. (2018), where the authors incorporate a soil–plant–atmosphere continuum model into the ecohydrological model Ech2o to study water stress in pine trees (the editor can verify that I have no affiliation to the paper). The topic remains of interest to the ecohydrological community and the readership of HESS.
The authors show that including more detailed plant hydraulics into STEMMUS-SCOPE increases model accuracy with regard to several observation data collected at the intermediate scale. The manuscript is sufficiently well written, however, I had some difficulties following the methodology section, specifically Section 2.4, that I will comment on below. The figures that have been included into the manuscript are clear and relevant.
In my opinion, the manuscript could be enhanced by further improving the discussion section to address some remaining open questions. I will list my major concerns about this below, addressing some of these may require extensive work. Thus, I recommend rejecting the manuscript and inviting a re-submission once the issues below have been clarified.
Alternatively, a version of this manuscript that focuses more on the technical aspects of the model might be suitable for Geoscientific Model Development.
Major comments
[1] STEMMUS-SCOPE-PHS presentation could be improved.
[1.1] Section 2.4 starts with the discussion of calculating the stomatal conductance, which depends on the calculation of net carbon assimilation An. It remains unclear how An is calculated until 2.4.3, where it is revealed that Farquhar's approach is used. I think it would be better to mention this directly where An is introduced (Eq. 1, Sec. 2.4.1).
[1.2] It is unclear to me what variable the model is solving for.
[1.2.1] Starting from Sec. 2.4.1, I initially assumed that the model computes transpiration (T) as a function of the water potential gradients. It then surprised me that T is calculated on the basis of an energy balance in Eq. 6 as T = LE/λ? Doesn't make this the whole plant hydraulics redundant? Or are water potentials fitted to the transpiration through Eqs. 2–5?
On a side note: Eq. 6 gives the unit mol s-1 m-2, which is fine, but is inconsistent with Eq. 2, where fluxes are expressed in m/s. It seems a transformation coefficient is missing here.[1.2.2] The stomatal conductance gsfrom Eq. 1 does not appears neither in the following equations nor in the appendix. This might be related to my comment 1.2.1 above.
[2] PSY-1 measurements need to be clarified.
This relates to Sec. 2.2. From my perspective, the PSY-1 measurements are the only direct observations of plant hydraulics and all other measurement data such as GPP, SIF, and PAR are proxies to it. Therefore, it is very important to me to understand how many trees were sampled. From the text and Fig. 1, it reads as if only a single tree has been instrumented for two months? If so, this feels quite inadequate and there needs to be some justification for the choice of tree and the measurement period. Similarly, at how many locations were soil sensors installed? The uncertainty introduced to the results and model evaluation might be significant. This is also relevant to the poor match of model results and observations shown in Fig. 7. I know that it might be unfair to criticise the lack of data in this study, because collecting data is time intensive. Perhaps the model implementation needs to be verified in a more heavily monitored site before moving to the Hutoucun site?
[3] Discussion should be improved.
[3.1] Disagreement of plant water potential is quite severe.
Looking at Fig. 7, the disagreement between simulated and observed plant water potential is quite severe. In the discussion, the authors state that diurnal dynamics have been captured. I must disagree. Even if we neglect absolute values, the simulated plant water potential both in leaf and root is much more erratic than the observed one. Further, observed diurnal dynamics on DOYs 208, 211, and 212 clearly follow different dynamics than the simulated ones. This implies that the in silico plant is showcasing a much more anisohydric behaviour than the in situ one. While intermediate scale observations have been matched quite well, this implies that the plant hydraulics and its pathway may not have been captured properly by the model. This has implications on the conclusions drawn in this study.
The authors demonstrate that including plant hydraulics into the model improves these site scale observations. This might be related to the additional degrees of freedom that are introduced into the model that allow for a better fit. It is then not clear to me, whether the model improvement is actually for the right reason or simply because of the expanded parameter space, especially considering that the plant water potential is not matched correctly. Perhaps the plant hydraulics module could be better calibrated? Because the model description is not entirely clear to me, I cannot draw any further conclusions on this point.
[3.2] Data scarcity hinders further discussions of any plant responses-related issue
Unfortunately, the single data point that I commented on in Sec. 2 hinders any deeper discussion of any plant responses-related issue and any interpretation of model results. In Sec. 4.3, the authors suggest that plant water storage might be a reason for the poor agreement. But at this point, any other reasoning, including measurement error, might be equally valid.
[3.3] Main questions of the manuscript are left unanswered
The issue of how these plants respond to water stress is not discussed in depth in the manuscript. Instead, only model performance with regard to observations are compared. It would improve the manuscript if additional insights that have been gained through this modelling exercise were discussed.
However, the issue in comment 3.1 is quite large and may make the claimed objectives of the paper on page 3, namely b) investigating the performance of the plant hydraulic model at a karst ecosystem, and c) answering how plants respond to drought from a perspective of plant hydraulics, not possible in this case study. Perhaps b) could be answered by saying the plant hydraulics model performs poorly on karst ecosystems. But because the reasons for the poor performance cannot be inferred, I don't think this is a good answer. The authors do not address the objective c) in the conclusions. As I reasoned above in my comment 3.1, I do not think this question can be answered on the basis of the current results. But I'm happy to be proven wrong.
References
Simeone, C. et al. (2018), Coupled ecohydrology and plant hydraulics modeling predicts ponderosa pine seedling mortality and lower treeline in the US Northern Rocky Mountains, New Phytologist, 221:1814–1830. doi:10.1111/nph.15499
Citation: https://doi.org/10.5194/egusphere-2024-2940-RC1 -
RC2: 'Comment on egusphere-2024-2940', Anonymous Referee #2, 24 Oct 2024
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egusphere-2024-2940 “Investigating Plant Responses to Water Stress via Plant Hydraulics Pathway” Z.Song, Y.Zeng, Y.Wang, E.Tang, D.Yu, F.Alidoost, M.Ma, X.Han, X.Tang, Z.Zhu, Y.Xiao, D.Kong, Z.Su
The article presents a very thorough investigation of an addition to the STEMMUS-SCOPE model to include the effects of plant water stress on the simulation of sub-daily soil-plant-atmosphere dynamics and transfers of mass and energy. The work needs a little reorganisation but is generally easy to read. There are several important questions to answer however that will require moderate revision to address.
Some of the details in the Supplementary material need to be included in the main text. For example, the fact that you ran the simulation for nearly one-year but show the results only for two particular weeks. What period do you estimate the fitting statistics over; I am assuming it is only the one week shown in each set of figures. How many soil layers were used (7) and the total depth of the soil column (1m), along with the range of time steps (1 second to 30 minutes as outlined in Zeng et al ?) should be explicit. This information can fit in a single paragraph at the end of §2.3 and allow the reader to better understand the scales involved.
The area shown in Figure 1 indicates the flux tower is located on the northern edge of the osmanthus plantation. How does this influence the measurements given the prevailing wind direction, for example, and what is the footprint of the tower and expected fetch given the height of vegetation and the flux tower? Given the small size of the plantation, is there any limitation to using remotely-sensed data at 500m and 1000m resolutions? How many trees were instrumented to represent these data?
The equation for SMWSF embedded in line 141 on page 5 needs to be expanded on its own line for clarity. It is not obvious what parts are in the exponential term, and I am guessing that SM(i) is the actual soil moisture in the i-th soil layer which is listed as qi in the text.
I am confused about estimation of the transpiration flux. It is indicated in Equation (6) that it is simply energy dependent, which may be good enough in well-watered and energy-limited systems but not in general, then on the next page we are told transpiration, stomatal conductance and carbon assimilation are coupled with the Farquhar et al method. Does Equation (6) represent a potential transpiration which is then modified? How does this affect the instantly equilibrated assumption in Equation (2) and which comes first?
Of the two formulae for plant water stress factor in Equations (8) and (9), which was used in the calculations shown in the subsequent figures and analyses?
It is not explicit in the text or figure caption, but can we assume that the data are half-hourly values in Figures 5, 6 and 7?
In Figure 8 the statistics listed are identical between well-watered and water-limited conditions, so if the statistics are being estimated only on the window shown I find that unusual. If the stats should be different please change one set, but if they are estimated over the entire simulation period then one set can be removed.
I agree with your conclusion that the assumption of constant internal water storage, as embodied in Equation (2), masks inertia in the system, internal redistribution, and limits the responsiveness of the formulation. It probably also limits any useful insights from the work.
Citation: https://doi.org/10.5194/egusphere-2024-2940-RC2 -
RC3: 'Comment on egusphere-2024-2940', Anonymous Referee #3, 06 Nov 2024
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Review Song et al. 2024
Major comments
This is a scientifically largely solid paper, and it is good to see the hydraulic continuum implemented into Stemmus-Scope. I would have liked to have seen the new model tested for more sites, e.g. for the PLUMBER dataset. Instead, the authors decided to use quite an unusual site, with shallow soil and ‘foggy’ atmosphere. Is this the best approach to test the improved model? Does this in fact explain some of your findings?
The estimates of the LE and H fluxes are barely improved by the PHS, yet the GPP is. This seems strange? There is a disconnect here. This needs to be discussed and explained.
Around Fig 5 and 6 it is not actually clear why Fig. 5 is called well-watered and Fig. 6 is water-stressed if we look at the values of the PHS water stress factor. These are fairly similar? Why is that? And the soil moisture based one never goes below 0.8 or so, so are these actually water stressed conditions?
Why are you not showing the simulations of soil moisture contents and soil temperatures? And even more importantly the land surface temperatures. Please add this and discuss?
Section 4.1 represents a somewhat confused narrative. The procedure to correct (not calibrate!) the 5cm soil heat flux should be described in the methods. Then you can discuss here the uncertainties related to this correction. Also, I cannot fully follow the discussion on surface temperature, and this is clearly a crucial state variable that could be incorrect in Stemmus-Scope, yet we are not being shown how it behaves. Moreover, I thought that Stemmus-Scope had a separate soil and canopy temperature?
Section 4.2: I know that the authors know that SIF is also affected by plant water stress via leaf potential. Why is this not mentioned here, or even better, implemented in the model?
In the Introduction quite a bit is made of this vegetation growing on a shallow soil, and about its foggy atmospheric conditions, yet you do not come back to this in the discussion. How does this affect things like the total soil moisture reservoir, capillary rise etc.
The English needs considerable improvement. I had a go at some things (see below), but a thorough check of your text is required.
Minor comments and technical comments
Line 34: Replace ‘stomata opening’ by ‘extent of stomatal opening’?
Line 36: Say: ‘as a result of limited data availability’
Line 40: Say: ‘the simulation and prediction of water stress’
Line 43: Does STEMMUS-SCOPE not also do the soil water balance? And say ‘the energy balance’
Line 46: Are you saying that for 2 out of the 172 sites it was overestimating the BR??
Line 46/47. It is not clear exactly what is mean by ‘indicating more energy was allocated to latent heat flux for transpiration’? Do you mean ‘according to the model calculations, compared to the measurements’?
Line 48: ...’for the same NEE’. The same as what? I think there is way to explain better what you are trying to say here. For example by breaking up the sentence.
Line 53: Perhaps say: ‘atmospheric’ instead?
Line 58: ‘The distance over which...’
Line 73: What is meant by the ‘big-tree conception’?
Line 74: water storage in the soil, plant or both?
Line 75: Is the word ‘trees’ missing after ‘an-isohydric (red-oak)’?
Line 79-80: I have trouble following this sentence.
Line 82: Replace ‘knowledge’ by ‘understanding’?
Line 97: What does it mean that ‘..the weather is moderate and rainy’. Please use a more detailed scientific description!
Line 99: This high relative humidity and fogginess makes for a reduced atmospheric demand, which combined with the plant water stress make for a unique feature for this site. Should more be made of this?
Line 100: What is the texture of this ‘thin soil layer’?
Line 104: Can you also give the English name of the vegetation?
Line 110: The data weren’t just averaged by this software but also corrected? List the type of corrections? Or refer to paper?
Line 113: Dd you mean to say that the SIF system was installed? Not ‘conducted’?
Line 115: The bit about the soil texture needs to go into the previous section
Line 115-117: Give the Manufacturers and versions of the probes? And their operating principle (e.g. Capacitance probes). Also, if this is a plantation, were there rows of trees? Were the sensors within or between rows?
Line 117: At what height in the canopy were the Plant Stem Psychrometers installed. And why was it installed so close to the edge of the field (see Figure 1)? Would that not have affected its readings?
Line 118: Say ‘obtained’ not ‘collected’
Line 128: Best to remove the word ‘elaborate’ because this tells us nothing about model quality.
Line 129: Is there ‘momentum transport’ in soil?
Line 133:: Remove ‘is considered’
Line 134/135: ‘multiple layers of spectra information’ sounds really vague. Explain more. Also, if you have all this spectral modelling, then say what other satellite products it could be compared to?
Line 138: What is ‘It’ referring to? And why/how does this ‘peristomatal water flux’ matter? This seems like a random remark.
Line 141: Where does the equation for SMWSF come from?
Line 148: ‘plant hydraulics (PHS) process’. Do you mean system?
Line 154: say ‘atmospheric vapour pressure deficit’
Line 161: Add ‘which equals transpiration, Trans.’ After ‘by roots to water released through stomata (Fig. 2).’ Then remove its explanation a few lines later.
Line 167: write ‘plant tissues and soil are assumed to be porous media’
Line 169: Darcy’s law uses conductivities, not conductances. Is this an issue?
Line 179: Eq. 6 is not an energy balance model, and how is Psi_air shown in Fig. 2 used to calculate transpiration?
Figure 2: Define Psi_air and explain how it is calculated/used. I cannot see it mentioned in the methodology?
Line 186-187: I would say: ‘multiplying the maximum carboxylation rate under a well-watered condition by the water stress factor’. Also you need to make it clear that this is now a different factor from the one presented earlier that was based on SMC. Properly introduce phwsf in the text
Line 190-197. You need to make it clear here that these equations were taken from the ED2 and CLM model, otherwise the subscripts don’t make sense.
Line 219 : why are you referring to these indices as ‘statistic proxies’?
Line 247: What is meant with ‘clearly diurnal dynamics’? Do you mean ‘clearly improved diurnal dynamics’?
Line 249: This should be ‘well-watered condition’.
Line 254: Also how does higher water consumption result in greater water stress? That also depends on the root-zone soil water store and hydraulic conductivity?
Line 254/255: The water stress is also relieved by replenishment of the root zone, including by capillary rise possibly. This should be mentioned.
Line 275: say ‘until they reach equilibrium’
Line 307: The equations relating to SIF should have been introduced in the Methods section.
Line 335: say ‘..for water-limited conditions’
Line 340/341: say ‘Neglecting the plant water storage’.
Line 342: There are various models that have already implemented this. Please refer to some of them?
Citation: https://doi.org/10.5194/egusphere-2024-2940-RC3
Model code and software
STEMMUS-SCOPE-PHS Zengjing Song, Yijian Zeng, and Bob Su https://github.com/Crystal-szj/STEMMUS_SCOPE_ZSo_t1
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