Vegetation in Arid Areas of the Loess Plateau Showed More Sensitivity of Water-Use Efficiency to Seasonal Drought
Round 1
Reviewer 1 Report
The manuscript presented here is a nice work that contributes to the understanding of the negative impacts of climate change on different vegetation types. The authors analyzed the effects of seasonal droughts on vegetation WUE through time and space in the Loess Plateau in China. Forecasted severe and more frequent drought events will limit the development of terrestrial ecosystems by seriously affecting vegetation growth and WUE.
They used the SPEI index to study the influence of drought in terrestrial ecosystems to carry out appropriate ecosystem management. They evaluated the severity of droughts and the sensitivity of WUE on a regional scale. They found divergent trends throughout the year. Spring drought increased WUE, whereas summer drought led to a decrease in WUE. Also, the sensitivity of WUE varied from arid to humid areas. The authors also give some future directions. The manuscript needs English to be revised, in some places, it is hard to follow ideas. The methods and statistical analysis employed are correct, and the references are appropriate and up to date. I found some mistakes throughout the text, and therefore I suggest some corrections that should be performed to improve the presentation of the manuscript.
Page 1, lines 19-20: the same increasing trend in humidity is described for fall and the other three seasons.
Page 1, line 40: replace the word “effect” with “affect”.
Page 1, line 45: replace the comma with a point.
Page 2, lines 46-51: this sentence is hard to follow and it lacks clarity.
Page 2, line 93: remove the word “be”.
Page 3, line101: replace analyzed with analyze.
Page 3, line 103: This study …
Page 3, line 104: ecosystemS
Page 3, line 111: is it the largest Loess Plateau? Are there more than one Loess Plateaus?
Page 3, line 129: do not start the sentence with the number 448.
Page 4, line 139: a space is missing between “to 2019”.
Page 4, line 146: Calculation OF the standardized …
Page 4, line 151: replace the comma with a point.
Page 4, lines 166-169: revise this sentence, it is had to understand.
Page 5, line 172: replace “an” with “a”.
Page 5, line 174: remove the extra “It is”
Page 5, line 198: The
Page 5, line 199: replace “significants” with “significant”.
Page 5, lines 205-207: the described increased trend in SPEI is not so evident in figure 2, it seems that SPEI fluctuates through the years and the final value is not higher that the previous ones. Also, the trend described for fall is not that clear.
Page 5, line 210: replace “were” with “was”.
Page 5, line 217: The SPEI (capital letters missing).
Page 5, line 218: it is stated that the SPEI value increased, although very slowly. Is that “trend” significant? Whit such a low value of change we could not say that it increased.
Page 6, line 231: the sentence “the more precipitation, the more severe the spring drought” is confusing. Did you mean that in more humid areas spring droughts are more severe?
Page 6, lines 236-238: this sentence is also confusing, as it is written it says that in the summer there is no drought.
Page 7, line 258: replace “the most regions” with “most of the regions”.
Page 7, figure 3: the legend bar is confusing, maybe inverting the colours could help, such as SPEI values >1 are in blue (no drought), and SPEI values <-2 are in red (severe drought). Adding the name of the regions as a reference (as in fig 1) could also help those who are not familiar with the geography of China, and makes it easier to follow the presented ideas.
The figure legend says “from years 2001 to 2019”, but the graph shows different seasons and not different years, is the average of those years represented in this figure?
Page 8, line 273: replace “is” with “was”.
Page 8, line 280: capital letter missing in “the”.
Page 9, lines 294-295: something is missing in this sentence.
Page 9, line 297: the word “occurred” is unnecessary.
Page 9, line 302, 309, and 317: replace “were” with “was”.
Page 9, line 309 and 320: replace “showed” with “shown”.
Page 9, line 318: WAS distributed.
Page 9, line 321: did not change.
Page 9, line 321: replace “litter” with “little”
Page 11, line 330: climate regions
Page 11, line 331: revise the sentence “in a humid areas” …
Page 11, lines 340-343: revise this sentence, it is hard to follow.
Page 11, line 371: a dot is missing between regions and On.
Page 11, line 379: capital letter missing.
Page 12, lines 389-392: revise this sentence.
Page 12, line 402: revise punctuations.
Page 13, line 415: capital letter missing at the beginning of the sentence.
Page 13, line 417: replace the comma with a point.
Page 13, line 418: “stop more”
Page 13, line 424: remove the word with
Page 13, lines 424-427: revise punctuation.
Page 13, line 427: replace litter with lesser.
Page 13, line 430: continues.
Page 13, lines 431-439: revise these sentences.
Page 13, line 444: revise the grammar.
Page 13, lines 456-458: revise this sentence.
Page 13, line 460: (which is a physical process).
Page 13, line 464: persistent.
Page 13, line 466: adapt to.
Page 14, line 471: replace “storage” with “shortage”
Page 14, line 472, 479: replace 2respond” with “response”.
Page 14, lines 474-476: revise this sentence.
Page 14, line 80: replace “with” with “while”.
Page 14, lines 483-485: this sentence is not sound.
Page 14, line 489: correct “. WUE,.”
Page 15, lines 525-530: revise this sentence, it is hard to follow.
Page 16: Tables in the appendix are not cited in the text.
Author Response
Point-to-point response to reviewers’ comments
Firstly, thank you very much for spending your time reviewing my manuscript. Secondly, thanks for your constructive comments on my manuscript, which played a great role in the revision of the manuscript. I will reply to your comments point to point.
Major concerns
lines 19-20: the same increasing trend in humidity is described for fall and the other three seasons.
Response: Thank you for your constructive comments. We have corrected the statements in revision. It should be that " the humidity in spring and summer on the Loess Plateau shows an increasing trend, while the aridity in fall shows an increasing trend. "
line 40: replace the word “effect” with “affect”.
Response: Thank you for your constructive comments. We have modified it following your suggestion, it should be that " affect the water supply conditions"
line 45: replace the comma with a point.
Response: Done.
lines 46-51: this sentence is hard to follow and it lacks clarity.
Response: Thank you for your constructive comments. We have modified this sentence following your suggestions, as follows:
"For example, the standardized precipitation evapotranspiration index (SPEI) is obtained through three steps, first, calculates evapotranspiration using meteorological data, then normalizes the cumulative probability of the difference sequence between precipitation and evapotranspiration, and finally reveals drought characteristics within the region by using the deviation degree between the difference between precipitation and evapotranspiration and its average state [1-2]."
lines 93: remove the word “be”.
Response: Done.
line101: replace analyzed with analyze.
Response: Done.
line 103: This study …
Response: Done.
line 104: ecosystems
Response: Done.
line 111: is it the largest Loess Plateau? Are there more than one Loess Plateaus?
Response: Thank you for your constructive comments. We modified the expression in revision. It should be that " It is the plateau with the largest loess coverage around the world and includes… "
line 129: do not start the sentence with the number 448.
Response: Thank you for your constructive comments. Following your suggestions, this sentence has been revised to: "This study used 448 climate grid points data of the Loess Plateau and its surrounding areas to calculate…"
line 139: a space is missing between “to 2019”.
Response: Done.
line 146: Calculation OF the standardized …
Response: Thank you for your constructive comments. We have modified it following your suggestion, " Calculation the standardized precipitation evapotranspiration index " has been revised as " Calculation of the standardized precipitation evapotranspiration index"
line 151: replace the comma with a point.
Response: Done.
lines 166-169: revise this sentence, it is had to understand.
Response: Thank you for your constructive comments. We have modified this sentence following your suggestions, as follows:
" To analyze the drought characteristics of the Loess Plateau, we divided five drought grades using China's meteorological drought grade standard, geographical characteristics of the Loess Plateau, and related research [3-4] (Table 1). The smaller the SPEI value, the more severe the drought. "
line 172: replace “an” with “a”.
Response: Done.
line 174: remove the extra “It is”
Response: Done.
line 198: The
Response: Done.
line 199: replace “significants” with “significant”.
Response: Done.
lines 205-207: the described increased trend in SPEI is not so evident in figure 2, it seems that SPEI fluctuates through the years and the final value is not higher that the previous ones. Also, the trend described for fall is not that clear.
Response: Thank you for your constructive comments. Indeed, the increased trend of SPEI is not so evident, we estimate the changing trend of SPEI according to the linear change slope fitted by the linear regression method, which is the overall increased or decreased trend of SPEI from 2000 to 2019. We modified the statements in revision, and add R2 (Square of correlation coefficient) in revision.
line 210: replace “were” with “was”.
Response: Done.
line 217: The SPEI (capital letters missing).
Response: Done.
line 218: it is stated that the SPEI value increased, although very slowly. Is that “trend” significant? Whit such a low value of change we could not say that it increased.
Response: Thank you for your constructive comments. We have modified the statement following your suggestion in revision, as follows: " The fluctuated SPEI was found in winter and all SPEI values from 2001 to 2019 were more than zero, which means there is no drought in winter during the study period."
line 231: the sentence “the more precipitation, the more severe the spring drought” is confusing. Did you mean that in more humid areas spring droughts are more severe?
Response: Thank you for your constructive comments.
Yes, you are right. We have modified it according to your suggestions, as following, " In spring, SPEI increases from southeast to northwest with the decrease of precipitation gradient, indicating that the spring drought in the southeast with more precipitation is more severe."
lines 236-238: this sentence is also confusing, as it is written it says that in the summer there is no drought.
Response: Thank you for your constructive comments.
We have modified it according to your suggestions, as following, " In the summer, the drought grade is no drought (2.5%) mainly distributed in Qinghai and southern Shaanxi, and most regions experienced mild drought (39.8%) and moderate drought (57.2%)."
line 258: replace “the most regions” with “most of the regions”.
Response: Done.
figure 3: the legend bar is confusing, maybe inverting the colours could help, such as SPEI values >1 are in blue (no drought), and SPEI values <-2 are in red (severe drought). Adding the name of the regions as a reference (as in fig 1) could also help those who are not familiar with the geography of China, and makes it easier to follow the presented ideas. The figure legend says “from years 2001 to 2019”, but the graph shows different seasons and not different years, is the average of those years represented in this figure?
Response: Thank you for your constructive comments.
(1) We have modified Figure 3 according to your suggestion, as follows:
Figure 3. Spatial distribution of seasonal SPEI average and drought magnitudes in the Loess Plateau from 2001 to 2019.
(2) Seasonal drought: Yes, you are right. In this paper, the spatial distribution of drought in the four seasons is the average of SPEI index of each season from 2001 to 2019 based on the grid. Therefore, we have modified the title of Figure 3 as: "Spatial distribution of seasonal SPEI average and drought magnitudes in the Loess Plateau from 2001 to 2019"
line 273: replace “is” with “was”.
Response: Done.
line 280: capital letter missing in “the”.
Response: Done.
lines 294-295: something is missing in this sentence.
Response: Thank you for your constructive comments.
According to your suggestion, we have supplemented the area proportion of sensitivity coefficient of WUE to seasonal SPEI that has passed the significance test in Fig.S1, as follows:
Figure S1. The area proportion of the sensitivity coefficient (WUE to seasonal SPEI) passing the significance test.
line 297: the word “occurred” is unnecessary.
Response: Done.
line 302, 309, and 317: replace “were” with “was”.
Response: Done.
line 309 and 320: replace “showed” with “shown”.
Response: Done.
line 318: WAS distributed.
Response: Done.
line 321: did not change.
Response: Thank you for your constructive comments. We have modified it following your suggestions, "didn’t changed…" has been revised as "did not change…"
line 321: replace “litter” with “little”
Response: Done.
line 330: climate regions
Response: Thank you for your constructive comments. We have modified it following your suggestions, " climates regions of …" has been revised as " climate regions of…"
line 331: revise the sentence “in a humid areas” …
Response: Thank you for your constructive comments.
We have modified the statement following your suggestion, as following, " the seasonal γ in humid, semi-humid, semi-arid, and arid areas were extracted, the results showed a clear seasonal divergence in four climate regions along with the increased magnitude of drought"
γ：the sensitivity coefficient of WUE to drought
lines 340-343: revise this sentence, it is hard to follow.
Response: Thank you for your constructive comments.
We have modified this sentence following your suggestion, as following, "The sensitivity coefficients of WUE to mild drought in arid, semi-arid, and semi-humid areas were −0.011, −0.039, and −0.023, respectively, whereas the coefficient was positive (γ = 0.017) in the humid area."
line 371: a dot is missing between regions and On.
Response: Done.
line 379: capital letter missing.
Response: Done.
lines 389-392: revise this sentence.
Response: Thank you for your constructive comments.
We have modified this sentence following your suggestion, as following, "Severe drought was in southeast–southwest and mild drought was in the northwest in spring and winter," whereas summer and autumn droughts had the opposite spatial distribution."
line 402: revise punctuations.
Response: Done.
line 415: capital letter missing at the beginning of the sentence.
Response: Done.
line 417: replace the comma with a point.
Response: Done.
line 418: “stop more”
Response: Done.
line 424: remove the word with
Response: Done.
lines 424-427: revise punctuation.
Response: Done.
line 427: replace litter with lesser.
Response: Done
line 430: continues.
Response: Done
lines 431-439: revise these sentences.
Response: Thank you for your constructive comments. We have modified this sentence following your suggestions, as follows: "Secondly, higher sensitivity of WUE to SPEI was found in grassland, which is because the grass is more sensitive to environmental and climate change than deep-rooted vegetation [5-6]. In other words, shallow-rooted herbs rely on precipitation to supplement soil water, whereas deep-rooted trees or shrubs primarily use deep soil water or groundwater [7-9]. Lastly, we found that the mild drought caused lower sensitivity of WUE to SPEI and moderate drought caused higher sensitivity of WUE to SPEI, implying that drought tolerance of vegetation may have a threshold effect, which is consistent with previous study [10-11]. "
line 444: revise the grammar.
Response: Thank you for your constructive comments. We have modified this sentence following your suggestions, as follows: "Drought can reduce soil water content as well as GPP and ET [12-13], which can explain why summer drought causes a decrease in WUE, whereas spring drought can cause an increase in WUE to some extent. "
lines 456-458: revise this sentence.
Response: Thank you for your constructive comments.
We have modified this sentence following your suggestion, as following, " On the other hand, because low vegetation coverage leads to more soil evaporation in arid regions, the higher sensitivity of ET to changed hydro-climatic conditions [14]. In contrast, in semi-arid and semi-humid regions, ecosystem functions and activities depend largely on water availability [6]. "
line 460: (which is a physical process).
Response: Thank you for your constructive comments. We have modified it following your suggestions, "which is physical a process " has been revised as " which is a physical process "
line 464: persistent.
Response: Done.
line 466: adapt to.
Response: Done.
line 471: replace “storage” with “shortage”
Response: Done.
line 472, 479: replace 2respond” with “response”.
Response: Done.
lines 474-476: revise this sentence.
Response: Thank you for your constructive comments.
We have modified this sentence following your suggestion, as following, "Some studies have also found a complicated relationship between vegetation activity and drought in humid regions with surplus water. The response of plants in humid regions to drought is affected by phenology aspects such as vapor pressure and the period of active leaf flushing [15]. Moreover, the tissue structure of plants in humid regions is very likely damaged by drought [16]. However, once dry spell is over, vegetation in humid regions can rapidly recover to its previous state [17]. "
line 480: replace “with” with “while”.
Response: Done.
lines 483-485: this sentence is not sound.
Response: Thank you for your constructive comments.
We have modified this sentence following your suggestion, as following, "Similar results were found in previous studies [12,17-18], such as Xu et al. [12], who found that the response of WUE to drought stress differed in arid, semi-arid, semi-humid, and humid regions. Furthermore, some researchers found that… "
line 489: correct “. WUE,.”
Response: Done.
lines 525-530: revise this sentence, it is hard to follow.
Response: Thank you for your constructive comments. The conclusions in the former manuscript had a little repeat with the obtained results. Therefore, we have revised the conclusion according to your suggestions, as follows: "In the current study, the impact of seasonal drought on WUE was analyzed using ridge regression, which appeared at the temporal and spatial distribution characteristics of seasonal drought and water use efficiency in the Loess Plateau from 2001 to 2019. The results show that from 2001 to 2019, there was significant spatial heterogeneity in seasonal drought and WUE, with drought levels generally being mild, and WUE values being highest in the summer. Seasonal drought had different effects on WUE, and the sensitivity coefficient also differed significantly as the SPEI changed. On the other hand, the sensitivity of seasonal WUE to drought magnitude was affected by regional dry and wet conditions. The impact of drought intensity on WUE in the four climate regions showed a clear seasonal divergence, and the sensitivity of WUE to drought magnitude in arid areas was generally higher than that in semi-arid, semi-humid, or humid areas. However, there are still many deficiencies in this study. Future research should pay more attention to the response processes of GPP and ET to drought, consider the impact of drought accumulation impact and lag effect on WUE. Our findings are crucial for understanding the impact of climate change on the Loess Plateau’s ecosystem carbon and water cycles. "
Page 16: Tables in the appendix are not cited in the text.
Response: Thank you for your constructive comments. The tables in the appendix have been cited in the appropriate places in the text according to your suggestions.
References:
- Yanyun Guo, Qi Hu, Weidong Fu, et al. Drought Trend over the Grasslands in the Tianshan Mountains, Xinjiang in Recent 55 Years Based on SPEI [J]. Arid Zone Research, 2019(3):670-676. https://doi.org /10.13866/j.azr.2019.03.18.
- Weiguang Li, Xue Yi, Meiting Hou, et al. Standardized precipitation evapotranspiration index shows drought trends in China [J]. Chinese Journal of Eco-Agriculture, 2012, 20(5):643-649.
- Liu ZP, Wang YQ, Shao MG, et al. Spatiotemporal analysis of multiscalar drought characteristics across the Loess Plateau of China. Journal of Hydrology, 2016,534. https://doi.org /10.1016/j.jhydrol.2016.01.003.
- Zhao A, Zhang A, Liu J, et al. Assessing the effects of drought and "Grain for Green" Program on vegetation dynamics in China's Loess Plateau from 2000 to 2014[J]. Catena, 2019. https://doi.org /10.1016/J.CATENA.2019.01.013.
- Yang Yuting, Guan Huade, Batelaan Okke, et al. Contrasting responses of water use efficiency to drought across global terrestrial ecosystems[J]. Scientific Reports, 2016, 6: 1－ https://doi.org /10.1038/srep23284.
- Ponce Campos Guillermo E, Moran M Susan, Huete Alfredo, Zhang Yongguang, Bresloff Cynthia, Huxman Travis E, Eamus Derek, Bosch David D, Buda Anthony R, Gunter Stacey A, Scalley Tamara Heartsill, Kitchen Stanley G, McClaran Mitchel P, McNab W Henry, Montoya Diane S, Morgan Jack A, Peters Debra P C, Sadler E John, Seyfried Mark S, Starks Patrick J. Ecosystem resilience despite large-scale altered hydroclimatic conditions. Nature,2013,494(7437). https://doi.org /10.1038/nature11836.
- Dodd M, Lauenroth W, Welker J. 1998. Differential water resource use by herbaceous and woody plant life-forms in a shortgrass steppe community. Oecologia, 117: 504-512. https://10.1007/s004420050686
- Ward D, Wiegand K, Getzin S. 2013. Walter’s two-layer hypothesis revisited: back to the roots! Oecologia, 172: 617-630. https://10.1007/s00442-012-2538-y.
- Jackson PC., Meinzer FC., Bustamante M., et al. 1999. Partitioning of soil water among tree species in a Brazilian Cerrado ecosystem. Tree Physiology, 19: 717-724.
- Yijie Sun, Xianfeng Liu, Zhiyuan Ren, et al. Spatiotemporal variations of multi-scale drought and its influencing factors across the Loess Plateau from 1960 to 2016 [J]. Geographical Research, 2019.
- Tingting Pei, Xiaoyan Li, Huawu Wu, et al. Sensitivity of vegetation water use efficiency to climate and vegetation index in Loess Plateau, China [J]. Transactions of the Chinese Society of Agricultural Engineering, 2019, 35(05):127-133+327.
- Hao-Jie Xu, Xin-ping Wang, Chuan-yan Zhao, et al. Responses of ecosystem water-use efficiency to meteorological drought under different biomes and drought magnitudes in northern China[J]. Agricultural and Forest Meteorology,2019. https://doi.org /10.1016/j.agrformet.2019.107660.
- Yu Z, Wang J, Liu S, et al. Global gross primary productivity and water use efficiency changes under drought stress[J]. Environmental Research Letters, 2017, 12(1):014016. https://doi.org /10.1088/1748-9326/AA5258.
- Zhang, Y. Q. et al. Multi-decadal trends in global terrestrial evapotranspiration and its components. Sci. Rep. 5, 19124. https://doi.org /10.1038/srep19124.
- Anderegg WRL, et al. (2012) The roles of hydraulic and carbon stress in a widespread climate-induced forest die-off. Proc Natl Acad Sci USA 109(1):233–237.
- Li, X., Li, Y., Chen, A., Gao, M., Slette, I.J., Piao, S., 2019a. The impact of the 2009/2010 drought on vegetation growth and terrestrial carbon balance in Southwest China. Agric. For. Meteorol., 239–248. https://doi.org/10.1016/j.agrformet.2019.01.036.
- Vicente-Serrano, S.M., Gouveia, C., Camarero, J.J., Beguería, S., Trigo, R., López-Moreno, J.I.,et , 2013. Response of vegetation to drought time-scales across global land biomes. Proc. Natl. Acad. Sci. 110, 52–57. https://doi.org /10.1073/pnas.1207068110.
- Liu Yibo, Xiao Jingfeng, Ju Weimin, Zhou Yanlian, Wang Shaoqiang, Wu Xiaocui. Water use efficiency of China's terrestrial ecosystems and responses to drought[J]. Scientific reports,2015,5. https://doi.org /10.1038/srep13799.
Author Response File: 
Author Response.docx
Reviewer 2 Report
The presented study of Pei et al. explores temporal and spatial changes in WUE of forests, grasslands and shrub communities within Loess Plateau in China. Moreover, the authors explored the impact of drought seasonality on the WUE by utilization of standardized precipitation evapotranspiration index and remote sensing data. I find the study as a great fit for Forests journal. The large scale analysis of ecosystem wide WUE response to drought significantly broadens our knowledge needed to tackle global aridization due to global climate change. The paper is well written with proper data analysis. Nevertheless, I would suggest to the authors to send the paper for language corrections as there were numerous grammatical and composition mistakes (random capital letters within sentences, no capital letters at the start of sentence, mixed tenses, random punctuations within the sentence etc.). The main concern I have is, if the units of WUE in this study are correct. It seems that WUE derived from GPP and evapotranspiration are generally ranging from 0-4 g per kg or mm (Dore et al. 2012, Guerrieri et al. 2016). Here in the presented study the WUE is ranging from 0-5 mg per kg/mm during vegetation season. Was there maybe some mistake in unit transformation within the calculation process? Please double-check that. Here are my further comments:
18: Autumn shows increasing aridity trend, not humidity.
117: I am missing information about the vegetation composition and/or distribution or density. Would it be possible to add a map which would show the distribution of different vegetation types in the Plateau? I see that you have the information as you calculated WUE per vegetation type. Moreover, as the paper is submitted into Forests, could you list the main tree species occurring in the Plateau?
Figure 3: Negative values of SPEI represent dry conditions and then in the figure they are visualized with blue and green colour? Seems confusing. I suggest to flip the colour gradient.
435-438: Please divide this large sentence into multiple ones, it is really hard to read. If I get this right, the mild drought caused lower sensitivity of WUE to SPEI and moderate high sensitivity of WUE to SPEI. The starting part of sentence: “the main magnitude of drought were no drought and mild drought” makes no sense to me.
484: This sentence is not finished, they found what?
515: I would suggest to avoid repeating of the obtained results and focus more on big picture and implications of the study. How the findings correspond to global climate change scenarios for the area? Synthesize your outputs into the most interesting facts. Arid areas suffered the most from the drought exposure, right? Therefore, their vitality could be exhausted and lead to their disintegration and maybe transformation into desert? What are the implications for the future studies? I see that you are mentioning the lag effect at the end of discussion, maybe I would move it into the conclusion.
I propose a minor revision of the paper and hope authors will successfully improve their manuscript.
Dore, S., Montes-Helu, M., Hart, S.C., Hungate, B.A., Koch, G.W., Moon, J.B., Finkral, A.J. and Kolb, T.E. (2012), Recovery of ponderosa pine ecosystem carbon and water fluxes from thinning and stand-replacing fire. Glob Change Biol, 18: 3171-3185. https://doi.org/10.1111/j.1365-2486.2012.02775.x
Guerrieri, R., Lepine, L., Asbjornsen, H., Xiao, J., and Ollinger, S. V. (2016), Evapotranspiration and water use efficiency in relation to climate and canopy nitrogen in U.S. forests, J. Geophys. Res. Biogeosci., 121, 2610– 2629, doi:10.1002/2016JG003415.
Author Response
Point-to-point response to reviewers’ comments
Firstly, thank you very much for spending your time reviewing my manuscript. Secondly, thanks for your constructive comments on my manuscript, which played a great role in the revision of the manuscript. I will reply to your comments point to point.
Minor concerns
Comments and Suggestions for Authors: I would suggest to the authors to send the paper for language corrections as there were numerous grammatical and composition mistakes (random capital letters within sentences, no capital letters at the start of sentence, mixed tenses, random punctuations within the sentence etc.). The main concern I have is, if the units of WUE in this study are correct. It seems that WUE derived from GPP and evapotranspiration are generally ranging from 0-4 g per kg or mm (Dore et al. 2012, Guerrieri et al. 2016). Here in the presented study the WUE is ranging from 0-5 mg per kg/mm during vegetation season. Was there may be some mistake in unit transformation within the calculation process? Please double-check that.
Response: Thank you for your constructive comments.
(1) We have carefully checked the grammar, capital, punctuation and content of the manuscript according to your suggestions, and have revised and improved it. And some awkward sentences have modified.
In addition, we are very sorry for the grammar and punctuation of the manuscript. We get help from International Science Editing (http://www.internationalscienceediting.) for editing this manuscript in revision.
(2) According to your suggestion, we carefully checked the units of WUE and found that the unit was correct. The specific explanations are as follows:
In this paper, the MODIS GPP (MOD17A2H) and MODIS ET (MOD16A2) products were derived from remote sensing data and came from the United States Land Processes Distributed Active Archive Center (https://lpdaacsvc.cr.usgs.gov/appeears/task/area). The spatial resolution is 500 m, and the time resolution is 8 days. According to the User’s guide GPP and ET products NASA MODIS land algorithm (https://www.researchgate.net/publication/; http://www.ntsg.umt.edu/project/modis/mod16.php), multiply each pixel of GPP and ET by a scale factor of 0.0001 and 0.1 to get the real GPP and ET values, and then used the maximum value synthesis method in ArcGIS to obtain the seasonal scale GPP and ET, with units of kg C m-2 and mm, respectively. The WUE (kg C mm-1 m-2) of each season was calculated using the
following formula. The unit was converted to mg C mm−1·m−2 based on the WUE values obtained.
There were several reasons why the WUE range of Dore and Guerrieri et al [1-2] differed.1) different data sources. Dore and Guerrieri measured ecosystem WUE using GPP and ET of flux stations, whereas this paper used remote sensing data to calculate WUE. 2) The time span was also different. this study covered a long period of time, from 2001 to 2019, whereas Dore and Guerrieri only showed data from the measurement data of 4-5 years. 3) The global WUE showed significant spatial heterogeneity, influenced by hydrological and climatic conditions as well as other factors, with forest ecosystems having higher WUEs than non-forest ecosystems [3]. WUE differences are also caused by differences in leaf and ecosystem scales. The study area has a diverse range of vegetation types, primarily grassland, whereas Dore and Guerrieri focus their research on forests. WUE was generally higher in forests in the southeast of the Loess Plateau, according to Liu et al. and Li et al. [4-5]. Furthermore, according to the findings of Zheng et al and Hou et al, the Loess Plateau's average annual WUE was 1.26± 0.28g C kg-1 H2O, and the average summer WUE was greater than 0.002 g m-2 mm-1[6-7]. It is consistent with the research results of this paper.
18: Autumn shows increasing aridity trend, not humidity.
Response: Thank you for your constructive comments. We have corrected the statements in revision. It should be that " the humidity in spring and summer on the Loess Plateau shows an increasing trend, while the aridity in fall shows an increasing trend. "
117: I am missing information about the vegetation composition and/or distribution or density. Would it be possible to add a map which would show the distribution of different vegetation types in the Plateau? I see that you have the information as you calculated WUE per vegetation type. Moreover, as the paper is submitted into Forests, could you list the main tree species occurring in the Plateau?
Response: Thank you for your constructive comments.
(1) We have supplemented the distribution of vegetation types on the Loess Plateau in Fig.1 following your suggestions.
Fig.1. Geographic location and vegetation types map of the Loess Plateau.
(2) The main tree species
The vegetation in the Loess Plateau is distributed horizontally from northeast to southwest, with the differentiation of broad-leaved forest, forest grassland, dry grassland, and desert grassland occuring sequentially from southeast to northwest. Moreover, forest resources are scarce, with a forest coverage rate of 7.16%. The main tree species are Pinus tabulaeformis Carr., Robinia pseudoacacia L., Armeniaca sibirica, Ailanthus altissima, Quercus wutaishansea Mary, Betula platyphylla Suk and others [8-10].
The main tree species are supplemented in the article as follows: "The vegetation coverage is low, and the vegetation from south to north is distributed in the forest belt, forest-steppe transition zone, and grassland zone in an obvious zoning pattern. The main tree species are Pinus tabulaeformis Carr., Robinia pseudoacacia L., Armeniaca sibirica, Ailanthus altissima and others (Fig. 1b). "
Figure 3: Negative values of SPEI represent dry conditions and then in the figure they are visualized with blue and green colour? Seems confusing. I suggest to flip the colour gradient.
Response: Thank you for your constructive comments.
We have modified Figure 3 according to your suggestion, as follows:
Figure 3. Spatial distribution of seasonal SPEI average and drought magnitudes in the Loess Plateau from 2001 to 2019.
435-438: Please divide this large sentence into multiple ones, it is really hard to read. If I get this right, the mild drought caused lower sensitivity of WUE to SPEI and moderate high sensitivity of WUE to SPEI. The starting part of sentence: “the main magnitude of drought were no drought and mild drought” makes no sense to me.
Response: Thank you for your constructive comments.
We have modified this sentence following your suggestions, as following "Lastly, we found that the mild drought caused lower sensitivity of WUE to SPEI and moderate drought caused higher sensitivity of WUE to SPEI, implying that drought tolerance of vegetation may have a threshold effect, which is consistent with previous study [11-12]. "
484: This sentence is not finished, they found what?
Response: Thank you for your constructive comments. We have modified this sentence following your suggestions, as follows: " Similar results were found in previous studies [13-15], such as Xu et al. [15], who found that the response of WUE to drought stress differed in arid, semi-arid, semi-humid, and humid regions. "
515: I would suggest to avoid repeating of the obtained results and focus more on big picture and implications of the study. How the findings correspond to global climate change scenarios for the area? Synthesize your outputs into the most interesting facts. Arid areas suffered the most from the drought exposure, right? Therefore, their vitality could be exhausted and lead to their disintegration and maybe transformation into desert? What are the implications for the future studies? I see that you are mentioning the lag effect at the end of discussion, maybe I would move it into the conclusion.
Response: Thank you for your constructive comments. We have revised the conclusion according to your suggestions, as follows:
" In the current study, the impact of seasonal drought on WUE was analyzed using ridge regression, which appeared at the temporal and spatial distribution characteristics of seasonal drought and water use efficiency in the Loess Plateau from 2001 to 2019. The results show that from 2001 to 2019, there was significant spatial heterogeneity in seasonal drought and WUE, with drought levels generally being mild, and WUE values being highest in the summer. Seasonal drought had different effects on WUE, and the sensitivity coefficient also differed significantly as the SPEI changed. On the other hand, the sensitivity of seasonal WUE to drought magnitude was affected by regional dry and wet conditions. The impact of drought intensity on WUE in the four climate regions showed a clear seasonal divergence, and the sensitivity of WUE to drought magnitude in arid areas was generally higher than that in semi-arid, semi-humid, or humid areas. However, there are still many deficiencies in this study. Future research should pay more attention to the response processes of GPP and ET to drought, consider the impact of drought accumulation impact and lag effect on WUE. Our findings are crucial for understanding the impact of climate change on the Loess Plateau’s ecosystem carbon and water cycles. "
The specific explanation is as follows: With the global warming, the intensity and frequency of drought worldwide are significantly increasing [16-17]. Drought is the main environmental factor limiting terrestrial ecosystem and has seriously affect the ecological security and human society [18]. Moreover, relationship between vegetation growth and water use of terrestrial ecosystems are significantly affected by drought [19]. For example, drought can constrain vegetation growth, lead to the decreased primary productivity and carbon absorption, affect the water supply conditions, impact on ecosystem structure and function [20-22]. Water use efficiency (WUE) is related to the amount of fixed CO2 or dry matter produced by plants consuming unit mass of water [22-23], which links the coupling of carbon and water cycles and it can be used as a feedback index of terrestrial ecosystem to cli-mate change terrestrial ecosystems [19, 24-26]. Moreover, increased frequency and magnitude of global drought events have a pro-found impact on the water-carbon coupling cycles [3,27]. Therefore, the results of this paper are of great significance for in-depth understanding of WUE’s sensitivity to drought, and will greatly help us better understand the ecological adaptation of vegetation and the feedback to climate change [28].
It is found that the sensitivity of WUE to drought magnitude in the arid area of the Loess Plateau was generally higher than that in semi-arid, semi-humid and humid regions. On the one hand, because low vegetation coverage leads to more soil evaporation in arid regions, the higher sensitivity of ET to changed hydro-climatic conditions [29]. In contrast, in semi-arid and semi-humid regions, ecosystem functions and activities depend largely on water availability [30]. Consequently, the change of WUE in arid areas is generally controlled by evaporation (ET) (which is a physical process), whereas that in semi-arid and semi-humid areas is primarily regulated by assimilation (GPP) (which is a biological process). Vicente-Serrano et al. [13] pointed out that persistent water deficit (i.e., the drought time-scale) in arid and humid areas affects the sensitivity of vegetation WUE to drought. Vegetation in arid areas has a mechanism that allows them to quickly adapt to the changing water resources, so the vegetation will rapidly reflect once it is less than the normal value in a short time [13]. In humid areas, vegetation is usually less adaptable to water stress, and the response to drought is different at short time-scales from those of physiological mechanisms operating in arid biomes. On the contrary, the response of biomes in semi-arid and semi-humid region to drought at long time-scales because plants can resist water shortage [13].
Compared with previous studies, the present study explored the impact of drought on WUE from a seasonal perspective, and further authenticated the function of ecosystem WUE in dealing with external environmental interference. It should be noted that, although previous studies pointed out that the response of ecosystem WUE to drought was affected by many factors, such as vegetation characteristics [31], drought magnitude, and regional characteristics [32-34], there are few studies on the impact of seasonal drought magnitude on different ecosystem WUE values, so more theoretical and model simulations are needed to verify this conclusion. On the other hand, this study does not consider the persistence of the water deficit (i.e., the drought time-scale) and lag effects of drought. Our findings are crucial for understanding the impact of climate change on the Loess Plateau’s ecosystem carbon and water cycles.
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Author Response File: Author Response.docx
