Loess Plateau Cropland: Evolution and Ecological Impacts over Four Millennia

Round 1

Reviewer 1 Report

Comments and Suggestions for Authors

Dear authors,

You manuscript is really interesting and useful for the land management, but some minor corrections or suggestions have been to consider.

I consider some more specific comments addressing the following points:

• What is the main question addressed by the research?

The Loess Plateau is the cradle of Chinese civilization with a long history of agriculture activities in relation with ecological changes. The study that the authors present in this manuscript analyzes the evolution of the cropland area and the ecological impact over the land during the last 4.000 years. The period is divided by three development stages of the cropland area: slow growth (2000-500 BC), fluctuating phase (500 BC-1000 AD) and rapid expansion (1000-2000 AD). 

Analysis of the historical conditions, and taken account the evolution of human (population growth, agriculture technology) and climate changes (forest cover, precipitation…), is crucial for understanding the changes in cropland on the LP.

The results of this study suggest that the upper limit of cropland area in fluctuating phases is 80.2 thousand km2, the maximum sustainable cropland area in LP.

The primary goal is to understand the relationship between land use and ecosystems in the YR Basin during historical periods. This understanding will provide a framework for evaluating the effectiveness of modern soil and water conservation policies and inform strategies to improve the social and ecological benefits of current conservation efforts.

• What does it add to the subject area compared with other published material?

Some previous articles such as Huang et al. (2023) and Chen et al. (2012) include similar aspects to this current manuscript. Please, indicate the novel proposal clearly in the aims of the article.

• What specific improvements should the authors consider regarding the methodology?

The authors use different variables to consider for the analysis and in every section the methodology is indicated. Overall, the methodology is the same to Huang et al. (2023). I think you have to cite these references during the methodology section and include the novel parts.

• Are the conclusions consistent with the evidence and arguments presented and do they address the main question posed? Please also explain why this is/is not the case.

Fig.3. This is very similar to the Fig. 5 in Chen et al. (2012). Please, if the Figure 3. Is modified from Chen et al. (2012) and Zhang (1995), it has be added and clarify which results are novel in this point.

The conclusion is the prediction of the most suitable cropland area based in historical data (with HYDE) and the evaluation of the modern soil and water conservation policies that can balance the social and ecological conditions and the conservation. 

The citations in the text are not in the journal's required format; please check all the citations and references.

I think that an abbreviations section is necessary.

In the line 52, please, includes that the LP is in north-central China.

Fig.1. Could the map of China be more detailed and distinct, allowing us to view two separate maps: one of China and another of the Loess Plateau?

In the line 138 could you explain what the meaning of ArcGIS is, I know it, but I think that, at least, the geographic information system should be cited.

In the line 203 the Fig 5B has to be Fig. 3, I think.

In the line 238 it is Chen et al., 2012. The same in line 236.

In the table 1 Ge is Ge et al., 2008

1661

Qing

608

(Ge, 2008)

 

The reference 22 is not in the correct format.

 

Author Response

Dear Reviewer,

We sincerely appreciate your valuable comments, which have significantly enhanced the reliability, logical coherence, and standardization of this paper. We have meticulously implemented all your suggested revisions. Please find our point-by-point response to your comments in the attached document.

Author Response File: Author Response.pdf

Reviewer 2 Report

Comments and Suggestions for Authors

Based on the datasets from public database and published literatures, the manuscript calibrated the change of cropland in the Loess Plateau in the past 4,000 years and further discussed the drivers and impacts of cropland evolution. The results provide a threshold value of cropland area for guiding future land use strategies. Overall, the structure and expression of the manuscript are clear. I have a few suggestions in the following.  

 

1. Temporal comparison between changes in cropland, population, temperature and precipitation has been made in the manuscript, and a regression method has also been adopted during the analysis. As we known, HYDE database also provides spatial distribution of cropland in the past 4,000 years. While the author has already got the population distribution data in the Loess Plateau (Fig. 8). Spatial continuous climatic factors data can be obtained from current climate simulation studies. Therefore, the dynamic of spatial evolution of cropland can be further revealed based on above data.
2. As for the impact of cropland on regional environment, Fig. 7 in the manuscript compares the changes of cropland with forest cover and sediment load. The resolution of these data is relatively low. It is better to make a more clear description about the information for these data, for example, what kind of proxy has been used for the forest cover reconstruction?

Comments on the Quality of English Language

he English could be improved to more clearly express the research..

Author Response

Dear Reviewer,

We sincerely appreciate your valuable comments, which have significantly enhanced the reliability, logical coherence, and standardization of this paper. We have meticulously implemented all your suggested revisions. Please find our point-by-point response to your comments in the attached document.

Author Response File: Author Response.pdf

Reviewer 3 Report

Comments and Suggestions for Authors

The article "Loess Plateau Cropland: Evolution and Ecological Impacts Over Four Millennia" by Tao Huang et al., spans 4,000 years of cropland evolution on the Loess Plateau (LP), providing a long-term perspective that is rare and valuable for understanding human-environment interactions. The division into three phases—slow growth (2000-500 BC), fluctuating phase (500 BC-1000 AD), and rapid expansion (1000-2000 AD)—offers a clear framework for analyzing trends and their ecological consequences. Despite the findings, here are a few potential limitations to note:

 

Major Issues

 

The reliance on the HYDE 3.1 dataset, with temporal resolutions varying from 1000 years (pre-0 AD) to 10 years (1700-2000 AD), introduces potential inaccuracies, especially for earlier periods. The study acknowledges discrepancies in global datasets for regional applications but does not fully quantify the uncertainty in its cropland estimates (e.g., ±23.4 thousand km² for the slow growth phase). How the authors face this challenge?

 

While the article discusses cropland area and food production per unit area (e.g., 139-172 t/km² in the Northern Song), it lacks detail on specific crops cultivated across the phases. This omission limits insights into how crop choices influenced ecological impacts or agricultural sustainability. The authors must offer information about crops dynamics

The ecological effects are assessed using forest cover, sediment load, and flooding frequency, which are reasonable but narrow proxies. Other critical indicators—such as biodiversity loss, soil nutrient depletion, or water quality—are not addressed, potentially underrepresenting the full scope of ecological degradation.

 

The method for estimating population in certain years (e.g., 25 AD, 105 AD) by assuming constant population ratios between the LP and the middle Yellow River introduces a potential bias. This assumption is not rigorously validated, and historical disruptions (e.g., wars, migrations) could skew these estimates.

 

While population growth is identified as the dominant driver (56.9%), the analysis may overstate its role relative to technological or climatic factors. For instance, the impact of innovations like the curved-shaft plow is noted but not deeply explored in terms of its potential to independently drive cropland expansion.

 

The discussion of modern soil and water conservation measures (e.g., "Grain for Green") is brief and lacks quantitative data on how these interventions have altered the sustainable cropland threshold since the 1950s. This limits the study’s relevance to current policy-making.

 

Incorporate higher-resolution regional datasets or additional historical records (e.g., archaeological evidence, tax records) to refine cropland estimates, particularly for the slow growth and fluctuating phases. Provide a detailed uncertainty analysis (e.g., confidence intervals) for each phase to strengthen the reliability of the findings.

Expand the study to include data on dominant crops (e.g., millet, wheat, maize) across the 4,000-year timeline. This could reveal how shifts in crop types influenced soil erosion, water use, and food security, adding depth to the ecological and agricultural narrative.

Include additional ecological metrics such as soil organic carbon levels, species diversity, or groundwater recharge rates. These could be modeled using proxy data from paleoenvironmental studies or modern analogs to provide a more comprehensive impact assessment.

Cross-reference population estimates with independent sources (e.g., census fragments, settlement patterns) or use sensitivity analyses to test the robustness of the constant-ratio assumption. This would improve confidence in the population-driven cropland expansion hypothesis.

Conduct a more detailed examination of how specific technologies (e.g., plow designs, irrigation) and policies (e.g., tax reforms, land tenure systems) independently shaped cropland dynamics. This could involve case studies of key periods like the Northern Song or Qing Dynasty.

Quantify the impact of modern interventions (e.g., terracing, check dams) on cropland sustainability using recent data (e.g., sediment load reductions post-1999). This would allow the study to propose an updated sustainable cropland threshold under current conditions, enhancing its practical utility.

Compare the LP’s cropland evolution and ecological impacts with other semi-arid agricultural regions (e.g., the Sahel, Central Asia) to contextualize findings globally. This could position the study as a broader reference for sustainable land use in similar environments.

Improve the interpretability of figures (e.g., Fig. 8) by adding color gradients or annotations to highlight population density shifts. Include a map showing the spatial distribution of cropland expansion over time, linked to ecological hotspots like erosion-prone slopes.

General Questions Across Sections

1. How does the study account for spatial variability within the Loess Plateau (e.g., northern vs. southern regions) in its cropland and ecological analyses?
2. What are the implications of the findings for current climate change scenarios, particularly regarding temperature and precipitation trends?
3. How can the study’s methodology be adapted or scaled to other regions with similar agricultural and ecological histories?

Detailed Minor issues

Abstract

1. What specific ecological impacts (e.g., soil erosion, flooding) are quantified in the study, and how do they vary across the three identified developmental stages?
2. How was the maximum sustainable cropland area of 80.2 thousand km² determined, and what assumptions underpin this threshold?
3. What are the key historical drivers of cropland expansion mentioned, and how do their contributions (e.g., 56.9% for population) inform modern land-use strategies?
4. How does the study’s focus on the Loess Plateau provide a reference for other regions with similar ecological and agricultural challenges?

 

1. Introduction

What specific characteristics of the Loess Plateau (e.g., soil type, climate) make it particularly vulnerable to ecological changes from cropland expansion?

How does the study define "ecological sustainability" in the context of historical land use without modern conservation measures?

What gaps in previous research on the Loess Plateau’s cropland history does this study aim to address, and why are these gaps significant?

How do global land use/cover change (LUCC) trends compare to the specific patterns observed in the Loess Plateau over the past 4,000 years?

2. Methods
3. How was the functional relationship between the HYDE dataset and historical literature estimates established, and what statistical methods were used to validate it?
4. What are the potential sources of error in the cropland area estimates for the Loess Plateau, and how were they addressed in the calibration process?
5. Why was the HYDE 3.1 dataset chosen over other global land-use datasets (e.g., SAGE), and what are its specific advantages for this study?

 

 2.2. Farming-Grazing Boundary

1. How were historical records of local customs and economic factors translated into a quantifiable farming-grazing boundary (FGB), and what uncertainties arise from this process?
2. What specific evidence supports the assumption that the FGB’s movement reflects changes in cropland area rather than other socio-economic factors?
3. How does the formula for calculating FGB distance account for the complex, interwoven nature of the boundary, and what are its limitations?

 

 2.3. Population

1. What primary sources were used to directly acquire population data for specific years (e.g., 2 AD, 609 AD), and how reliable are these sources?
2. How was the assumption of constant population ratios between the Loess Plateau and the middle Yellow River validated, and what alternative methods could be considered?
3. What are the potential impacts of historical events (e.g., wars, migrations) on population estimates, and how were these accounted for?

 

 2.4. Other Datasets

1. How were forest cover, precipitation, and temperature data reconstructed for the past 4,000 years, and what are the associated uncertainties?
2. What criteria were used to select sediment load and flooding frequency as indicators of ecological impact, and why were other indicators (e.g., biodiversity) excluded?
3. How were population density maps standardized across historical periods, and what adjustments were made for changes in administrative boundaries?

 

 2.5. Calculation of Contribution Rates

1. What independent variables were included in the multiple regression model for cropland area, and why were these chosen over others (e.g., soil fertility, irrigation)?
2. How were the standardized regression coefficients calculated, and what does the unexplained variance (1-R²) suggest about unmodeled factors?
3. What are the limitations of the Földi formula in attributing contribution rates, and how sensitive are the results to changes in input data?

 

3. Results

 3.2. Drivers of Changes of Cropland Area

1. How did specific agricultural technologies (e.g., curved-shaft plow) directly influence cropland expansion, and what evidence quantifies their impact?
2. What role did policy reforms (e.g., abolition of the head tax) play in population growth, and how was this effect isolated from other factors?
3. Why does the study find a negative correlation between cropland area and climate factors (rainfall, temperature), and how does this align with expected agricultural responses?

 

 3.3. Local and Spillover Effects of Cropland Area Changes

1. How was the causal link between cropland expansion and increased sediment load in the Yellow River established, and what alternative explanations were considered?
2. What specific mechanisms connect reduced forest cover on the Loess Plateau to increased flooding frequency in the lower Yellow River?
3. How do the ecological effects during the fluctuating phase (500 BC-1000 AD) differ from those in the rapid growth phase (1000-2000 AD) in terms of severity and reversibility?
4. Discussions
5. How do the ecological impacts of cropland expansion on the Loess Plateau compare to those in other semi-arid regions, and what lessons can be drawn from these comparisons?
6. What evidence supports the claim that 80.2 thousand km² is the maximum sustainable cropland area, and how might this threshold change with modern conservation measures?
7. How did the interplay of population growth and declining food production per unit area during the rapid growth phase lead to extensive farming practices?
8. What are the trade-offs between ecological restoration (e.g., "Grain for Green") and food security on the Loess Plateau, and how can they be quantified?
9. Conclusions
10. How do the findings on cropland evolution over 4,000 years inform specific land management policies for the Loess Plateau today?
11. What are the key ecological risks of exceeding the proposed 80.2 thousand km² cropland threshold, and how can these risks be mitigated?
12. How do the contributions of population, technology, and climate to cropland changes provide a framework for balancing agricultural development and ecological sustainability?
13. What further research is needed to refine the sustainable cropland threshold and address limitations in the current study?

References

Ok

Author Response

Dear Reviewer,

We sincerely appreciate your valuable comments, which have significantly enhanced the reliability, logical coherence, and standardization of this paper. We have meticulously implemented all your suggested revisions. Please find our point-by-point response to your comments in the attached document.

Author Response File: Author Response.pdf

Reviewer 4 Report

Comments and Suggestions for Authors

The study presents a 4,000-year analysis of changes in the area of cultivated land on the Loess Plateau, representing a long-time perspective. The identification of three distinct phases of development and the establishment of a sustainability threshold based on historical data represent an original contribution to the literature. The work is relevant to understanding the relationship between land use and ecosystems in an ecologically sensitive and culturally important region. In this context, I am very excited to discover in this paper the direct links between historical policy decisions, population growth, environmental change and changes in the area of cultivated land. The overall structure of the article is acceptable and provides a comprehensive analysis change. However, I have some suggestions to further improve the manuscript:

• In the introduction, a large section dealing with the characteristics of the study area could be moved to the methods section. After this reorganisation, the introduction needs to be improved to better contextualise the subject.
• Present only the results of the study in the results section. Any discussion or comparison with existing literature (L187-L192 ; L220-222; L231-245; L289-L295…) should be moved to the discussion section.
• Discussion:
• the discussion should be organized in sections. The different sections should be more interconnected, with smoother transitions to maintain a logical flow.
• better explain how the sustainable or reasonable historical upper limit of 80.2 thousand km² for cultivated land on the LP was chosen and discuss how current and modern conditions (agricultural technologies, soil conservation practices etc.) could, in some cases, impact this sustainability threshold
• present and discuss the limitations of the study
• set aside a section to present and discuss the theoretical and practical implications of the results, among others, on current sustainable agricultural development policies in the region
• Review the usefulness of inserting figure 8. In any case, it is recommended not to insert figures in the discussion but rather to discuss the results with the literature. If the figure is essential, it could be placed in section 2.3. Population or in results 3.2. Drivers of Changes of Cropland Area;

 

 

 

Author Response

Dear Reviewer,

We sincerely appreciate your valuable comments, which have significantly enhanced the reliability, logical coherence, and standardization of this paper. We have meticulously implemented all your suggested revisions. Please find our point-by-point response to your comments in the attached document.

Author Response File: Author Response.pdf

Round 2

Reviewer 3 Report

Comments and Suggestions for Authors

Dear authors, I appreciate your willingness to improve the document.