Human expansion into Asian highlands in the 21st Century and its effects

Most intensive human activities occur in lowlands. However, sporadic reports indicate that human activities are expanding in some Asian highlands. Here we investigate the expansions of human activities in highlands and their effects over Asia from 2000 to 2020 by combining earth observation data and socioeconomic data. We find that ∼23% of human activity expansions occur in Asian highlands and ∼76% of these expansions in highlands comes from ecological lands, reaching 95% in Southeast Asia. The expansions of human activities in highlands intensify habitat fragmentation and result in large ecological costs in low and lower-middle income countries, and they also support Asian developments. We estimate that cultivated land net growth in the Asian highlands contributed approximately 54% in preventing the net loss of the total cultivated land. Moreover, the growth of highland artificial surfaces may provide living and working spaces for ∼40 million people. Our findings suggest that highland developments hold dual effects and provide new insight for regional sustainable developments.

1. The authors have identified a valid trend in increasing human activity in highlands, this is interesting and significant since the human pressure on highlands can have different and serious impacts on these ecosystems while also contributing to human development. However, it is noteworthy that these trends are not universal as in the case of the Hindu Kush Himalayan region where the population movement to the low lands is still very significant (e.g. as in Ref: P 31 in The Hindu Kush Himalaya Assessment: Mountains, climate change, sustainability and people). 2. To this effect, the questions raised by this paper are noteworthy and contributes significantly to the evidence necessary to account for our response to climate change. 2. The authors have provided sufficient evidence through spatial approaches to answer the questions they raised. The methodology is accessible and easily predictable given the data is shared along with the paper. 3. However, it is not clear what is driving these trends especially when developing highlands is still more costly today than ever given many countries are making it difficult to disturb mountain slopes and mountain ecosystems. There is a need to provide a robust argument for this trend. 4. Though sounds trivial, the authors need to clarify the distinction between hills and mountains (the criteria), especially the criteria used to demarcate/identify them in the spatial datasets they used. 5. Line 116-119: Many of these countries may have the least amount of low land to expand into and are largely hilly in nature and hence the expansion in the highlands is the only natural. There appear to be no interesting or contrary observations to be made here? If some of these countries have plenty of flat lands to expand into, why are they not doing it? It would be interesting to see some arguments on these lines. 6. The paper is good in identifying what is happening but lacks strength in providing an argument on why it is happening. Is it the saturation of lowlands for further expansion, is it the increasing economic and policy ability to occupy highlands more than before? Obviously, these questions are beyond the scope of this paper but are very important offshoots of the questions the authors raised. 7. Though most observations are obvious, the paper identifies the pertinent socio-economic and environmental implications of human expansion into highlands. These observations have important policy implications. 8. Line 157-177: Interesting trends between developing and developed countries in terms of ecological lands being converted to human use. Why developing countries are increasingly relying upon highlands? What is driving them? Policy failures or a conscious decision to engage natural resources for livelihood generation which appears to be the case based on the arguments made in the section starting from L186 but needs to be clearly stated. Do we have evidence supporting these cause-effect relations? 9. It is good that some reasons for expansion are discussed in L 209-212. We need more such possible reasons to make this paper more usable for policy purposes and by establishing causeeffect relations. 10. L 219-224: The question here is why? What is driving these seemingly contrary trends? 11. The text from Line 243 provides some good explanations for trends but not for all the regions, it would be good to provide a clear explanation for all the trends in a tabular form, for e.g.
Reviewer #2 (Remarks to the Author): Mountains provide 50% of global fresh water, offer habitat for some 33% of terrestrial biodiversity, harbor half of all glbaol biodiversity hotspots (Rumbine & Xu 2021, Front Ecol Environ). Asian highlands are the most dynamic, diverse and complex landscapes at worldwide. This papper provides timely insights for land expansion in the Asian highlands. However it needs to well definitely defined highland boundary. The commonly used definition is from WCMC-UNEP 2002 and Kapos et al. 2000. According to their definition, only around 1/4 of world terrestrial ecosystem is called mountains or hills. It woudld be nice to tell the difference between hills and mountains for readers.
Editorial Note: Parts of this Peer Review File have been redacted as indicated to remove third-party material where no permission to publish could be obtained. 1

Dear editor and reviewers,
We thank all of you for your comments and suggestions, which are very helpful for us to improve this manuscript. We carefully revised the manuscript (especially drivers of the trends) according to your comments and suggestions (Please see our point-to-point response below in blue and purple text). We also improved the English with the help of native speakers (see Language certificate). We hope that this revised version could meet the requirements for publishing consideration.
Your sincerely,

Reviewer #1 (Remarks to the Author):
1. The authors have identified a valid trend in increasing human activity in highlands, this is interesting and significant since the human pressure on highlands can have different and serious impacts on these ecosystems while also contributing to human development. However, it is noteworthy that these trends are not universal as in the case of the Hindu Kush Himalayan region where the population movement to the low lands is still very significant (e.g. as in Ref: P 31 in The Hindu Kush Himalaya Assessment: Mountains, climate change, sustainability and people). To this effect, the questions raised by this paper are noteworthy and contributes significantly to the evidence necessary to account for our response to climate change.
Response: Many thanks for your acknowledgement and positive comments.
2. The authors have provided sufficient evidence through spatial approaches to answer the questions they raised. The methodology is accessible and easily predictable given the data is shared along with the paper. provide a robust argument for this trend. Response: Thank you for the important and constructive comments. We analyzed and discussed the driving forces of the trends in a tabular form according to your suggestion (Please see our response to the question of NO.11, and we provided a clear explanation for all the trends in a tabular form).
4. Though sounds trivial, the authors need to clarify the distinction between hills and mountains (the criteria), especially the criteria used to demarcate/identify them in the spatial datasets they used. Response: We apologize for the insufficient details and thank you for your suggestions for improvement. We improved and clarified the criteria used to demarcate/identify highland (i.e. hills and mountains) according to Digital elevation model (DEM) in Methods part.  40.35% highlands but its expansion rate in highlands was more than 55% (Fig. 2d), which is an unusual contrary observation in Yemen compared with other countries.

Added sentences in Discussion part:
Notably, Yemen has the least highland areas (only accounts for 40.35%) among these countries, while the highland developments was > 55% (Fig. 2d) an argument on why it is happening. Is it the saturation of lowlands for further expansion, is it the increasing economic and policy ability to occupy highlands more than before? Obviously, these questions are beyond the scope of this paper but are very important offshoots of the questions the authors raised. Response: Many thanks for your comment and sharing your insights on the topic. We provided more arguments on why it is happening from multifactor perspective, including topographic conditions, marketing demand, population growth/livelihood and dynamic political economy/ecology (dynamic political economy/ecology is also the second reviewer"s suggestion; Please see our response to the raised question NO.11, and we provided a clear explanation for all the trends). We also realized the importance of the offshoots proposed in this article, and we added two sentences (hints) of future work in the penultimate paragraph of the Discussion.

Added sentences in Discussion part:
Although Asian highlands hold great potentials for social developments, we should explore the threshold of balancing highland developments and ecological protection in future works. Moreover, considering that highlands are rich in biodiversity, we need to further explore the impacts of highland developments on biodiversity.
7. Though most observations are obvious, the paper identifies the pertinent socio-economic and environmental implications of human expansion into highlands. These observations have important policy implications.
Response: Many thanks for your acknowledgement.
8. Line 157-177: Interesting trends between developing and developed countries in terms of ecological lands being converted to human use. Why developing countries are increasingly relying upon highlands? What is driving them? Policy failures or a conscious decision to engage natural resources for livelihood generation which appears to be the case based on the arguments made in the section starting from L186 but needs to be clearly stated. Do we have evidence supporting these cause-effect relations? Response: Many thanks for your constructive comments. We provided some evidence in Discussion part (at the beginning of the third paragraph) from the perspective of environmental Kuznets Curve of low income and middle-high income countries. Moreover, our response to the raised question NO.11 also provided some evidences supporting these cause-effect relations).

Added some sentences in Discussion part:
We observed that highland developments caused considerable ecological land loss and fragmentation. Transforming ecological lands into human use in highlands exhibited higher ecological costs in low and lower-middle income countries (∼ 1.9 times the rate of ecological land loss) than in high and upper-middle income countries, which corresponds to the law of environmental Kurtz curve in the process of economic development in developing and developed countries. Previous studies have proved that, for countries just in the stage of economic take-off (e.g. low and lower-middle income countries), reducing poverty and promoting rapid economic growth force these countries to make extensive use of natural endowment to realize industrial take-off, resulting in ecological damage in the early stage of development [42][43][44] . In addition, international trade and investment can lead to the situation of "producing products and consuming natural resources in low-income countries, while enjoy benefits in high-income countries", which improves the ecological environments of developed countries and exacerbates the ecological problems of developing countries 45 .Therefore, the above evidences may well explain the large ecological costs caused by the highland developments in low and low-income countries. 9. It is good that some reasons for expansion are discussed in L 209-212. We need more such possible reasons to make this paper more usable for policy purposes and by establishing cause-effect relations. Response: Thank you for acknowledgement and suggestion. We added more possible reasons by establishing cause-effect relations in Discussion part (Please see our response to the raised question NO.8 &11, and we provided more explanations for possible reasons).
10. L 219-224: The question here is why? What is driving these seemingly contrary trends? Response: Many thanks for your comment. Actually, the population carrying rate in highland expansion areas in most Asian countries is relatively low (much lower than that in lowlands), thus we improved the original sentences to make readers understand it more clearly. We also gave explanations on driving forces (Please see our response to the question NO.11).

Original Line 116-119:
The population capacity rate in the highlands in 16 countries exceeded the Asia level ( Fig. 4d, 16 . 2d and Supplementary   Fig.8a).
11. The text from Line 243 provides some good explanations for trends but not for all the regions, it would be good to provide a clear explanation for all the trends in a tabular form, for e.g. Response: Many thanks for your suggestion. We improved this part through literature review, expert consultation and referees" suggestions (from more dynamic political economy/ ecology analysis). The revised details are shown in second paragraph and Table 1 of Discussion part. Table 1 in Discussion:

Improved sentences and added
We analysed the dominant and potential drivers affecting the human activity expansions in the highlands in different Asian countries during 2000 to 2020 from multifactor perspective (Table 1). Generally, for the countries with extremely high (≈90%) or very low human activity expansion rates in the highlands (<2%) (only one-fifth countries), topographic constraint may be the dominant factor affecting highland developments, considering that these countries possess extremely high or low proportion of highlands (Table 1). However, other four-fifths Asian countries had 5%-95% highland areas (Supplementary Fig 12), and thus explaining the human activity expansion rates in the highlands in these countries using topographic factors is unfair. Highland developments may be driven by different needs, and the superposition of multiple factors could result in the highland developments with 7 different degrees in these countries, including topographic condition, marketing demand, population growth/livelihood and dynamic socio-political economy/ecology factors (details in Table 1). In Bhutan, flat land resources are scarce and more than 95% of its land is highlands (Fig.2d). Thus, highland development is obligatory for maintaining social and economic developments. Such situation explains the extremely high human activity expansion rate in the highlands in Bhutan (≈90%). In contrast, the scarce highlands in some countries (highland proportion <5% in Qatar, Bahrain, Bangladesh, Maldives, Iraq, Turkmenistan, Kuwait and United Arab Emirates, Supplementary  Fig.12a) can explain their low human activity expansion rates in the highlands (<2%). Therefore, topographic factors significantly impact highland developments in the abovementioned countries (R 2 = 0.9981, p < 0.01, Supplementary Fig 12b).  22 . Mongolia"s western, northern and northeastern parts are upland and suitable for farming, and a national project for the development of farming was started since 1997 to improve food security and increase family incomes to alleviate poverty, which has largely driven Mongolia's highland developments 23 . Notably, Yemen has the least highland areas (only accounts for 40.35%) among these countries, while the highland developments was > 55% (Fig. 2d), which is mainly due to following reasons: most of the lowlands (e.g coastal plains) are semi-desert, and the highlands are fertile and suitable for developments; and Yemen"s civil war exacerbated long-term political instability and economic backwardness, thus Yemen strengthened the developments and utilizations of highlands (including agricultural developments, oil /gas /mineral explorations) to promote livelihoods and economic recovery [24][25][26] . from lowlands to highlands, away from areas vulnerable to drought and flooding from sea level rise) drove highland developments in these countries 28 .
Since the 21st century, the global urbanization has been accelerating significantly (urban population has increased from 30% in 1950 to 56% in 2019), especially in Asia and Africa 29 .
To meet the needs of urban population growth and rapid urbanization as well as people's pursuit of fresh air, landscape, exclusivity and closeness to nature, the hillsides around urban regions are being developed at an accelerating rate in different Asian cities 7,15,30,31 , which drove the highland developments in many Asian countries.
China"s "One Belt One Road" initiative increased direct investments and the construction of various infrastructures in Asian countries (e.g. railways, highways, hydropower stations, natural gas pipeline projects), which contributed to the highland developments in Asian countries to a certain extent 32 .
Highlands have beautiful natural landscapes, and Asian countries have accelerated the developments of mountain tourisms in the 21st century, which also drove highland developments to a certain extent 33 . These countries have relatively scarce highlands (Fig. 2d), therefore most human activities are concentrated in lowlands.

Improved sentence in Introduction:
The geographically diverse highlands are ecologically fragile regions, which play important roles in biodiversity conservation (offering habitat for ∼ 33% of terrestrial biodiversity), carbon sequestration, water supply (over 50% of global fresh water) and soil and water conservation 8-12 . (1) we obtained the highland boundaries in Asian with above two standards (as shown in the figure below). Asian highland proportion is 40.41% for the standard applied by this study, while 35.30% for WCMC. There is a little difference between the two results.
Considering the large area of China in Asia, we calculated the proportion of China"s highlands considering the two standards (as shown in the figure below). According to China's survey and common standards, China's highlands are more than 2/3 (∼69%), and the result from the standard we used is closer to the China's situation. (2) WCMC did not give a detailed numerical standard for dividing mountains and hills in highlands, while the standard we used presents detailed values (Supplementary  Table 9). Nevertheless, we also added a sentence in Discussion to discuss this issue. 12 2. I like to the statement of "Highland development supports sustainable development in Asia", but authors have failed to provide alternatives on how to do it. Balmford et al. (2016) talked "Getting Road Expansion on Right Track" as a good example. His team is also developed "land sharing" and "land sparing" approach for agricultural expansion. Response: Many thanks for your comments. We added one paragraph in Discussion part to provide alternatives on how to do it.

Added paragraph in Discussion:
Although improving and planning transport links to substantially increase food production at relatively limited environmental cost) could enhance livelihoods and regional food production and reduce overexploitation of cultivated land while helping safeguard vital ecosystem services and significant biological diversity 56 . Therefore, integrating the strategies of "land sharing", "land sparing" and planning transport links in Asian highland developments may help save land resources and protect the ecological environment in highlands, and make highland developments achieve sustainability. Response: Thank you for your constructive comments. Following your suggestion, we improved drivers of land expansion in highlands considering topographic conditions, marketing demand, population growth/livelihood and dynamic socio-political economy/ecology factors. We also improved the English with the help of native speakers (see Language certificate).

Improved sentences and added Table in Discussion part:
We analysed the dominant and potential drivers affecting the human activity expansions in the highlands in different Asian countries during 2000 to 2020 from multifactor perspective (Table 1). Generally, for the countries with extremely high (≈90%) or very low human activity expansion rates in the highlands (<2%) (only one-fifth countries), topographic constraint may be the dominant factor affecting highland developments, considering that these countries possess extremely high or low proportion of highlands (Table 1). However, other four-fifths Asian countries had 5%-95% highland areas (Supplementary Fig 12), and thus explaining the human activity expansion rates in the highlands in these countries using topographic factors is unfair. Highland developments may be driven by different needs, and the superposition of multiple factors could result in the highland developments with different degrees in these countries, including topographic condition, marketing demand, population growth/livelihood and dynamic socio-political economy/ecology factors (details in Table 1). In Bhutan, flat land resources are scarce and more than 95% of its land is highlands (Fig.2d). Thus, highland development is obligatory for maintaining social and economic developments. Such situation explains the extremely high human activity expansion rate in the highlands in Bhutan (≈90%). In contrast, the scarce highlands in some countries (highland proportion <5% in Qatar, Bahrain, Bangladesh, Maldives, Iraq, Turkmenistan, Kuwait and United Arab Emirates, Supplementary  Fig.12a) can explain their low human activity expansion rates in the highlands (<2%). Therefore, topographic factors significantly impact highland developments in the abovementioned countries (R 2 = 0.9981, p < 0.01, Supplementary Fig 12b).  22 .
Mongolia"s western, northern and northeastern parts are upland and suitable for farming, and a national project for the development of farming was started since 1997 to improve food security and increase family incomes to alleviate poverty, which has largely driven Mongolia's highland developments 23 . Notably, Yemen has the least highland areas (only accounts for 40.35%) among these countries, while the highland developments was > 55% (Fig. 2d), which is mainly due to following reasons: most of the lowlands (e.g coastal plains) are semi-desert, and the highlands are fertile and suitable for developments; and Yemen"s civil war exacerbated long-term political instability and economic backwardness, thus Yemen strengthened the developments and utilizations of highlands (including agricultural developments, oil /gas /mineral explorations) to promote livelihoods and economic recovery [24][25][26] . highways, hydropower stations, natural gas pipeline projects), which contributed to the highland developments in Asian countries to a certain extent 32 .
Highlands have beautiful natural landscapes, and Asian countries have accelerated the developments of mountain tourisms in the 21st century, which also drove highland developments to a certain extent 33 .
Low human activity expansion rate in the highlands (<Asia level) (∼2% -5%) Israel, Singapore, Cambodia, Sri Lanka, Kazakhstan These countries have relatively scarce highlands (Fig. 2d), therefore most human activities are concentrated in lowlands.

Added some sentences in Discussion part:
We observed that highland developments caused considerable ecological land loss and fragmentation. Transforming ecological lands into human use in highlands exhibited higher ecological costs in low and lower-middle income countries (∼ 1.9 times the rate of ecological land loss) than in high and upper-middle income countries, which corresponds to the law of environmental Kurtz curve in the process of economic development in developing and developed countries. Previous studies have proved that, for countries just in the stage of economic take-off (e.g. low and lower-middle income countries), reducing poverty and promoting rapid economic growth force these countries to make extensive use of natural endowment to realize industrial take-off, resulting in ecological damage in the early stage of development [42][43][44] . In addition, international trade and investment can lead to the situation of "producing products and consuming natural resources in low-income countries, while enjoy benefits in high-income countries", which improves the ecological environments of developed countries and exacerbates the ecological problems of developing countries 45 .Therefore, the above evidences may well explain the large ecological costs caused by the highland developments in low and low-income countries.
Improved and Added References: