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Impact of Spatial and Temporal Scales on Afforestation Effects: Response to Comment on “Why Large-Scale Afforestation Efforts in China Have Failed to Solve the Desertification Problem” As Drs. Yang and Ci note, there were some errors in my previous manuscript. Some of these problems resulted from the deletion of certain key references as a result of efforts to shorten the original manuscript in response to the original review comments. In this response to their letter of comment, I will attempt to correct these errors and provide additional support for and clarification of my opinions. Afforestation is the Priority of China’s Ecological Restoration Projects. To alleviate the problem of grassland degradation, the Chinese government has invested huge amounts of money in planting trees over the past 30 years (1). Because of their expertise with trees and China’s desperate need for new sources of wood fiber, China’s Bureau of Forestry has naturally prioritized afforestation, and has been much less interested in grassland restoration (1). From 1949 to 2005, 2.2 billion ha was converted into forest by planting trees and shrubs, amounting to nearly 22.91% of China’s total area (Supporting Information Figure S1). An additional 30 million ha were regenerated by aerial seeding (ref 2, p 352). Although government policies have encouraged largescale afforestation in semiarid and arid areas in the past, such projects have proven economically costly and ecologically unsustainable (1). The overall survival of afforestation areas has been only 53 million ha in 2005 across China (3). Including afforestation in the semihumid northeastern region of China, the survival of trees planted during the Three Norths Shelter Forest System Project was only 15% (ref 4, p 44). In addition, monoculture plantations have encountered serious insect infestations. A total of 4 million ha of Populus tremula monoculture in northern China have been affected, and 120 000 ha yr-1 of plantations have died, as a result of infestations by Anoplophora glabripennis Motsch. and Anoplophora nobilis Ganglb., two wood-boring beetles (ref 5, p 40). Simultaneous with this tree mortality, deserts have expanded (6) and sandstorm frequency has increased from an average of 0.5 per year during the 1950s (7) to 2.3 per year in the 1990s (ref 6, p 15 and ref (7)) and 10.3 per year between 2000 and 2006 (refs 7 and 8, p 12). Although the real cause of increased desertification and sandstorm frequency is still debated, it is indisputable that afforestation projects have decreased local vegetation cover (9) due to shading by the trees or actual removal of vegetation to promote tree survival, and where large numbers of trees have subsequently died, the bare soil is far more vulnerable to erosion by wind (in arid areas) and water in the north Loess Plateau. From south to north, precipitation decreases and wind erosion increases in the Loess Plateau. The loss of vegetation cover in this area that results when plantations of trees have died can lead to greatly increased wind erosion in the denuded areas. However, it seems logical to predict that in parts of the Loess Plateau where wind velocity is sufficient to entrain the surface sediments, wind erosion will increase until vegetation cover is re-established. Although there have unquestionably been some afforestation successes, particularly in areas with abundant water, these statistics demonstrate the potential risks associated with overly aggressive afforestation in drier parts of northern China. 7724
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Many Chinese ecologists have published books and articles in both Chinese and English to point out these problems (e.g., refs 1 and 6). In response to these criticisms, planting of shrubs has increased dramatically in all afforestation-related projects in recent years due to changes in the government’s forestry law (10). In the fourth stage of the Three Norths Shelter Forest System Project (2001-2010), planting of shrubs increased from less than 20% of the total area to around 40% (10). However, it is unclear whether the 60% of the remaining area in which trees are the priority and 40% of shrubs are the priority can be truly sustainble in the long term. Impact of Spatial and Temporal Scales on the Effect of Afforestation. The problem of afforestation in arid and semiarid China was addressed by Dr. Ci several years ago (6). In response to afforestation failures in arid and semiarid areas, Dr. Yang and his colleagues (11) at the Chinese Academy of Forestry designed a new planting technique based on the use of a “microcatchment water harvesting system with microphytic crust treatment” in semiarid parts of the Loess Plateau, and this approach has been used since 1992. This approach is designed to catch precipitation so that it can irrigate planted trees, and to further increase water availability to trees, the original vegetation was removed and the soil surface was compacted and smoothed (11). Unfortunately, large-scale use of the technique has decreased vegetation cover by an average of 30.5% in afforestation plots (including the shrub planting plots) by the fifth year after planting in grassland areas of northern Shanxi Province that had undergone afforestation during the Grain for Green Project (ref 9, p 974), sometimes even producing entirely bare ground (Supporting Information Figure S2). In contrast with natural grassland and forest, for which water use was historically in equilibrium with the regional water supply, soil moisture content to a depth of 6 m in such afforestation areas (include the shrub planting plots, too) has decreased by 37.2% (ref 9, p 974) by the seventh year. It would be more correct to state that degraded grassland was not restored to a natural grassland community, but was instead, in many cases, planted with trees. Application of a new technique has particularly significant consequences when the scale expands from a few test projects to large areas. As afforestation expands and more trees begin growing in an area with inadequate precipitation, the trees will gradually deplete the groundwater to compensate. For example, during the 1970s, the initially successful afforestation used to restore the environment of Jingbian County in the Mu Us Sandland was used as a model for the rest of China, but 20 years later, almost all of the planted Hippophae rhamnoides L. and >70% of the planted Populus davidiana Dode. had died, and vegetation cover had declined below the levels that existed before the afforestation (Leader Liu of Bureau of Forestry, Jingbian County) as renewed desertification erased the early gains and soil moisture shortages were exacerbated. As a result, the overall tree survival rate decreased to around 15% (Supporting Information Figure S3), and the level of groundwater decreased to between 40 and 50 m over a period of 30 years (Leader Liu of Bureau of Forestry, Jingbian County). As Dr. Yang and Dr. Ci note in their commentary (reference numbers changed to follow the sequence in my response): “Desertification is a complex combination of geobiophysical processes, geobiochemical cycles, and geobiosocial relationships (12), and it must be controlled with integrated dryland development paradigms (13).” It is important to note that forestry ecosystems include more than just trees and shrubs; 10.1021/es801569c CCC: $40.75
2008 American Chemical Society
Published on Web 09/19/2008
they also include grasses and other steppe or prairie vegetation, herbaceous species, and lichen species, all of which form a complex plant community. The low resilience of dryland ecosystems means that tree planting cannot be the only measure used to restore degraded ecosystems. Sustainable strategies for environmental restoration and management must explicitly integrate ecological, economic, and societal issues in the overall framework (1). It is not clear to me that government policy has recognized these needs and changed the afforestation policy sufficiently in response. The comments by Drs. Yang and Ci. Dr. Yang and Ci are, of course, correct that the Taihang Mountain Afforestation Project was not the correct project related to efforts to reduce dust storms in Beijing. The correct project is actually the Cleaning the Air of Beijing project (1). They are certainly correct that some small-scale or short-term assessments in arid and semiarid areas have shown afforestation to be successful, and it is also true that longer-term results in areas where water availability is sufficient to support forest vegetation types have been successful. The more important point I was trying to make in my original paper is that the overall success of afforestation is not clear, particularly in arid and semiarid areas, and that there is sufficient evidence of failures (some quite dramatic) that the Chinese policy emphasis on afforestation must be opened to debate rather than allowing the current largely uncritical attitudes go unchallenged. More generally, the central authorities have failed to realize the importance of the natural ecological structure of many ecosystems targeted for afforestation (14). Although there is a growing realization in China that afforestation alone is not appropriate, the pro-afforestation attitudes have been slow to change (14). The Forest and Grassland Taskforce noted that the implementation of many afforestation projects has not been tailored to local conditions, and that tree planting has been overemphasized at the expense of restoring a region’s original vegetation cover (1). This inflexible approach must change.
Supporting Information Available Three additional figures. This material is available free of charge via the Internet at http://pubs.acs.org.
Literature Cited (1) Jiang, G.; Han, X.; Wu, J. Restoration and management of the Inner Mongolia Grassland requires a sustainable strategy. AMBIO 2006, 35, 269–270. (2) Bureau of Forestry of China. China Forestry Yearbook; China Forestry Press: Beijing, 2006 (in Chinese). (3) Bureau of Forestry of China. China Forestry Report 2005; http://www.forestry.gov.cn(in Chinese). (4) Su, Y. Review of 25-year’s result of Three Northern Regions Shelter Forest System Project, China. Sci. Culture 2004, 3, 42–44 (in Chinese). (5) Lu, W. Discussion on severity and control of Asian longhorned beetle of poplar trees in the Three North Protection Forest Program. For. Sci. Technol. 2004, 58 (1), 39–41 (in Chinese). (6) Ci, L. Present status of desertification and restoration strategy in China. For. China 1999, 5, 14–15 (in Chinese). (7) Sandstorm Climate in China at Recent Years. http://baike.baidu.com/view/2097.htm(in Chinese) (8) Wang, W.; et al. Never-failing Sand-storm Coming: China’s Sandstorm Climate in 2006. Weather Knowledge 2006, (3), 12–17 (in Chinese). (9) Cao, S.; et al. Impact of Grain for Green Project to nature and society in north Shaanxi of China. Sci. Agric. Sin. 2007, 40, 972–979 (in Chinese). (10) . The general plan for the fourth stage of the Three Norths Shelter Forest System Project; State Forestry Administration, China: Beijing, 2000. (11) Yang, X.; et al. Afforestation using micro-catchment water harvesting system with microphytic crust treatment on semiarid Loess Plateau: A preliminary result. J. For. Res. 2005, 16 (1), 9–14. (12) Schlesinger, W. H.; et al. Biological feedbacks in global desertification. Science 1990, 247, 1043–1048. (13) Reynolds, J. F.; et al. Global desertification: building a science for dryland development. Science 2007, 316, 847–851. (14) Xu, J.; et al. China’s ecological rehabilitation: Unprecedented efforts, dramatic impacts, and requisite policies. Ecol. Econ. 2006, 57, 595–607.
Shixiong Cao Beijing Forestry University, College of Soil and Water Conservation, No. 35, Qinhuadong Road, Haidian District, Beijing 100083 China ES801569C
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