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Response to Comment on “Why Large-Scale Afforestation Efforts in China Have Failed to Solve the Desertification Problem” As Prof. Liu noted, the forested areas of China have traditionally been distributed east of the 500 mm precipitation isoline, which comprises China’s humid or semihumid regions (1). Sankaran et al. report in Nature that trees will not become established without irrigation if rainfall is below 100 mm and that they are unlikely to form closed-canopy woodland with less than 650 mm of rainfall (2). Obviously, understanding the amount of precipitation that is required to sustain forests is crucial, as this will define the boundary between areas capable of supporting forestry and those that are only capable of sustaining grassland or other vegetation types. This has clear relevance in identifying areas suitable for large-scale afforestation. In this response, I will provide additional support for and clarification of the opinions I expressed previously.
The Precipitation Isoline for Afforestation When precipitation is lower than potential evaporation, surface soil moisture cannot sustain forest vegetation, and shrubs or steppe species replace the forest to form a sustainable natural ecosystem that will exist in a stable equilibrium with the available water supply. Unfortunately, afforestation is being practiced in many areas of China where precipitation is insufficient to support forests, greatly reducing tree growth or even leading to extensive tree mortality (3, 4). Water requirements of tree species varies widely. For example, Populus tremula is ecologically similar to Populus tremuloides, which is typically found where precipitation is at least 180 mm per year, and more often found with precipitation nearer to 1000 mm (5). This suggests that planting it in areas with low precipitation is unlikely to be a wise choice. Inappropriate afforestation in parts of arid northern China has decreased soil moisture (3, 6), and inappropriate species selection seems unlikely to produce a stable equilibrium with the available water supply (3). For many of the trees species that have been used (e.g., Populus tremula), the trees are initially capable of exploiting deep soil water when surface water is inadequate, but this only delays the inevitable; drying of both the surface soils and deep soil water ensues, leading to the lowering of the water table, and when tree roots can no longer reach deep soil water, tree mortality occurs (6, 7). Historically, large-scale afforestation in areas where precipitation is lower than potential evaporation has failed or shows signs of future failure (4, 6).
Implications for Practice Drought is a major constraint worldwide to the production of common vegetation types, and revegetation of arid regions such as those in northern and northwestern China is primarily water-limited (8). Moving from the Loess Plateau to the Mongolian Plateau, forest vegetation is replaced by shrubs, grasses, and other steppe or prairie vegetation, as well as by
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herbaceous species, as the annual precipitation decreases (1). As Prof. Liu notes, the climate of much of northwestern China appears to be unsuitable for afforestation owing to the low rainfall. Recognizing this, some scientists and local governments have implemented successful engineering efforts (9) and grassland rehabilitation (10) as alternatives to afforestation (11). These successes illustrate that there are alternatives to afforestation, and that recognizing where water availability may be too low to support afforestation makes it possible to seek alternatives that are better suited to the local site and environmental characteristics. Important steps in any such effort would be to learn what the natural vegetation community is in a region, and to compare the available water with the water requirements of the species being proposed for use in afforestation. Where the natural climax community is steppe or other nonforest vegetation, or where water requirements of a species seem to be higher than the long-term mean precipitation in a region, this is a strong clue that alternatives to afforestation (or the use of species with greater water-use efficiency) should be considered. To support such decision-making, Chinese researchers should develop a resource such as the USDA’s Silvics of the forest trees of the United States (5) that can guide them in species selection.
Literature Cited (1) Liu, X. The Problem of Forestry Ecology. China Agric. Res. Plan 2005, 26( (2)), 14–17, in Chinese. (2) Sankaran, M.; et al. Determinants of woody cover in African savannas. Nature 2005, 438, 846–849. (3) Wang, G; Liu, Q.; Zhou, S. Research advance of dried soil layer on Loess Plateau. J. Soil Water Conserv. 2003, 17( (6), 156–169, 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) United States Department of Agriculture. Silvics of North America. Agriculture Handbook 654, http://www.na.fs.fed.us/ spfo/pubs/silvics_manual/table_of_contents.htm. (6) Cao, S.; et al. Impact of Grain for Green Project to nature and society in north Shaanxi of China. Sci. Agric. Sin. 2007, 40, 764–771, in Chinese. (7) Duan, Z.; et al. Evolution of soil properties on stabilized sands in the Tengger Desert, China. Geomorphology 2004, 59, 237– 246. (8) Jackson, R. B.; et al. Ecosystem carbon loss with woody plant invasion of grasslands. Nature 2002, 418, 623–625. (9) Feng, L.; Lu, J.; Di, Y. Review on the prevention of sand damage to railway lines in desert areas of China. J. Desert Res. 1994, 14 (3), 47–53. (10) Jiang, G.; Han, X.; Wu, J. Restoration and management of the Inner Mongolia Grassland requires a sustainable strategy. AMBIO 2006, 35, 269–270. (11) Xu, J.; et al. China’s ecological rehabilitation: Unprecedented efforts, dramatic impacts, and requisite policies. Ecol. Econ. 2006, 57, 595–607.
Shixiong Cao College of Water and Soil Conservation, Beijing Forestry University, Beijing, China ES801979E
10.1021/es801979e CCC: $40.75
2008 American Chemical Society
Published on Web 09/23/2008
