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Carbon Sequestration May Have Negative Impacts on Ecosystem Health Yafeng Wang† and Shixiong Cao*,‡ †
State Key Laboratory of Urban and Regional Ecology, Research Center for Eco-Environmental Sciences, CAS, Beijing 100085, P. R. China ‡ Key Laboratory of Soil and Water Conservation & Combat, Beijing Forest University, Beijing 100083, P. R. China
Figure 1. Afforestation practice in a wet region of China’s Fujian Province, where precipitation averages 1730 mm yr-1 and the annual temperature averages 18.3 °C, documents that overall vegetation cover and species richness will decrease and soil erosion will increase when natural ecosystems are afforested using inappropriate species.
Rhonda Saunders
C
hina, the world’s most populous country, has become a major emitter of greenhouse gases.1 To reduce the carbon footprint, China would combat climate change by planting more trees, and commit to expanding the country’s forested area by 40 million ha and its stocking volume by 1.3 billion m3 from 2006 to 2020.2 Yin et al. 2 estimated that China’s cumulative carbon emissions from fossil fuel combustion between 2006 and 2020 will be more than 32 Gt, with a baseline of 1.6 Gt in 2005. Thus, the official target of increasing the carbon stock by 683 Mt of woody biomass accounts for only 2% of these cumulative CO2 emissions, even after excluding emissions of other greenhouse gases (e.g., methane, nitrous oxide). The carbon sequestration expected to result from China’s large-scale afforestation projects was a good goal, but may have negatively affected ecosystem health. For example, extensive efforts to plant trees in many arid and semiarid regions have caused environmental deterioration when the trees consumed too much of the limited soil moisture, resulting in reduced overall vegetation cover and species richness and increased wind erosion.3 Monitoring data show that northern and northeastern China’s drier regions are receiving less and less precipitation (a 12% decline since 1960).1 Hence, the areas that are currently marginally suitable for afforestation may soon become unsuitable. Thus, excessive reliance r 2011 American Chemical Society
on afforestation to implement China’s ecological and carbon sequestration programs is risky in arid and semiarid regions. Also, the mean forest stocking level in 2005 (67.2 m3/ha) is very low compared to the international average of 110 m3/ha.2 China’s afforestation programs require a closer examination. Even in parts of southern China with high rainfall, the rapid growth of the trees can prevent the establishment of ground vegetation cover and reduce species richness (Figure 1). Although afforestation may initially increase forest cover, its impact on biodiversity and ability to achieve ecological restoration will depend on the suitability of the tree species for the local environment. In addition, large portions of the afforestation areas are dominated by monocultures that are susceptible to insect and disease problems.3 Planting trees without protecting the understory vegetation can also potentially increase water’s erosive energy by increasing raindrop size through rain-canopy interactions, thereby accelerating soil erosion.3 Unfortunately, the ecological and socioeconomic impacts of the coupling between humans and nature may not be immediately obvious or easily predictable because of time lags in their impacts. Published: February 10, 2011 1759
dx.doi.org/10.1021/es200042s | Environ. Sci. Technol. 2011, 45, 1759–1760
Environmental Science & Technology Conventionally, preventing deforestation and promoting afforestation are considered to be ways to slow global warming. Unfortunately, latitude-specific deforestation experiments4 have revealed that afforestation projects may be counterproductive at high latitudes and may offer only marginal benefits in temperate regions; this is because the climate effects of CO2 storage in these forests may be offset by albedo changes. Thus, large-scale afforestation in temperate zones may not mitigate global warming.4 In addition, forests are believed to reduce flooding by acting as sponges that trap water during heavy rains, then release it slowly into streams, thereby lessening the severity of floods and maintaining stream flows during dry periods. However, Laurance5 concluded that the evidence that forests reduce flooding is weak, especially for the largest and most devastating floods. Laurance suggested that retaining or regenerating large forest areas was an economically dubious strategy for developing nations from a flood-reduction perspective. The above-mentioned factors suggest that the preservation or restoration of existing ecosystems should be the primary goal of carbon sequestration, and that the destruction of these ecosystems by large-scale afforestation may be counterproductive.4 Ecosystem management and the promotion of human well-being should be integrated based on the recognition that biodiversity conservation and human needs are complementary goals. This integration requires building the capacity to deal with multiple objectives, the use of deliberative processes, learning from research on “the commons”, and developing a complexity-based approach for governance of the commons.3 Thus far, however, governments have emphasized administrative campaigns and have failed to consider socioeconomic incentives or the importance of market-based mechanisms (e.g., contracting, open bidding) for implementing their policies.2 There are no panaceas, so it is necessary to explore a wider range of alternatives. Monolithic and inflexible solutions such as wide-scale afforestation can increase risk. The potentially limited carbon sequestration provided by large scale afforestation and the potentially large risk to ecosystem health suggest that other scientific, technical, and socioeconomic solutions may be a more effective way to combat climate change.
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synthesis, and applications. Crit. Rev. Environ. Sci. Technol. 2011, 41, 1–19. (4) Bala, G.; Caldeira, K.; Wickett, M.; Phillips, T. J.; Lobell, D. B.; Delire, C.; Mirin, A. Combined climate and carbon-cycle effects of large-scale deforestation. Proc. Natl. Acad. Sci. U.S.A. 2007, 104, 6550–6555. (5) Laurance, W. F. Forests and floods. Nature 2007, 449, 409–410.
’ AUTHOR INFORMATION Corresponding Author
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[email protected]; Tel.: 86-10-6233-6324; Fax: 8610-6233-7156.
’ ACKNOWLEDGMENT This work was supported by the Fundamental Research Funds for the Central Universities (BLJC200908) and National Natural Science Foundation of China (Nos. 40901098). We thank Geoffrey Hart (Montreal, Canada) for editing an early version of this paper. ’ REFERENCES (1) Piao, S.; Ciais, P.; Huang, Y.; Shen, Z.; Peng, S.; Li, J.; Zhou, L.; Liu, H.; Ma, Y.; Ding, Y.; Friedlingstein, P.; Liu, C.; Tan, K.; Yu, Y.; Zhang, T.; Fang, J. The impacts of climate change on water resources and agriculture in China. Nature 2010, 467, 43–51. (2) Yin, R.; Sedjo, R.; Liu, P. The potential and challenges of sequestering carbon and generating other services in China’s forest ecosystems. Environ. Sci. Technol. 2010, 44, 5687–5688. (3) Cao, S. Impact of China’s large-scale ecological restoration program on the environment and society: Achievements, problems, 1760
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