In their Correspondence, Cao et al. suggest that afforestation projects in drylands could be exacerbating environmental degradation (Nature 465, 31; 2010). However, afforestation in China does not only involve tree planting: planting shrubs for environmental protection is one of the many afforestation activities defined by the Forest Law of the People's Republic of China (see http://go.nature.com/pgZqRY). Environmental changes resulting from tree planting in specific regions cannot be extrapolated to entire afforestation projects.

Most water from atmospheric precipitation cannot be effectively conserved in situ even if it is not partially consumed by natural or artificial vegetation. Serious soil erosion and environmental disasters, such as floods caused by run-off, are key environmental degradation problems in drylands, especially in the Loess Plateau in northwestern China. Scientists and policy-makers need to recognize the inherent complexities of dryland ecosystems so that the effects of the indisputable trade-off between soil-erosion control and water conservation can be minimized.

The low resilience of dryland ecosystems means that natural restoration is extremely difficult. Afforestation is one way to reconstruct almost natural dryland ecosystems. We do not deny that some afforestation efforts have had suboptimal results, caused by the use of unsuitable tree species, or inappropriate initial densities or afforestation methods. Also, some official reports or data from case studies have overestimated or exaggerated the environmental effectiveness of afforestation activities in drylands. However, forest ecology projects have undoubtedly contributed to environmental improvement and have therefore been promoted for the sustainable development of drylands in China.

A more scientific and feasible assessment system of these projects should be established to provide more accurate information for the public.