Are restoration efforts in China halting soil erosion?

Immense effort has been put into combating soil erosion in China over recent decades, but how effective have these been?

A new study published in Critical Reviews in Environmental Science and Technology found a strong positive relationship between vegetation cover and soil erosion reduction benefit.

Interestingly, the authors discovered that when vegetation cover is below 60 per cent, the soil erosion reduction benefit increases sharply with increasing vegetation cover. But when cover exceeds 60 per cent, the benefits may be diminished.

Says co-author, Jianchu Xu from the World Agroforestry Centre, “This implies that vegetation restoration programs should probably aim for only partial vegetation cover”.

Xu warns, however, that because of the variation which occurs in the natural environment, one has to be careful not to universally apply this percentage. “Each location has its own specific characteristics in terms of geographical conditions, vegetation species, forest/grass ratio, age of species and management regime.”

The critical vegetation cover percentage is also determined by factors such as rainfall characteristics, soil type, slope and rainfall intensity. The spatial distribution of vegetation is also significant.

In the study, conducted by scientists from the World Agroforestry Centre, National University of Singapore and the Kunming Institute of Botany, a wide range of different sites were analysed to represent the varying climatic zones in China.

Soil erosion is second only to population growth as the biggest environmental and public health problem facing human society; costing an estimated US $400 billion per year globally. In China, top soil is being lost 30 to 40 times faster than the natural replenishment rate as a result of rapid development. Soil erosion is of particular concern in the upper and middle reaches of large river basins such as the Yangtze River, Yellow River and Pearl River.

“Soil erosion is seriously affecting China’s agricultural productivity, with degraded soils and water quality hindering economic and social development,” explains Xu. “While efforts to treat soil erosion have accelerated in the past two decades, the situation is still far from optimistic.”

Xu and colleagues found that the species chosen for vegetation restoration were instrumental in determining the success of soil erosion reduction programs. For example, in the arid and semi-arid north of China, grasses with only a few trees are preferred as these do not consume large amounts of water. In the relatively humid south China region, trees are preferred, especially because many can also provide economic benefits to local communities.

The age of vegetation is also important. The scientists found that soil erosion reduction benefit increases gradually with the age of certain trees, highlighting the importance of preserving older trees or shrubs from clearance.

The study also attempted to review the vexed issue of whether vegetation restoration or engineering measures – such as terracing and silt check dams - is more effective in controlling soil erosion; both of which are given equal importance in China.

“Overall, vegetation measures could not completely replace engineering measures, especially in large areas with strong gully and/or gravitational erosion,” outlines Xu. “Both have their own advantages and disadvantages, and each approach has a role to play in tackling soil erosion and water loss in different locations of a basin”.

Several recommendations are provided in the study, including ensuring only the most suitable species are used in vegetation restoration. The authors emphasize that successful restoration in one location cannot necessarily be duplicated in another area. They recommend a mix of different species, each playing to the others’ strengths in controlling soil and maximizing conservation benefits.

The scientists believe that to more fully understand the complex relationship between vegetation cover and soil erosion, more research should be conducted on a larger spatial scale as well as investigations into shorter dynamics such as on a seasonal, monthly or event-basis. They also recommend the development of a better assessment system than the current one based on simple statistical methods.

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Ran L, Lu X Jianchu Xu. 2012. The effects of vegetation restoration on soil erosion in China: a review. Critical Reviews in Environmental Science and Technology p 1-28.