New evidence proves diverse forests control erosion

Scientists in China have used a new method to tease apart the causes of erosion in a landscape. The results show that diverse forests reduce erosion dramatically

 

Researchers at the World Agroforestry Centre’s East and Central Asia program have designed a new way to use a locally calibrated model of water and sediment flows in a given landscape to explore the relative impact on a watershed of recorded land-use changes as well as climate variables.

Forest, watershed, erosion, protection, Viet Nam, Ba Be

Lake Ba Be in northern Viet Nam: diverse forests protect watersheds. Photo: World Agroforestry Centre/Thomas W. Wilson

“The specific attribution to terrain, climate and human activities of changes in sediment transfer is important for policy-making”, said Dr Ma Xing, first author of the study and a scientist at the Yunnan Institute of Environmental Sciences who has been seconded to the World Agroforestry Centre’s East and Central Asia program. “It has wide implications for watershed management. In our research, we analyzed a unique data set from the Kejie watershed in the upper Salween River in Yunnan Province, China, where river flow and sediment concentration data go back to 1965, along with weather records and maps that allow the analysis of land use change”.

The researchers split a data set of various measurements into two periods: 1970–1985; and 1986–2010. There were some big differences. Most dramatically, the net annual sediment transport in waterways in the second period (8.3 t ha1 yr1) was 40% of that in the first (13.7 t ha1 yr1), with a trend towards further reduction.

By recombining land-use and climate data for the two periods in model scenarios, the researchers found that 47.8% of the decrease was due to land-use and land-cover changes, that is, increased diverse forest cover, 19.8% to climate change (owing to milder rainfall patterns) and 26.1% to watershed engineering measures. The remaining 6.3% was due to the simulation bias in the model.

“Moreover, mean annual suspended sediment yield decreased drastically with the increase of forest cover, making diverse forest cover one of the most effective ecosystems to control erosion”, said Dr Jianchu Xu, co-leader of the research study and coordinator of the Centre’s program in East and Central Asia.

Dr Meine van Noordwijk, Chief Science Advisor with the Centre and co-leader of the study, added that, “Of course, the approximate 1:2:1 ratio that we found in this study for climate change, land-cover change and engineering is specific to the case study but the approach we used for teasing apart the various contributions to the recorded impact is replicable. It can help make the public debate more based on evidence”.

Put simply, erosion is soil particles moving in a landscape. It is an enormous problem (but also a beneficial legacy) in many part of the world. It leads to reduced soil fertility and lower yields from crops, which restricts food supply. Erosion also clogs rivers and water reservoirs with sediment, reducing the quantity and quality of water for domestic and industrial uses. However, downstream benefits from past erosion and sedimentation processes are also evident in a large part of the global rice production systems.

Still, the primary association of erosion is soil degradation and a loss of soil fertility. Globally, a lot of effort has been spent on controlling sediment levels in streams but there is continuing debate on how effective the various measures are and how engineering interventions in a river bed (including check dams and reservoirs) interact with changing land cover and a changing climate.

Teasing apart the relative contributions of these factors in actual case studies is not easy, hence the importance of this new way of organizing the data.

Read the journal article

Ma X, Lu X, van Noordwijk M, Li JT, Xu JC.  2014. Attribution of climate change, vegetation restoration, and engineering measures to the reduction of suspended sediment in the Kejie catchment, southwest China. Hydrological Earth System Science 18:1979–1994

 

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This work links to the CGIAR Research Program on Forests, Trees and Agroforestry

 

rfinlayson@cgiar.org'

Rob Finlayson

Robert Finlayson is the Southeast Asia program's regional communications specialist. As well as writing stories for the Centre's website, he devises and supervises strategies for projects and the countries in the Southeast Asia region, including scripting and producing videos, supervising editors and translators and also assisting with resource mobilization.

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