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Agriculture dries out the Cerrado

Satellite images of Cerrado region: agricultural area doubled between 2003 (top) and 2013

Mustard Lab / Universidade BrownSatellite images of Cerrado region: agricultural area doubled between 2003 (top) and 2013Mustard Lab / Universidade Brown

The more the native vegetation of the Cerrado gives way to agriculture, the lower becomes the volume of rainwater available to these crops.  The warning comes from a study conducted by Stephanie Spera of Brown University (USA) and her collaborators, including Brazilian ecologist Marcia Macedo from the Woods Hole Research Center (Global Change Biology, March 29, 2016).  The researchers analyzed satellite images taken over an 11-year period in the Matopiba, a region spanning the Brazilian states of Maranhão, Tocantins, Piauí and Bahia.  The photos revealed a major encroachment of the agricultural frontier into the area.  In 2003, 1.2 million hectares of crops were visible.  By 2013, agriculture had occupied 2.5 million hectares.  Three quarters of the expansion in agricultural areas took place on lands once covered by native vegetation, almost all of it Cerrado.  Also based on the satellite images, the researchers assessed the amount of water released into the air by plant leaves in a process called evapotranspiration.  During the rainy season, from October to April, when crops are growing, evaporation in agricultural areas is similar to that seen in areas covered by native vegetation.  The problem emerges during the dry months, between harvests.  In the dry season, the volume of evapotranspiration in agricultural areas averages 60% lower than in areas with native vegetation.  The risk is that this lack of moisture in the air may worsen droughts and ultimately postpone the start of the rainy season, shortening the productive period of the year.  Because moisture circulates through air currents, the authors fear that the effects of these droughts may not be limited to the Cerrado and could end up affecting the Amazon.  One way of mitigating the problem would be to plant two crops per year on the same soil, such as summer-harvest soybeans followed by late-harvest corn.  This practice makes for a longer period of plant growth and may cause less of a reduction in evapotranspiration.