The Pampas of Argentina and Uruguay is one of the largest uncultivated grasslands in the world. Grasses have dominated the Pampas for at least three thousand years. Starting in the 19th Century eucalyptus trees were planted on small areas, for shade on cattle ranches and for construction materials. Today, the pulp and paper industry and the carbon offsets industry are expanding their operations in South America. Increasingly, they are targeting grasslands for conversion to large-scale industrial tree plantations.
Robert Jackson, Professor of biology at Duke University, has spent many years researching the impacts of plantations on water. "Extensive tree establishment could compromise groundwater replenishment at the landscape scale, making its use transient and producing a widespread depression of groundwater", he wrote in a 2004 paper published in Global Change Biology. Written with his colleague Esteban Jobbágy the paper is based on a comparison of grassland and adjacent plantations in the Pampas in Argentina. As well as lowering ground water, they found that "The conversion of grasslands to plantations in the Pampas triggered intense soil and groundwater salinization in areas with intermediate texture sediments, the most common soil type in the region."
In the Pampas, shallow freshwater lenses are used to provide drinking water, but below these lenses is brackish groundwater below plantations. Tree plantations suck up the deeper groundwater bringing salts to the surface. Plantations also affect soil nutrients, depleting calcium, magnesium and potassium but enriching sodium, leading to more salty soil.
"A landscape with deep and salty groundwater would be a likely outcome of a massive tree establishment in the Pampas," Jackson and Jobbágy warn.
In December 2005, Jackson was the lead author of a report published in Science magazine, titled "Trading Water for Carbon with Biological Carbon Sequestration". Jackson and his colleagues looked at data comparing the chemistry of soils in grasslands or shrublands with those in adjacent plantations in 16 countries. They studied stream flow data from 26 long-term catchment studies (with more than 500 annual observations) comparing grassland, shrubland or agricultural catchments with plantations. They also conducted their own research in Argentina.
"Carbon sequestration strategies highlight tree plantations without considering their full environmental consequences," Jackson and his co-authors wrote. Their report documents that replacing grassland and shrubland with plantations results in "substantial losses in stream flow, increased soil salinization and acidification".
"Within a decade," Jackson said in a 2005 interview with National Public Radio, "tree plantations reduce stream flow by about one-half compared to the shrublands or grasslands they replaced and about one out of every eight streams dried up completely for a full year or more." More than one-fifth of the catchments experienced reductions in runoff of 75 per cent or more for at least one year.
"Plantations not only have greater water demands than grasslands, shrublands, or croplands," note Jackson and the international team of scientists in Science, "they typically have increased nutrient demands as well. These demands change soil chemistry in ways that affect fertility and sustainability."
In another report published in 2005, Jackson and his colleagues found that "Eucalypts had a larger impact than other tree species in afforested grasslands, reducing runoff by 75 per cent, compared with a 40 per cent average decrease with pines."
Particularly important is the impact of tree plantation on dry season water flows: "Changes in low flow may be even more important than changes in annual flow, as the dry season is when reduced water supply will have the most severe effects for users, particularly in arid and semiarid regions."
In a report published last year, Jobbágy and Jackson looked at the impact of eucalyptus plantations on soil chemistry in the Argentinian Pampas. Their findings upheld what they'd previously found. Tree plantations "showed a widespread and homogeneous salinization of groundwater and soils at all study sites". Jobbágy and Jackson report that "Compared to their surrounding grasslands, tree plantations . . . had shallow ground waters that were 15 to 30 times saltier."
To farmers and villagers living near industrial tree plantations, all these statements from peer reviewed scientific journals are statements of the obvious. But farmers and villagers tend not to dig boreholes and collect samples of soil and ground water to be send off for analysis in laboratories. Neither do they produce reports for publication in scientific journals. Instead, they notice when their crops won't grow or when their wells dry up. The best way of preventing these problems is to stop the further expansion of the industrial tree plantations - before Jackson and Jobbágy's warning of "A landscape with deep and salty groundwater" becomes a reality in the Pampas.
By Chris Lang, http://chrislang.org
