Melanie Lenart is an environmental scientist and writer who lives in Tucson, Arizona. She initially came to Tucson to continue her studies in forests and climate, and in 2003 received a Ph.D. in Natural Resources with an emphasis in Global Change from the University of Arizona, where she now works on a variety of climate-based projects and teaches environmental writing. Her 2010 book Life in the Hothouse: How a Living Planet Survives Climate Change takes a Gaian view that the Earth is a living system to explore the interactions between plants, climate and the landscape. For more on her, see www.melanielenart.com. Follow on Twitter @MelanieLenart. Follow on Twitter
Mention Argentina to someone outside of the country, and chances are the discussion will turn to the mouth-watering quality of the beef. I heard all kinds of recommendations about enjoying this Argentinian delicacy when I was preparing to travel to this South American country.
Farmers are quickly replacing ranchers throughout the La Plata Basin (shown in gray). Graphic courtesy of Karl Musser.
Yet once there, I learned that Argentinian ranchers have been fading from the landscape in recent decades, much like their U.S. counterparts. In fact, a switch from ranching to farming is occurring throughout South America’s La Plata Basin, which includes north-central Argentina, Paraguay, a slice of Bolivia, much of Uruguay, and a large chunk of southern and central Brazil.
In the last four decades alone, landowners have converted a third of the basin’s 760 million acre area from grasslands and forests into crops – especially soy. The economy, along with the availability of genetically engineered crops, has driven the transformation. Profits from the export of corn and soy have kept landowners in business, while the taxes on the exports provide crucial support for the governments involved.
An unexpected yield
Still, the large-scale shift to crops is having unexpected repercussions. According to research by scientist Celina Santoni and her colleagues at the San Luis laboratory headed by Esteban Jobbágy, we’re looking at one of them on this blustery winter day.
Alberto Panza, facing camera, shows Celina Santoni (front) and other scientists the river that carved through his land in 2008. Photo by Melanie Lenart.
It’s a 40-foot-deep chasm where the entrance to Alberto Panza’s 1,150-acre ranch once stood. The ripping birth of a new river – aptly named the Rio Nuevo – carved the gorge during an intense storm and flood in 2008.
Some three years after its emergence, Panza was showing the gaping results to Santoni and a few other scientists supported by the Inter-American Institute for Global Change Research, a funding group that works to unite efforts among its 19 member countries. The institute was supporting her dissertation research, and my efforts to help synthesize results from the many IAI-supported research projects in the La Plata Basin.
By then, the river had relaxed into a calmer version of its earlier ground-tearing self. It meandered quietly along a bottom about as wide as a two-lane highway, but steep slopes prevented us from reaching it. On the other side of the gash, a dozen cows placidly nibbled the grass under a stand of native calden, a drought-tolerant tree related to desert mesquites of the U.S. Southwest.
In the past, ranchers would scatter their cattle among these dry forests as well as in the country’s lush grasslands. But cattle grazing is becoming less common as both forest and grassland give way to crops.
This, in turn, is raising water tables and loosening up underground pockets of salt.
The salt deposited by the Rio Nuevo is so thick it looks like snow in some places, as on this stretch upstream of Alberto Panza’s land. Photo by Celina Santoni. Used with permission.
Santoni points out white layers sitting like a sprinkling of powdered sugar on the mocha-colored banks below. The telltale crystals marked past levels of river flow, providing compelling evidence of its heavy salt load.
Although we can’t reach the river from Panza’s land, Santoni has taken hundreds of water samples from other stretches of the still-growing Rio Nuevo, which has ripped up roads as well as ranches over the years. Her analysis of the water’s conductivity – a way of assessing salinity – suggests the Rio Nuevo contains roughly five times the amount recommended by the World Health Organization for human consumption.
Similar problems occur in other regions of dryland agriculture throughout the world, from Texas to Canada to Australia. The emerging problem in Argentina seems to stem from the same beginnings, she explains: Salt that had been accumulating under dry forests and prairies for millennia can move into the topsoil as the water table lifts them to the surface.
She asks Panza if he had observed a difference in the height of the water table on his land over the years. The change in water table height was “taller than me,” responded Panza, who stands about 6 foot tall. “The water has risen two and a half meters in the last 15 or 10 years,” he said – roughly 8 feet.
Argentina could learn from Australia’s harsh lessons, Santoni suggested, because Australians began converting another type of dry forest, eucalyptus, to agriculture more than a century ago. The results are especially apparent in the southwestern section of Western Australia.
“In the end,” Santoni said, “what this produced were huge areas of soil that was salinized, with salt crusts that made them completely useless.”
Salty soils in the western pampas
Gonzalo Laborde showing an area of his farm where salt levels are too high for crops to tolerate. Photo by Melanie Lenart.
It’s obvious when farm soil reaches the threshold of too much salt, as I learned a couple of days later. Nothing grows there.
We’re in the pampas, a classic grassland region, standing on a low-lying section of a 5,000-acre ranch-turned-farm owned by Gonzalo Laborde. Recently flooded, it still feels spongy underfoot. Rows of desiccated corn stalks shows evidence of recent growth nearby, but this parking-lot-sized patch of soil is barren. White crystals glisten in the noon light, looking from some angles like a thin sheet of ice on the brown soil.
Laborde is struggling to keep the salt patch from expanding. He and his consulting agronomist, Pablo Etcheverry, keep tabs on the water table height using a nearby measuring station marked by an upright PVC pipe. The height had dropped slightly since last month, down to 1.37 meters below the surface. Four and a half feet. Still a bit too close for comfort.
He has seen the water table rise in recent years, as he reported during a chat around the fireplace of the 1905 ranch house where he grew up, and now raises his own family.
From the perspective of former ranchers like Laborde, the government pushed ranchers into farming by its political efforts to keep beef prices low and restrict exports. In 1989, the Laborde family began making the switch from raising alfalfa and cattle to growing crops, enjoying the freedom to export all of their corn and about 90 percent of their soy. By 2000, they had sold off all their cows to focus on crop production.
The yellow highlighting shows where native forests grow at each time frame for the El Morro watershed that includes the Rio Nuevo. Black lines show where rivers and washes have appeared over the years. Graphic by Sergio Contreras, Celina Santoni and Esteban Jobbágy. Used with permission.
Laborde is enthusiastic about efforts by researchers from Jobbágy’s lab, which on this day include agricultural consultant Jorge Mercau and graduate student Eva Florio, to work with farmers to compile and analyze data at a much larger scale than an individual farm. The problem can’t start or end at the scale of an individual farm. It takes a watershed to create changes as extreme as the rise of the Rio Nuevo.
Back at the lab, Santoni showed an adapted satellite view at the scale of the El Morro watershed, where the Rio Nuevo made its appearance. Forest cover in the area had been reduced to 10 percent. Judging from aerial photos, she said, forest cover was only a fifth of what it had been in 1960, when these dry forests covered about half the watershed.
Un mapa de la napa
“The vision we have is to collaborate with the water table,” Laborde said. He and Etcheverry take monthly water table height measurements at 16 different places on the hacienda, using a satellite-based Geographic Positioning System to locate the sites.
Mapping how it fluctuates under the landscape is the first step in attempting to regulate the height of the water table, known in Spanish as la napa.
Pablo Etcheverry (front) and Gonzalo Laborde measure the height of the water table. Photo by Melanie Lenart.
The idea of creating un mapa de la napa came from the farmers themselves, as opposed to being part of the original research plan, explains Jobbágy, an ecologist and agronomist whose many publications include papers in Ecology, Nature and Science as well as Argentina’s popular Ciencia Hoy. A principal investigator who has been receiving IAI support since 2002, he said the institute encouraged him to collaborate with “stakeholders,” an English word adopted into Spanish to mean any person with a stake in issues that could benefit from scientific investigations.
“Some of the amazing things that happened is that today, farmers are collecting data that we are using in our research – really good data. They are partners in this research. This was not conceived in the beginning but it became an opportunity that we took,” Jobbágy told me back at the Environmental Studies Laboratory he heads in San Luis. In a later interview, he added, “Suddenly we can have 10 times more sampling points.”
Interpreting the data
Now comes perhaps a bigger challenge: explaining to collaborating and otherwise curious farmers how the rise of a river and emerging patches of salty soil relate to the switch from ranching to farming. Most locals blame the river’s rise on an increase in rainfall and a 1977 earthquake in San Juan Province.
I asked Jobbágy about this one afternoon around the blue ping-pong table that serves as the lab’s lunch table between occasional friendly competitions. Jobbágy considers it extremely unlikely that the earthquake shifted the underground topography enough to set the stage for the emergence of the river more than 30 years later. Earthquakes have been known to unleash new rivers, he acknowledged, but they typically do so within days or weeks, not decades later.
Meanwhile, scientists and farmers generally agree that an increase in regional rainfall has contributed to the rise of the water table. But they differ in their opinions on the extent of this influence. Average annual rainfall in the past few decades has increased by 20 percent or more compared the long-term average of about 24 inches a year, based on records going back to the 1920s.
But Jobbágy and other scientists see the rainfall increase itself only as a contributing factor, not the main reason for the river’s creation. If anything, he’s more likely to see a connection because several decades of above-average rainfall have helped inspire the expansion of crops in the region. The farms we visited and most others in the area lack any irrigation systems, and so rely mainly on rainfall.
Unlike many farmers in the area, Laborde already saw a connection between land use changes and the height of the water table.
“The water table is close to the surface throughout this region. Lately, it has risen,” Laborde explained. “And the vision that we have is that it’s risen a bit – or a lot – because of the management by humans. With pasture or forest, the water table is lower. With pure agriculture, the groundwater has moved its ceiling a bit higher. At any moment, the water table can rise enough to bring problems of salinization.”
Where’s the salt?
Laborde’s understanding fits in well with the science coming out of Jobbágy’s lab. There, researchers have found that native vegetation, especially trees, will latch onto incoming water, leaving little or none to trickle down into the water table below.
The green-barked chañar tree is another native species (Geoffroea sp.) being displaced by deforestation. Photo by Melanie Lenart.
In the past, native forests would respond to years of higher than usual rainfall by growing faster. Annual growth of many trees registers in their tree rings, with wet years showing up as years of abundant growth. This is why researchers often use variations in tree-ring width to estimate precipitation rates for the years before historical record-keeping. It’s also why the water table under dry forest on its own remains roughly stable regardless of typical climate variability.
Crops, however, don’t use nearly as much water as trees. They even use far less than perennial grasses, whether alfalfa or pampas plants. For one thing, crops grow for only a season. More water gets past bare soil than roots to seep into the water table below.
But where does the salt come from? Salt occurs naturally in the soil, and also can arrive with maritime winds. In humid environments, rejected salts wash out with the water draining through the soils. In dry environments, though, vegetation can get in the middle and interrupt this would-be downward flow.
Forests not only mine all the water that rains down on the site, they also reject most salts in the process. According to research reported by Santoni, Jobbágy and others in a 2011 Ecological Applications article, these salts build up under the root zone, creating pockets some 10 to 40 feet below the surface. Salts can also accumulate under grasslands, especially in low-lying areas.
By comparing the soil profile under intact forests and nearby crop parcels with known dates of initiation, the researchers were able to show the general dissolution of the salt pockets at 15 to 30 years after the switch from native forests to crops.
The transformation of land from ranching to farming began in the humid Pampas in the 1960s and spread outward in the 1970s. It has accelerated in the past 15 years, in the El Morro watershed and throughout the La Plata Basin. This means it could just be reaching critical mass for watershed-scale changes in the water table height and associated delivery of salt.
Native calden trees (Prosopis caldenia) bulldozed in San Luis will be converted into charcoal, leaving more land clear for potential crops. Photo by Melanie Lenart.
“The idea is to anticipate this,” Santoni explained. “Then, perhaps we can propose management techniques to conserve part of the natural forest within agricultural zones. Having islands of forest within crop zones might help avoid these drastic changes.”
None of the farmers the researchers have talked to are interested in turning back the clock and shifting back from crops to cattle, or in planting trees on land they could otherwise be farming. And it’s a fine line between having water near the surface, where crops can benefit from it, and too near the surface, where it can flood and bring damaging salt into topsoil. A dry summer in 2012 left wilted corn and reduced crop yields in the area, Laborde said. This could allay some farmers’ concerns about the rising water table in the short term.
Still, Jobbágy and others hope to work with farmers and government officials to come up with some acceptable solutions that could keep agriculture in the region viable over the long term.
It would be much easier to stabilize the water table before it crosses a threshold than try to correct it afterward. For instance, modeling research in Australia suggests a 15 or 20 percent cover of well-placed perennial vegetation across the landscape might do the trick. On the other hand, Jobbágy noted, Australians have found they may need to return 70 percent or more of an area to forest to restore it once the water table has gotten too close to the surface. Once the salinity threshold has been crossed, the soil will support only salt-tolerant species – which do not include corn and soy.
A less salty solution
The researchers are still working out the percent of perennial plant cover that might be needed to stabilize the area around San Luis. They recognize that farmers might be more willing to consider planting forage species than trees, so they’re trying to provide some agricultural solutions.
Santoni, for instance, is modeling whether planting alfalfa – a high-quality feed that yields high-priced beef – could help alleviate or prevent the kind of dramatic changes faced by some local farmers. Alfalfa roots can penetrate some 20 feet down, making them useful in the effort to move water up and out of the soil.
Jobbágy is working with colleagues to consider whether double-cropping – which typically means planting winter wheat during the region’s mild winters – could help consume enough water to keep the water table at bay. Although most of the annual rainfall comes during the summer, the lower evaporation rates during winter can permit rainfall to slip below the root zone – especially if ground cover is sparse or non-existent.
“One of the messages is that farmers, knowingly or unknowingly, are responsible for some of the hydrology in the region,” Jobbágy said. While some may not be thrilled to hear that their activities could have negative consequences, there’s a positive side to this knowledge. “The good news is that we are helping them to manage an amazing resource.”
Laborde, for instance, was making efforts to adopt new management techniques to protect the water table. He was leaving plant residues – such as the desiccated corn husks covering the ground near the salt patch – as a layer of beneficial litter that protects the soil. He was growing winter wheat on some of his land to help transpire water during the cool season, when many farmers don’t plant anything. He even had alfalfa growing on a small area, mainly to indulge the horses he keeps because of his passion for polo.
He’d like to see other farmers join in with such efforts, even though growing winter wheat and alfalfa do little to boost the bottom line.
While it’s not likely that farmers or the governments involved will be interested in braking agricultural engines that are driving economic recovery, Jobbágy and his team hope to generate interest in maintaining enough year-round plants in place – even if they’re alfalfa rather than native trees – to keep agriculture sustainable.
“For me, a world that’s producing more and more grain – I like that” because it can feed more people, he said. “I don’t see it as a bad idea to get more from our ecosystems. We have a lot to gain, and we can mitigate some of the losses.”
Links and References:
Inter-American Institute for Global Change Research.
Environmental Studies Laboratory (GEA, Grupo de Estudios Ambientales) in San Luis, Argentina
Jayawickreme, D.H., C.S. Santoni, J.H. Kim, E.G. Jobbágy, and R.B. Jackson, 2011. Changes in hydrology and salinity accompanying a century of agricultural conversion in Argentina. Ecological Applications 21(7): 2367-2379.
Jobbagy, E.G., and R.B. Jackson, 2004. Groundwater use and salinization with grassland afforestation. Global Change Biology 10: 1299-1312.
Santoni, C.S., E.G. Jobbágy, and S. Contreras, 2010. Vadose zone transport in dry forests of central Argentina: Role of land use. Water Resources Research 46. Doi: 10.1029/2009WR008784.