When María Muñoz wasn’t taking care of her 4,000-acre cattle ranch in southeastern Uruguay, she liked to sit on the couch and look out at the rolling landscape. “You could see hills: hills and hills and more hills,” her eldest son, Pancho, remembers. “You could see very far.” This was 20 years ago, when cattle and sheep picked their way between splotchy gray and white rocks on the family land, and the occasional stubby tree stood amid tough grasses and prickly bushes.

Then, in the early 1990s, beef prices dropped. With two of their three sons still in college, Muñoz and her husband, Francisco Ferber, were struggling to make ends meet. So Ferber suggested a new business plan: planting 1,000 acres of trees, which could be harvested within a decade and sold for firewood, paper, poles, and posts.

The benefits and subsidies offered by Uruguay’s Forestry Law of 1987 made the option tempting. The law forgave property taxes on land covered with native forest or a tree farm—so, by covering half their land with eucalyptus, the Ferbers could dramatically reduce their taxes. Furthermore, the law subsidized the cost of planting the trees and guaranteed loans from the national bank, so the Ferbers would hardly have to put down any of their own cash. And they wouldn’t have to give up ranching. Once the eucalyptus saplings were strong enough to withstand the cows’ clomping hooves, the Ferbers would be able, once more, to graze animals all over their land—perhaps with better results than before, since the trees could provide shade in the summer and act as a windbreak in the winter.

Today, the Uruguayan Forestry Law that spurred the Ferbers to plant trees on their land holds a new, international interest, as environmental policy-makers at the UN Climate Change Conference in Paris consider a similar plan: Initiative 20×20, a plan to plant new forests and tree farms on 50 million acres (20 million hectares) of South American land by 2020. So far, the initiative’s organizers have secured commitments from governments and investors to cover 28 million hectares, or 70 million acres, of land with trees. That’s a land area one and a half times that of the entire country of Uruguay.

Planting trees locks up carbon in the trunks—for a long time, if they’re left standing or used to make furniture or buildings. If they’re used for toilet paper or paper napkins, the carbon returns to the atmosphere as soon as the product starts to decompose—but even that can have its advantages, because tree farms can help take pressure off old-growth forests. While nearly 100 percent of the world’s wood products came from naturally occurring forests in 1950, less than 50 percent do today, says Roger Sedjo, the director of the Forest Economics and Policy Program at Resources for the Future, a Washington, DC-based think tank. A practice called tree—planting trees to harvest and sell—makes up the difference.

Policy-makers around the globe have tried numerous approaches to try to keep old trees standing and to plant new ones: They’ve paid locals to care for trees, paid countries to store carbon, paid communities to preserve biodiversity. And yet deforestation in Latin America, Asia, and Africa is advancing at an alarming rate. It releases 2.9 gigatons of carbon into the atmosphere every year—more than emitted by all the world’s cars, trucks, trains, and airplanes combined. In Uruguay’s giant neighbors, Argentina and Brazil, at least 15 percent and 10 percent of the forests, respectively, have disappeared since 1990. “Almost half of Latin America’s greenhouse gases come from land use in Latin America,” says Walter Vergara, who has worked at the World Bank and is now spearheading Initiative 20×20 as a senior fellow at the World Resources Institute. “Planting trees, reforesting, restoration are an essential element for Latin America to be able to reduce its carbon footprint.”

Initiative 20×20 is a new stab at the problem: an attempt to use a combination of private funding and carbon credits to reforest large swathes of South American land. Vergara hopes the 70 million acres will sequester about 1.8 gigatons of carbon.

But a close look at the Uruguayan experience reveals that planting a tree is not always a good thing. It depends on what species gets planted and where, who plants it, and who owns the land. Thus, even with the best of intentions, without local land ownership and a mechanism to ensure that each tree-planting project under the umbrella of Initiative 20×20 is proceeding as planned, the unintended consequences could be severe.

* * *

At La Cascada, María Muñoz objected to planting trees. She was concerned they might drain the estate’s groundwater, invade the natural grassland, and erode the topsoil. But, more than anything, she worried about destroying the landscape.

“If tomorrow, I want to look out and all I can see are trees… she didn’t really like that idea,” Pancho explains. The idea of change “worried Mamá the most, because she was the one who enjoyed the view the most.”

The Ferbers’ argument about trees dragged on throughout 1994. The thing is: Their decision would be final. “You can plant a pasture for your cattle today and then leave it fallow next year,” Pancho explains. But a single cycle of planting and harvesting trees takes a decade or more, and both eucalyptus and pine trees acidify the soil, so, even after harvesting them, other crops won’t grow. As Pancho says, “Once you plant a tree farm, there’s no going back.”

All over the country in the early ’90s, cattle ranchers struggling to get by were debating their options. At the time, few of them considered planting their own trees; more often, if they decided they couldn’t make it on ranching, they “sold their land, paid off their debts, and then bought land elsewhere,” says Fernando García Préchac, an agronomy professor at Uruguay’s University of the Republic. Cattle ranches gave way to tree farms, especially in western Uruguay, close to the rivers where companies would soon build pulp mills, and in northern Uruguay, close to the railroads.

In those regions, multinational companies such as the Anglo-Dutch Shell, the Finnish United Paper Mills-Kymmene Corporation, and the Washington State–based forestry company Weyerhaeuser snatched up as many contiguous estates as possible and planted huge swathes of trees, in a fever to take advantage of the subsidies, start profitable enterprises, and secure carbon credits in a world newly concerned about climate change. Ninety-eight percent of Uruguay’s tree farms are eucalyptus and pine, species native to Australia, Europe, and North America, because they grow fast—for paper—and straight—for lumber and posts.

Soon, the Uruguayan countryside was transformed. In 1980, there were 500,000 acres of tree farms and about 1.6 million acres of native forest in Uruguay according to the Ministry of Livestock, Agriculture, and Fisheries.Today, tree farms cover nearly 2.7 million acres, 6.2 percent of Uruguay’s total land area. Over the same period, the native forest has barely increased. While environmentalists sometimes question the veracity of the government’s statistics about natural forest cover, no one contests the fact that the amount of land devoted to tree farms in Uruguay has multiplied more than five times since the passage of the 1987 Forestry Law.

“The new landscape is much tidier,” says Diego Piñeiro, an Uruguayan sociologist and long-time dean of the University of the Republic’s School of Social Sciences. “The diversity is gone. The wildness has been lost.”

According to a report commissioned by the UN Forum on Forests and written by the Finnish sustainable forestry consulting firm Indufor, as of 2010, Uruguay’s trees sequestered over 70 percent of the carbon emitted most significantly by the country’s transportation sector. And that percentage may be even greater now. An analysis by the Uruguayan company CarboSur, which provides consulting for fossil fuel companies and others looking to reduce their carbon footprint by planting tree farms, suggests that Uruguay may be one of the only carbon-neutral countries in the world.

But not everyone approves. A tree farm isn’t a forest, some environmentalists argue. While diverse, native forests sequester carbon in the trees, the undergrowth, and the soil, tree farms sequester carbon only in the trees themselves. In that case, “the only way to add to the carbon stock is by adding land,” says Matt Palmer, an ecologist at Columbia University—and “you’ve set yourself a pretty low ceiling for what sequestration is possible.” Furthermore, environmentalists argue, the focus on a global calculation of atmospheric carbon can obscure local effects on ecosystems, economies, and families.

“Just because you can plant a tree somewhere doesn’t mean you should,” Palmer concludes.

For the Ferbers, it ultimately came down to economics. “O hago esto, o no seguimos. O no podemos seguir,” Ferber told his sons one afternoon about twenty years ago. Either we plant trees, or we have to give up. We can’t go on.

So, in 1995, they planted.

When you look out the window of the Ferbers’ 150-year-old farmhouse, there are no longer rocky hills as far as the eye can see. Long, even rows of spindly eucalyptus, with white trunks and red, peeling bark, now block the view.

* * *

When Uruguayan lawmakers voted nearly unanimously for the 1987 forestry law, environmentalists said the government had sold out to the economic interests of the Global North. Why, they asked, was a country with nearly universal literacy selling its resources of land, water, and soil to foreigners so they could make, sell, and consume paper—and reap the profits? Industry proponents slung the insult “antipatriotic” right back, claiming environmentalists cared more for Uruguay’s earth than its people.

The two sides came to blows in Rocha, in southeastern Uruguay, in 2011 and 2012, when forestry companies came looking for more land to forest. An alliance formed between environmentalists and hotel and hostel owners worried that logging trucks rumbling in and out of the port of La Paloma, which is smack dab in the middle of Rocha’s scenic beaches, would chase away summer sunbathers.

Ricardo Rodríguez, a biologist, was on the front lines of the fight. He delivered speeches, wrote letters, gave interviews. He called for assemblies, sat on panels, organized public meetings. When I hitched a ride with him in January, he was headed out of the city of Rocha to visit the place he views as the only victory from that fight. It’s an area a few kilometers in diameter where, over the past decade, Uruguayan cattle ranchers, together with hippies from the United States and Europe, have bought up land, swearing never to plant trees. Driving along Highway 109, Rodríguez rolled down both front windows—the old-fashioned way, with a hand crank—to try to get some relief from the summer afternoon heat and humidity.

Environmentalists have a laundry list of concerns about tree farms. First, and perhaps most importantly, they say tree farms don’t necessarily deliver the carbon sequestration environmental policy-makers seek. They sequester only a limited amount of carbon—perhaps even less than native grasslands, which can sequester carbon in the soil.

Furthermore, there are all the local effects. Eucalyptus and pine acidify the soil. They’ve drawn down Uruguay’s streams, rivers, and groundwater. On average, tree farms planted on native grasslands reduce the water flow 50 percent, says Gervasio Piñeiro, an ecologist from Uruguay who now works at the University of Buenos Aires. In drier regions, rivers can disappear entirely, leaving cattle ranchers with nothing for their animals to drink.

And tree farms have changed the ecology of Uruguay, affecting which species survive and which disappear. They break up the habitat of Uruguay’s native weasels and raccoon-like coati, its scissor-tailed swallows and ostrich-like rheas, all of which need large expanses of prairie. Introduced grasses like gorse and ragwort thrive in between rows of eucalyptus and pine and can later spread across pastures and stifle native grasses. Wild boars and parrots can use tree farms to hide from predators, either among the trees or in their highest branches—and both have become pests in Uruguay in recent decades, the former preying on ranchers’ sheep and the latter descending on farmers’ fields, threatening investments in soy and rice.

Rodríguez and I pulled up at the gate of a small cattle ranch, on the top of a hill from which we could see the surrounding farms and, on the horizon, the timber plantations. “Here in the Rocha Mountains, tree farms are forming a border all around this isolated area,” he says. Creeks run through the deep, tree-filled ravines between the rocky hills. Cattle meander and grassland birds flit across the fields.

“The tree farms are a kind of latent threat,” Rodríguez says. “Like a rug, the dark green, uniform tree farms are smothering the diverse natural landscape.”

But the future of South America’s landscapes does not have to be as dark as Rodríguez fears.

Uruguayan environmentalists generally object to large-scale tree farming: the Weyerhaeusers and Shells of the world sweeping into their country, buying up land, opening pulp mills. But now there’s a new wave of tree farming sweeping Uruguay, a model closer to the Ferbers’ decision to plant trees on only part of their land, leaving wide margins by rivers and streams and stands of native trees. Instead of buying land themselves, forestry companies operating in Uruguay, including UPM and Montes del Plata, have started signing contracts with small landowners. The details vary (who puts down how much money, who reaps what percentage of the profits), but the crucial point is this: These tree farms don’t blanket the landscape. They speckle it.

* * *

In some ways, policy-makers have come a long way since 1987. The thinkers behind Initiative 20×20 are promoting the use of native species, focusing on landscape restoration, and including grasslands in addition to forests and tree farming. In some cases, local landowners with a stake in outcomes will even maintain control over projects. But, as the World Resources Institute’s website acknowledges, “Now comes the hard part: taking commitments and pledges to the ground.”

Initiative 20×20 “is not a legal treaty,” explains WRI’s Walter Vergara. “It’s an agreement to undertake certain goals in restoration. Those goals have been established by the governments themselves.”

In other words, international policy-makers spearheaded the plan. National governments set their own forestation goals. And, except in the case of public lands, private investors will make the final decisions about what trees to plant where based on potential profits from selectively thinning excess trees, selling products like tropical fruits and nuts, and earning carbon credits on a global carbon market.

In Paris, policy-makers are continuing to discuss Initiative 20×20, which this weekend announced an additional $365 million in funding—doubling the $365 million previously pledged. Some governments included their 20×20 plans in their Intended Nationally Determined Contributions. Furthermore, whether negotiators come to any agreement about a global carbon market—and, if so, what that agreement looks like—could dramatically change the initiative going forward.

So far, investors have pledged $115 million for reforestation; $360 million for tree farming projects, including plantations of bamboo, cacao, and coffee, which Vergara says will be interspersed with tropical trees; and $255 million for other projects. Those pledges are not set in stone, however, and might change before the projects are implemented.

One way to tip the balance in favor of ecosystems and biodiversity over monocultures might be to structure the global carbon market such that it incentivizes forests over tree farming. Currently, the UN’s own clunky definitions—the Food and Agriculture Organization labels all masses of trees as “forests”—makes it impossible to decide what to fund. A drippy tropical rainforest in Brazil that’s more than 50 million years old and contains 16,000 tree species? That’s a forest. A 500-acre plantation of two-year-old eucalyptus dunnii trees destined to become toilet paper? That’s also a forest.

Beyond structuring carbon markets and UN definitions to incentivize true conservation, another important step would be to put in place a mechanism to effectively monitor projects. Vergara says the World Resources Institute is working on a tool that would “monitor the net increase in vegetation, which will be a proxy for soil conservation, hydrology, biodiversity, and carbon” and will also measure the land area of restored cropland and rangeland. But the Uruguayan experience shows that measuring vegetation is not the same as studying soil and water quality, biodiversity, or even carbon sequestration. For more detailed monitoring, the initiative will rely on governments and on individual companies, which Vergara says “have their own monitoring systems and want to make sure that whatever they commit to they deliver.” But the Initiative itself will do nothing to ensure that the promised species get planted.

This is of particular concern in regions with a long history of tree farming of exotic species. Chile, for example, has pledged to reforest 250,000 acres and to use 1 million new acres for tree farming and silvopasture projects. Currently, 92 percent of Chile’s tree farms are eucalyptus or pine.

The case of Uruguay again makes it clear how important it is to have regionally specific verification measures in place. Ninety percent of Uruguay’s tree farms have sustainability certifications from the Forest Stewardship Council and similar organizations. But these certification schemes were created to protect the native forests of the United States and Northern Europe, not to conserve the world’s ecosystems. Thus, as long as property owners don’t cut down native trees in order to plant eucalyptus or pine, FSC inspectors give their stamp of approval—often to paper that comes from a tree farm that’s draining groundwater and harboring invasive species. For the Initiative 20×20 stamp to have meaning, an independent and knowledgeable observer will have to monitor conditions on the ground.

* * *

“Thanks to the decision to forest part of the land, my parents could stay in the countryside, and they could send their children to school,” says J.J., the youngest Ferber. “I’m very grateful to the forestry sector.”

A new generation now runs La Cascada: The three Ferber brothers jointly manage the estate. They’d make more money if they scrapped cattle ranching altogether and just planted trees, J.J. says. But they’d rather not. “My father, my grandfather, my great-grandfathers, my great-great-grandfathers, and all their ancestors were cattle ranchers,” J.J. says. “My goal is to care for the estancia for my daughters,” the middle son, Nacho, agrees. “For my brothers and me, the estancia goes beyond the tangible, the material.”

When Francisco Ferber passed away in October 2013, María Muñoz stayed at La Cascada, caring for the farmhouse and watching over the estate. But her health was deteriorating, and, one night in March 2014, Pancho and J.J. dined with her and tried to persuade her to move to Montevideo, where she’d be less isolated, closer to medical care.

No, no me voy a ninguna parte,” she told her sons. I’m not going anywhere. “¿Ven esto? Éste es mi lugar en el mundo.” Do you see this? This is my place in the world.

That very night, or maybe it was the night after—the brothers remember it differently—she had a heart attack and died on the Ferber ranch.

“Now that our parents aren’t with us anymore,” J.J. says, “when you go to the house, to the estancia, it’s as if they’re there, surrounding you.” Whenever possible, he and his brothers, their wives and their kids, dine together at La Cascada. They sit at the table that María Muñoz once set, a table that has now been sequestering carbon for more than 200 years.

Editor’s note: This article originally referred to the practice of “planting trees to harvest and sell” as “agroforestry.” That practice is more accurately described as “tree farming.” Additionally, this article originally stated that parts of Initiative 20×20’s funding will go toward planting “monocultures of cacao and coffee.” Since these crops will be interspersed with tropical trees, it is more accurate to describe these as plantations of cacao and coffee.