Israel’s Yatir Forest has been hailed as a green refuge in the Negev that is helping fight climate change. But some Israeli ecologists now contend that it has wiped out important desert ecosystems and shows that forestation projects are not always an unalloyed environmental good.

Two hours’ drive out of Tel Aviv, on the southern slopes of Mount Hebron, the Yatir Forest is the country’s largest planted woodland, with 4 million trees spread across 7,400 acres. Dense stands of Aleppo pine gird the hillsides, in vivid contrast to the dun-colored Negev desert. The forest’s high points have views east toward the Dead Sea and south across the Negev, toward the town of Be’er Sheva and a solar power plant that glints through the haze.

Yatir was planted in the 1960s by the Keren Kayemeth LeIsrael-Jewish National Fund (KKL-JNF), a private, non-profit group that was created in 1901 to buy and develop land for the Zionist movement and now owns around 13 percent of Israeli territory. KKL-JNF is currently extending the Yatir Forest along narrow embankments designed to hold water on the land and help the trees grow.

KKL-JNF has made wide-ranging claims for the environmental benefits of the Yatir Forest, saying it is holding back the desert, recharging soils with moisture, preventing floods in Be’er Sheva, and fighting climate change by capturing carbon dioxide from the air. But this showcase forestation project also has its share of critics, with some Israeli ecologists saying that, whatever the benefits, the collateral damage has been too great. The trees, the ecologists say, are obliterating grasslands that contain rare endemic species. There is also evidence that the new Israeli desert forests have so far caused more warming than cooling, as the dark mass of the Yatir Forest’s trees is absorbing solar radiation, while the lighter colors of the desert once reflected the sun’s heat back into space. The Yatir, some experts say, is an example of the ecological damage that can occur when large-scale forestation projects are undertaken in places that have not had forests in recent times.

Creating new forests is an emotionally and politically charged issue in Israel. Planting trees is, as one scientist told me, “a way of saying we are here.” And most Israelis are proud of their country’s forestation projects in the Negev. Some Israeli climate scientists see its new forests as a model for greening the dry lands of the Earth and soaking up atmospheric carbon dioxide. Other supporters of the Yatir argue that, overall, in an area that has experienced intensive human habitation for thousands of years, the creation of the Yatir Forest is, on balance, a good thing.

“The Negev desert is not some sort of pristine wilderness that is sullied by over-zealous forestry policies,” Alon Tal, an environmental policy expert and chairman of the Department of Public Policy at Tel Aviv University, said in an email. “Rather, it is an extremely degraded dryland region, whose soils show the impact of centuries upon centuries of overgrazing and mismanagement. Our success at tree planting is nothing less than an ecological makeover.”

The benefits and drawbacks of forestation projects like the Yatir have global significance, since tree planting is not always an unalloyed environmental good. Such lessons are especially relevant now, as scientists worldwide are calculating how to help stave off climate upheaval by planting hundreds of billions of trees across an area of the planet almost the size of the United States.

Semi-arid southern Israel is made up of loess soils and grasslands, the earth a dusty mix of sand and clay, devoid of trees but harboring often-rare sages and other herbaceous grasses and shrubs. Being frequently used for sheep pastures, they may not look much like prized ecosystems, says Alon Rothschild director of biodiversity at the Society for the Protection of Nature in Israel (SPNI). But, he says, they are a meeting place between desert and Mediterranean ecosystems and comprise “two of the rarest and most threatened habitats in Israel.” They teem with ground-nesting birds and specially adapted reptiles.

Plants that Rothschild says are at risk from the spreading tree cover at Yatir include the dark-brown iris, Iris atrofusca; an endemic daffodil, Allium kollmannianum; and remnants of the most southerly population of a species of wild wheat. Among the animals disappearing as the trees encroach are endemic Be’er Sheva fringe-fingered lizards, Acanthodactylus beershebensis. Several species of ground-nesting birds — including pin-tailed sandgrouse (Pterocles alchata) and shrub-nesters such as the spectacled warbler (Sylvia conspicillata) — are also in decline, preyed upon by crows and jays.

“Reptiles need sun,” says Rothschild. “Trees take away the sun and provide branches where predatory birds can sit looking for lunch.”

Many of these species are officially protected. But Rothschild claims the KKL-JNF is rarely called to account for damaging their habitats, only 4 percent of which have any national protection. And Rothschild notes that Yatir is not the only KKL-JNF desert forestation program. Others include smaller woodland projects such as Lahav and Beeri, both also in the Negev.

KKL-JNF did not respond to requests for comment.

Some contend that the northern Negev was forested until the trees were cut by the Turkish occupiers of Palestine to fuel steam trains, so Israel’s tree-planters are only putting back what has been lost. But Rothschild says there is no basis for this claim. Certainly, British maps of the Negev in the 1880s do not mark forests.

Many Israelis and Jews around the world celebrate the annual “holiday of trees,” or Tu BiShvat. Tree planting in Israel is a popular way to honor a loved one, says Rothschild’s colleague Jay Shofet, director of partnerships and development at the SPNI. “It’s practically a Zionist commandment,” says Shofet. “The image of a forested Israel has always fired the imagination of well-intentioned Zionists. But the truth is most of Israel is a shrubland ecosystem, which is a high-value area for biodiversity and it must be preserved, not carpeted in forest.”

Shofet, who grew up in New York, admits to “a deep desire to get American Jews to stop planting trees with KKL-JNF.”

The Yatir Forest, which abuts the barrier between Israel and the occupied territory of the West Bank, is on land once controlled by Bedouin herding communities. After a history of land conflicts, the Israeli authorities now allow the Bedouin to graze their sheep in the forest each spring, which researchers say reduces the risk of fires by removing understory.

Most forest planting and management in Israel is carried out by the KKL-JNF, which is the nation’s biggest private landowner, developing farms, building housing, and constructing roads, as well as planting around 250 million trees to date. But the organization has been plagued by controversy. Earlier this year, it emerged that memorial trees planted outside Jerusalem to honor a Japanese diplomat, known as the “Japanese Schindler” for saving 6,000 Jews in Lithuania during the Holocaust, had been felled by the organization to make room for housing.

Despite the political and ecological concerns, many will argue that tree planting has to take priority in the fight to hold back climate change. Planting trees on a large scale across the world is increasingly seen as essential to reducing the accumulation of carbon dioxide in the atmosphere. A recent heavily publicized paper in Science, carried out by researchers at the Crowther laboratory of the Swiss Federal Institute of Technology in Zurich, estimated the amount of land potentially suitable for new trees — and not currently occupied by forests, agriculture, or human settlements — at 2.25 billion acres. That’s enough for half-a-trillion trees.

Israel claims to be one of the few places in the world with more trees at the end of the 20th century than at the beginning. Arguably the trees at Yatir could show the way to creating forests in arid lands. Tal says that “in a world where as much as much as 70 million acres of forested land continue to disappear annually,” Israel’s afforestation efforts are “worthy of recognition and, in many areas of the world, emulation.”

“The Yatir Forest,” he continued, “may be imperfect, but after half-a-century we can see that the trees have dramatically increased the carbon content of the area’s soil and produced a lovely series of parklands.” The last point, he said, is key, as the Yatir provides recreation and respite to Israelis in the Negev. The forest is of special benefit to the Negev’s Bedouin population, Israel’s poorest ethnic group. “Yatir provides innumerable ecosystem services to these communities — from firewood in winter to grazing lands for the beleaguered remnants of the Bedouin’s shepherding culture,” says Tal.

So how have those trees fared in the half-century since their planting? Researchers at the Weizmann Institute for Science in Rehovot have for 20 years been intensively monitoring the pines in the heart of the forest. I joined Ph.D. student Yakir Preisler on one of his weekly trips to check the equipment and harvest the data. Dozens of trees are strapped with measurement devices to follow their growth and metabolism, including sap flow, leaf temperature, and rates of photosynthesis and transpiration of water. Above the trees, a tower carries instruments to measure the forest’s breath.

The Aleppo pine trees, which are naturally found in wetter regions of the Mediterranean, have adapted well to the harsh arid conditions, says Preisler. “They are surviving on the edge,” he says. “They behave differently from those in Greece, for instance. They grow like crazy in the short wet season, when water is available for photosynthesis,” and shut down when it is not. With an annual average rainfall below 12 inches, shutdown is most of the year.

“What we are finding out is timely and relevant for the world as climate changes,” says Preisler. “This is how forests will be in southern Europe in a few decades.”

But the Yatir Forest’s adaptability has limits. New saplings rarely grow without irrigation, and recent dry years have seen massive tree losses. A yearlong drought in 2010 killed 50,000 trees, with more than 80 percent tree loss in places. Almost a decade on, I saw few signs of regeneration in these barren spots. It seems unlikely that, without new planting, the forest will survive beyond the lifetime of its founding trees.

So, is all this effort and ecological disruption worth it? The doubts are growing. Dan Yakir, a specialist on the relationships between the biosphere and atmosphere, at the Weizmann Institute, has calculated that the Yatir Forest has so far not helped cool the planet. In fact, if anything, it has so far caused warming.

How come? The growing trees are certainly taking up carbon dioxide. KKL-JNF estimates that each tree in the forest will eventually store 500 to 800 kilograms of carbon. The problem is that the dark foliage of the forest canopy is replacing a light-colored desert surface. The dark surfaces absorb more solar radiation than the desert, re-radiating it into the air, which warms.

Yakir estimates it will take 80 years of growth, until the 2040s, before the global cooling from the accumulated carbon in the trees of the Yatir Forest exceeds the global warming caused by its dark foliage — assuming the trees survive that long. Climate models predict declining rainfall in the Negev. More frequent droughts like the one in 2010 could kill the forest and return its carbon to the air.

Yakir believes the warming effect he finds from the Yatir Forest is probably an extreme case. The high rates of solar radiation in the Negev and the strong contrast between the foliage and desert surface means the warming effect is great, while the slow growth of trees in the desert environment means the countervailing carbon capture is slow. He compares it with forests in Siberia, where growth in the cold climate is similarly slow, while the contrast between forest canopy and snow in is very high.

But while the Yatir may not be a great advertisement for forests as climate saviors, Yakir says similar forests planted on a larger scale in other arid environments could work much better for climate. Last year, he and Weizmann colleague Gil Yosef made the case for planting similar forests in the dry grasslands of the Sahel region of Africa and in northern Australia. They found that, if planted across a sufficiently wide area, such forests could shift wind systems and make rain that would encourage more natural forest growth and carbon uptake.

In the case of the Sahel, if an area the size of Mexico could be planted with forests — a gargantuan undertaking — it would change local air temperatures enough to push the East African Jet Stream, a fast upper-air flow, northwards. That would allow wetter air from the Atlantic to penetrate inland as far as the Sahel — something that may not have happened since 6,000 years ago, when the Sahel was lush with vegetation and supported swamps and giant rivers. The Sahel megaforest, Yakir and Yosef’s modeling study concluded, would double average rainfall over an area almost twice that of the planted forest. More trees might then be able to expand to the north.

Yakir and Yosef maintain that the hydrological changes created by the trees would mean that, unlike the Yatir, a Sahel megaforest could recoup the warming effect of dark foliage in double-quick time — within about six years. “We argue that if you want to plant trees in new areas, the semi-arid regions are the ones to look at,” Yakir says.

Adds Tal, “In a planet which desperately needs to mitigate greenhouse gas emissions and find new areas for carbon sequestration, Israel’s forests offer proof of concept — that even areas with only 250 millimeters [10 inches] of rain [a year] can be home to pleasant woodlands and provide local communities with a response to the relentless loss of soil.”

Rothschild is more skeptical. He says the recent claim that the world has room for another 500 billion trees is “a joke so far as Israel is concerned.” The land is simply not there. The paper from the Crowther laboratory singled out for planting 300,000 acres in Israel, he says. “But when we checked what was included, it turned out that 87 percent of that was not available for planting, because it was too dry, or was built on, farmed, or already had natural trees,” says Rothschild. Most of the rest, he says, comprises “unique natural areas” that should be preserved.