Will tropical dry forests survive the next 50 years?

In 1978, renowned ecologist Dan Janzen jumped into a ravine in Costa Rica, broke three ribs, and spent the first month of the rainy season watching the tropical dry forest from inside a shack. At night, a simple 25-watt bulb drew in so many moths that they plastered the walls like a live wallpaper. The forest teemed with caterpillars, so many that whole trees stood defoliated and the ground lay carpeted in their droppings.

“There has never been such a high-density caterpillar year since then,” Janzen told Mongabay about Costa Rica’s Guanacaste Conservation Area. “And there has been a continual decline after.”

That decline represents more than just the global phenomenon of insect decline. It signals the quiet collapse of one of Earth’s most overlooked and endangered ecosystems: tropical dry forests. These forests once dominated vast swaths of the planet, but after decades of development and now accelerating climate change, what remains is dwindling and lacks protection.

The forgotten half of tropical forests

Stretching across Latin America, Africa, Asia and the Pacific — from Madagascar’s thorny forests to Brazil’s Caatinga, from South America’s Chaco to India’s seasonal woodlands — tropical dry forests account for nearly half of all tropical and subtropical forests worldwide.

The scale is staggering. Up to 60% of all forests in India and approximately 30% of the forests in mainland Southeast Asia are classified as dry forests. In southern Africa, the miombo woodlands alone contribute to the livelihoods of more than 100 million people.

“Hundreds of millions of people depend on dry forests,” Phosiso Sola, regional coordinator for the Center for International Forestry Research (CIFOR) in East and Southern Africa, said in a research brief. “Destroy these forests and you destroy their well-being.”

Yet, these vast ecosystems are often overlooked in the conservation community. While the Amazon rainforest captures headlines and funding, tropical dry forests wage a less visible battle for survival.

“There’s not been the kind of focus on them that they deserve,” Stuart Pimm, a leading conservation biologist at Duke University, told Mongabay.

Unlike their perpetually green rainforest cousins, tropical dry forests undergo drastic changes between seasons, receiving 500-1,500 millimeters (19.6-59 inches) of annual rainfall during summer monsoons, followed by pronounced dry seasons. During dry seasons (lasting five to eight months), most trees shed their leaves, creating landscapes that appear almost barren. Then, with the first rains, the forest explodes back to life.

Tropical dry forest canopies remain more open than dense rainforests, and they support plants and animals adapted to these extreme seasonal swings. Some are familiar, such as jaguars (Panthera onca), tigers (Panthera tigris), lemurs and tapirs, while many are more obscure.

Gerardo Ceballos, senior researcher at the Institute of Ecology at the National Autonomous University of Mexico, described some of his favorite dry forest inhabitants in Mexico: The tiny pygmy skunk (Spilogale pygmaea), an arboreal species “so amazing, so beautiful, and so tiny;” the Magdalena rat (Xenomys nelsoni); the mouse opossum (Marmosa mexicana); and the critically endangered green macaws (Ara ambiguus), majestic birds that grace the forest canopy. The Mexican beaded lizard (Heloderma horridum), he said, produces venom-containing compounds crucial for modern diabetes medication.

“Every time we lose a species, we’re losing so many possible things that can help humans to survive,” Ceballos said.

A landscape vanishing into fragments

“Tropical dry forest was the major ecosystem in the tropics … it’s all gone now, because it has been turned into is agriculture and pastures,” said Janzen. Although these forests have not vanished completely, in some regions, CIFOR research indicates as little as 5% of the original dry forest extent remains.

A 2022 study published in Nature Sustainability provided the most comprehensive global assessment of tropical dry forest destruction to date. Using high-resolution satellite imagery, researchers estimated that around 18 million square kilometers (6.9 million square miles) of tropical dry forests and woodlands were standing in the year 2000. By 2020, humans destroyed more than 710,000 km2 (274,000 mi2) of dry forest in just two decades — an area about twice the size of Germany.

The destruction occurred because humans find tropical dry forests particularly attractive. “There’s many, many reasons why tropical dry forest sites are more comfortable for humans,” renowned ecologist Dan Janzen told Mongabay. “They have good roads, they have reasonable weather. They don’t have as many diseases [as rainforests].”

The scale of loss is global. Janzen remarks on the vast dry forests in India, East Africa and the northern half of Australia. “[They have] been trashed completely,” he said. The 2022 study identified major deforestation hotspots concentrated in South America — such as the Gran Chaco in Argentina, Paraguay and Bolivia, and Brazil’s Cerrado — as well as in Asia, particularly the dry forests of Cambodia, Laos and Vietnam.

A lack of comprehensive protection is stark: Less than one-third of the world’s dry forests are located within protected areas. The study also found that one-third of all remaining dry forests are located in areas where deforestation is progressing rapidly, the so-called “frontier areas.” Over half of all these frontier areas are in Africa’s dry forests.

What remains in many areas are small, isolated patches scattered across human-dominated landscapes. In Central and South America, dry forests are now fragments of a massive, contiguous forest that once stretched from Mexico to Northern Argentina. These fragments face what Janzen calls an almost impossible conservation challenge

“The survivors are those little patches of forest that still exist out there and the species that live in those little patches,” he said. But conserving them is “almost an impossible task because the little fragments are not spectacular. They don’t look like gorgeous rainforests.”

The ecological collapse carries profound human consequences. Dry forests provide essential services to some of the world’s poorest populations, according to the CIFOR. These ecosystems contribute wood for cooking, wild fruits, vegetables, nuts, edible insects and bushmeat for local diets, especially during times of scarcity. Dry forests also provide economic opportunities through products such as beeswax, honey, medicinal plants and charcoal, which can be gathered and sold.

Much of the recent deforestation happens as capital-intensive agriculture spreads into dry forests, the 2022 study found. This represents a shift from traditional small-scale farming to large-scale commercial operations that can rapidly clear vast areas.

Climate change accelerates the crisis

When Janzen arrived in Liberia, Costa Rica, in 1963, an average of 116 days per year reached temperatures of 32° Celsius (89.6° Fahrenheit) or greater. Today, there are 193 such days, a rise of 66%.

“When I got there [in 1963] there was just a four-month dry season. Today, it’s six months,” Janzen said. “That means two more months of heat and two more months of no rain.”

The temperature increase, caused by global climate change, affects species in complex ways. Many animals have “very specific limits, upper and lower limits on the temperature where they can function,” Ceballos said. If temperatures rise too high, critters need to hide in the shade and have less time to do the things they need to survive, such as hunting and foraging.

For reptiles like sea turtles, the stakes are even higher. “Their sex is defined by the temperature,” Ceballos said. “And those changes, even the small changes, may … skew the sex ratio really hard, and this may cause the local or the global extinction of the species.”

Climate change effects extend vertically, too. Rising temperatures are pushing hot air masses up mountainsides, evaporating the cloud forests that provide crucial refuge for dry forest species during dry seasons. In Costa Rica’s Cordillera Guanacaste, clouds that once reliably formed at certain elevations are now 100 to 500 meters (328 to 1640 feet) higher, leaving lower-elevation cloud forest areas hotter and drier. This vertical migration has profound consequences for the countless species that depend on these “refrigerators in the sky” for refuge during the harsh dry season.

Stuart Pimm emphasized another escalating threat: fire. He said that as climate heats up, dry forests are “going to be even more flammable … that’s a very considerable worry.”

The combination of extended dry seasons, higher temperatures and human activity creates conditions ripe for catastrophic fires. Fires that could eliminate remaining forest fragments.

Another devastating consequence of climate change is the disruption of rainfall patterns that have governed these ecosystems for millions of years.

“More of a problem, I think, in the tropics than in the temperate zone, is that much of the biodiversity and intricacy of tropical forests relies on cues that the participating organisms can all detect,” Winnie Hallwachs, a famed tropical ecologist who helped to establish and expand the Guanacaste Conservation Area told Mongabay. “And of course, in dry forest, they had the major cue of the beginning of the rains. And now there’s a lot of stuttering in the queues.”

The consequences cascade through entire food webs. “There can be one or two heavy rains, three weeks early, or the rains can start and go for ten days and then stop again,” Hallwachs said. “And since we’re focused on insects, we particularly see the impact on this of wiping out insect populations.”

For example, a large moth called Manduca dilucida typically produces offspring during the early months of the rainy season, after which the pupae remain underground for 10 months. But climate change has created an extra rainy period in September that tricks some moths into emerging early. The caterpillars born in September and October don’t survive due to increased predation, tougher leaves and parasites. This means fewer moths survive to reproduce the following May. The problem has gotten so bad that a wasp species that depended on these moth caterpillars for survival has gone locally extinct in the area.

The pace of current climate change leaves little time for adaptation. Unlike past climate shifts that occurred over thousands or millions of years, current changes are happening within decades.

“The speed [at which climate change] is moving and accelerating, is a major problem,” said Hallwachs. “The extreme events we have had both severe dryness, [which killed] many trees, and severe wetness that has washed away trees, washed away insect pupae, basically scraped off a lot of the topsoil.”

Ceballos notes that while some species may have genetic variability to survive, “there are many, many species whose variability is very strict” and cannot adapt quickly enough.

“The fact that it’s dry forest, as opposed to rain forest or cloud forest, does give it some insulation,” Janzen said. In other words, dry forest organisms are, to some degree, accustomed to a dry season. But at the same time, “They can’t just tolerate any amount of dryness that you throw at them, just any kind of heat that you throw at them. They can’t do that.”

Fighting for time: conservation efforts and emergency responses

Despite the dire outlook, some conservation efforts offer hope. Janzen pointed to the Guanacaste Conservation Area as a model.

“[Guanacaste Conservation Area] was deliberately chosen to be a collection of small fragments, and we deliberately went out and bought a whole lot of farmland and pastures to allow those fragments to grow and become connected to each other,” he said. “So, we have now a big area [around 700 km2, or 270 mi2] of tropical dry forests that’s very young and very regenerating.”

Ceballos’s team created the Calakmul Biosphere Reserve in Mexico, “one of the largest reserves in tropical forests on the planet.” The reserve now supports an estimated 500 jaguars and has seen the recovery of tapirs, peccaries and other wildlife.

However, the economic challenges remain formidable. Most tropical dry forests have been converted to pastures, homes, roads and other areas for human use. They aren’t likely to be converted back.

Tropical dry forests have also not been widely studied compared to tropical rainforests. The 2022 study noted that improved deforestation monitoring and better land-use planning are urgently needed.

Janzen and Hallwachs argued that traditional scientific approaches are insufficient for the current crisis. Instead of more studies, they advocate for action. This emergency approach might include active intervention such as supplemental feeding of wildlife, irrigation systems and assisted migration of species to more suitable habitats.

In northern Mexico, Ceballos’s team already practices intensive management, providing supplemental food to prairie dogs and considering the implementation of irrigation systems for wild areas.

“We are irrigating millions of acres [for crops],” Ceballos said. “Why don’t we irrigate this and … keep them going for the next 50 or 100 years?”

Looking toward 2075, the future of tropical dry forests appears to hang in the balance. “We know, like the rest of the globe, it’s [tropical dry forests] going to be badly whacked,” Hallwachs said. “We know for certain it will be much less diverse than it is right now, we feel that the hope lies in the preservation as much as possible of forests.”

Ceballos said he predicts that while some areas will maintain recognizable dry forest characteristics, “the composition of the ecosystem will be very different.”

His prognosis includes widespread local extinctions as “many species won’t be able to move up, or they will move away to high up in the mountains or North or South, according to their capabilities.” Many endemic species with restricted ranges face global extinction, representing the loss of millions of years of evolutionary history.

“It’s the little fragments that are going to exist 50 years from now,” Janzen said, “unless somebody has given them land to grow back and become bigger areas.”

Climate change may create new ecosystems with unpredictable characteristics.

“Parts of what is now dry forest will develop into having the conditions of deserts, but not necessarily with at all the seed sources or the insects that are associated with present deserts,” Hallwachs said.

Despite the grim projections, Ceballos said he maintains cautious optimism. If conservation efforts can reduce stressors and help species survive the transition, he said he believes many may be able to adapt. “Most of these species have been alive for millions of years … if we manage to help them for the next 100-200 years, I think most of them will be able to survive.”

The key lies in acting decisively now, Ceballos said. “The future is here now … we need to start.”

The Bottom Line

The survival of tropical dry forests — and the hundreds of millions of people who depend on them — hinges on immediate action. “[It’s] going to depend on how much land is given back to it, as well as climate change,” Janzen said.

These ancient ecosystems now face their greatest test in millions of years of existence, Ceballos said. The next few decades will determine whether the world will support these remarkable forests, with their tiny pygmy skunks, lemurs and tapirs or merely read about them as victims of humanity’s failure to act in time.

The science is clear, tropical dry forests “face an uncertain future under climate change,” Ceballos said, “the question now is whether humanity will listen to this warning in time.”

Banner image: An Indri lemur in Madgascar. Image by Rhett Butler / Mongabay.

Liz Kimbrough is a senior staff writer for Mongabay and holds a Ph.D. in Ecology and Evolutionary Biology from Tulane University, in New Orleans, Louisiana where she studied the microbiomes of trees. View more of her reporting here.

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Citations:

Buchadas, A., Baumann, M.,  Meyfroidt, P., & Kuemmerle, T. (2022). Uncovering major types of deforestation frontiers across the world’s tropical dry woodlands, Nature Sustainability, 5. 619-627. doi:10.1038/s41893-022-00886-9

Blackie, R., Baldauf, C., Gautier, D., Gumbo, D., Kassa, H., Parthasarathy, N., Paumgarten, F., Sola, P., Pulla, S., Waeber, P., & Sunderland, T. (2014). Tropical dry forests: The state of global knowledge and recommendations for future research. Center for International Forestry Research (CIFOR): Bogor, Indonesia. doi:10.17528/cifor/004408

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