Late one afternoon last October, Scott Saleska encountered a weirder than usual welcome to the remote Brazilian research station he’d been coming to for 17 years to study how the Amazon rainforest breathes. Arriving at the base of a 67-meter (220-foot) flux tower that juts up through the rainforest canopy, he clipped his climbing harness to a galvanized steel safety cable and started up. Dragging electronic replacement parts in a haul bag, Saleska rose rung by rung on his way to troubleshoot two malfunctioning sensors clamped high on the triangular structure, which measures just eighteen inches wide on each side, and rises like a vertical proboscis above the teeming green forest.
The dry season was normally in full force by now, but when Saleska reached the pinnacle of the jungle lightning rod, he saw a portentous grey wave crashing toward him, targeting the galvanized steel spire on which he stood. Fearing a lightning strike like one that had recently fried some of the tower’s instruments, Saleska scrambled down and unclipped his harness just as a monsoon-quality downpour swamped the research station with an hours-long tropical deluge.
The unseasonable storm was just the first punch in Saleska’s latest bout of Amazon fieldwork, an endeavor comprised of equal parts logistics, groundbreaking science, and gambiarra, a Brazilian Portuguese word that means essentially what MacGyver does when he improvises his way out of thorny situations with his beloved Swiss army knife and a roll of duct tape.
The primary aim of Saleska’s research is to assess the implications of the climate dilemma we’re in—essentially, to chart the Amazon’s respiratory state—using significantly more sophisticated tools than MacGyver ever had: sonic anemometers for micro-meteorological measurements, spectroradiometers to gauge leaf age, and hand-held porometers that measure the rate of water evaporating from the surfaces of individual leaves.
Saleska has chosen this place to do his work because the world’s largest contiguous rainforest contains clues that can help scientists diagnose the implications of Earth’s changing vital signs. By taking various measurements of the jungle’s life-sustaining functions, such as the water loss and carbon uptake of individual leaves during photosynthesis, and cross-referencing on-the-ground measurements of leaf production against satellite imagery, Saleska and his many collaborators are creating a detailed portrait of the Amazon’s biogeochemical functions—as Saleska puts it, “from the leaf to the landscape.”
This ecological research station in the Tapajós National Forest in northern Brazil has been literally instrumental in helping Saleska and his multinational colleagues explore three vital and interrelated scientific questions: Which factors control carbon dioxide and water vapor fluxes, or exchanges, between the forest and the atmosphere during “normal” seasonal dry periods and during extreme drought periods of El Niño years? How will climate changes affect the 5.4-million-square-kilometer (2.1-million-square-mile) Amazon forest’s ability to absorb atmospheric carbon, including the increased carbon from fossil fuel burning that’s causing the planet to warm in the first place? And how will feedback from these changes alter environmental systems and patterns around the planet?
Climate scientists can now project with confidence that an increasingly warmer world will produce more extreme weather patterns that have the potential to dramatically affect the life cycle of tropical forests. Higher temperatures are likely to dry out some parts of the rainforest and contribute to more frequent droughts and catastrophic wildfires. Disturbed forests that have been logged, or forests such as the Tapajós that are already relatively dry because of their location may experience increased stressors on tree health that reduce those forests’ ability to absorb CO2.
The question is, “How, and by how much?”
Among Saleska’s surprising findings are indications that this part of the Amazon rainforest tends to “green up” and continue to absorb carbon dioxide even during the dry season—in fact, the trees here absorb more carbon in the dry season than in the wet season. One implication for this is that the forest may be more resilient than previously thought in the face of at least some of the changes climate scientists and ecologists predict as the planet warms.
That’s the good news. In order for that to happen, however, the Amazon rainforest has to survive widespread land clearing as well as the rising temperatures that are projected to reach levels by the end of the century that the region hasn’t seen in 10 million years. Exactly where is the tipping point at which climate change would cause the forest to become a source instead of a repository, or sink, of atmospheric carbon? That’s a question Saleska and his collaborators are trying to answer.
A tree falls in the forest
Even under ideal circumstances, field research ranges from the mundane to the arduous, whether in the tropical rainforest, Norwegian Arctic, or in Colorado’s Rocky Mountains, where Saleska also works. To generate anything useful from a field season in the Amazon, researchers must possess the usual traits of endurance, stubbornness, perseverance, resourcefulness, physical stamina, patience, and an immense capacity to transform abstract data points into original thought and insightful analysis.
For Saleska’s Brazilian rainforest work, he adds another suite of job descriptions to his main occupation as professor of ecology and evolutionary biology at the University of Arizona in Tucson: logistics coordinator, computer programmer, electrical engineer, human resources expert, diplomat, budget administrator, linguist, climber, and, on this late October day, lumberjack.
After descending from the flux tower just as the storm hit, Saleska hung up his harness for the day. With his Brazilian colleague, 33-year-old Kleber Silva Campos, in the passenger seat, Saleska drove down the 8-kilometer (5-mile) rutted dirt track that led to a two-lane asphalt road, which would take them to the relative comfort of the team’s “base camp” compound, another half hour drive from the Tapajós forest entrance.
As he drove, Saleska turned to Silva Campos, a local who had recently completed his Masters degree in environmental science in nearby Santarém at the Federal University of Western Pará, known by its Portuguese acronym, UFOPA. In the pidgin Portuguenglish the two of them used with each other, Saleska quietly said he hoped no trees had fallen across the road during the downpour. Silva Campos retorted, “Não fala assim,” which translates to “Don’t say that!”
Not a hundred meters later, they came upon two Cecropia trees that had fallen across the muddy track during the early part of the storm, which had still not let up. With the crepuscular light dissipating with tropical swiftness, about to add darkness to the dripping damp, Saleska and Silva Campos took stock. They had one machete in the car, which seemed like a comically undersized tool to chop through the fallen trees, each the diameter of a telephone pole. There was an occasionally functioning chain saw at base camp 24 kilometers (15 miles) away, but no reliable way to communicate with anybody there, even if a car was available. Inside their vehicle, there was no rope strong enough to drag the deadfall.
As Silva Campos drove back to the flux tower to retrieve a second machete, Saleska started hacking. Tepid rain cascaded from a seemingly endless source, and the enveloping darkness screeched equatorial menace. After the nocturnal baton dropped, a riot of squawks and chirps mixed with rain spatter, creating a multi-sensory forest orchestra complemented by the musty aroma of rain-soaked leaves and decomposition.
It was vaguely reassuring to know that jaguars rarely attack humans, and that the most venomous pit vipers, black scorpions, wolf spiders, white-kneed tarantulas, and Brazilian giant centipedes that call this forest home tend to stay away from big clearings.
The trees had fallen on a perfect perpendicular axis to the road, which meant it would be necessary to cleave two breaks in the trunks to allow the car to pass. Silva Campos, who had grown up in a nearby village, returned with the second machete and worked like a machine, hacking non-stop with strategically angled chops every couple seconds. Saleska, swinging with no less intent but perhaps a little less practice, was only a third of the way through his side when Silva Campos had broken through his.
Sweating but unruffled, Saleska paused and invoked Archimedes, the ancient Greek father of physics, to plan the next step. “If I had a lever large enough,” Archimedes famously said, “I could move the Earth.”
They sheathed the machetes and engineered a system of pre-industrial mechanical advantage, using a stump as a fulcrum and a smaller tree as a lever. The lever snapped. As Archimedes might have suggested, they needed a larger lever.
Despite the setback, the two eventually cleared the impasse and, exhausted, sweaty, and soaked to the pores, emerged from the rainforest.
After a brief stop at base camp, Saleska then drove another hour to Santarém to attend a Halloween party hosted by his Brazilian colleagues. They had all gathered to welcome “Scotchy” (as the name Scott is pronounced here) back to the Amazon.
A carbon trove in trouble
Ever since British naturalist Alfred Russel Wallace traveled to Santarém in the 1850s to study life in “some far land where endless summer reigns,” the Amazon has captivated scientists from a wide range of disciplines. Geographically, the rainforest spans nine countries and is two-thirds the size of the continental United States. Ecologically, it hosts the most fecund biodiversity on the planet: 20 percent of the world’s bird species, more than 400 mammal species, 40,000 plant species, and two and a half million identified insect species—not to mention myriad other invertebrates and microbial and fungal life forms. Hydrologists know that the Amazon River, by far the largest aqueous artery on the planet, feeds one-fifth of all the freshwater that flows into the oceans, more than the next eight largest rivers in the world combined. Biophysicists estimate that a comparable amount of water flows upwards from the rainforest, each tropical tree pumping a geyser of water into the atmosphere, doing its part to drive the global water cycle.
If your field is biogeochemical ecology, as Saleska describes his hybrid research discipline, the Amazon contains a “treasure trove of carbon and water interactions.”
Biogeochemical ecology attempts to explain the complex exchanges and relationships taking place in the natural world—essentially, to determine how chemicals and biological systems interact to sustain life. Most of us give little thought to the tiny pores, called stomata, on the surface of a leaf. But those openings act like tiny mouths, allowing the leaf to consume carbon dioxide and release water. Through photosynthesis, the leaf uses the sun’s energy to convert CO2 into sugars and complex carbohydrates that the plant then uses to drive its functions and build its structures. When the plant is photosynthesizing, it releases oxygen; when it’s burning sugars, it produces CO2. Those same processes at work within a single leaf are at play throughout the forest, the biome, and the planet.
But it all starts with the leaf.
Each leaf is its own biogeochemical factory, producing a series of interactions that help drive global water and carbon cycles. Saleska’s job—and that of his colleagues—is to take what they’re observing and measuring in the forest and figure out how these single-leaf and single-plant interactions “scale up” to create global impacts that affect life on Earth, now and into the future.
The Amazon’s biogeochemical exchanges are important on multiple fronts. For starters, the region’s immensity, unparalleled biodiversity, and critical role in the planet’s carbon and water cycles means that what happens in the Amazon doesn’t stay in the Amazon. The fates of this and other tropical forests are intimately connected to the fate of the rapidly changing planet in the Anthropocene, the current geological age in which humans have become a dominant force affecting Earth’s climate and environmental processes. As increases in global temperatures unleash ecological and meteorological mayhem around the planet, understanding how this iconic biome works has become more important than ever.
“We have a huge task ahead to improve our scientific knowledge of the ecosystem,” said José Mauro S. Moura, a professor of biogeochemistry at UFOPA who has worked with Saleska’s group. This task is even more pressing because the Brazilian Amazon forest has shrunk by about one-fifth since 1970 due to large-scale land disturbances, especially those caused by logging and industrial agricultural of crops and cattle. “We already know it’s not a good place to plant soy beans,” he told me over a meal of pirarucu, a giant Amazon River fish, at a restaurant near Santarém.
Moura put his fork down and added: “We also know it’s a really good place for a forest.”
To the flux tower
To get to this research site, approximately 350 kilometers (218 miles) south of the equator, it’s a multi-hop flight from wherever you start in North America to Santarém. From the Santarém airport, it’s another hour south by car on a rural two-lane road, passing increasingly smaller villages dotted with lojas(little stores), borracharias (repair shops) and churrascurias(barbeque buffets), nearly every one sporting its own shaded veranda. As human settlements thin, the road follows the lush border of a protected forest on the right, and a razed forest planted with soy and manioc on the left.
At the entrance of the Floresta Nacional de Tapajós, an armed guard with a clipboard stops us at a locked gate. The guard, who monitors not just scientific access but also potential illegal loggers, greets Saleska with a warm handshake and opens the gate.
The research team today includes the machete machine Silva Campos and 32-year-old Deliane Penha, a PhD student at UFOPA. Silva Campos’s main job is to maintain the flux tower’s infrastructure and instruments, while Penha painstakingly gathers data on the flows of water through the forest’s trees. She does so by taking measurements of individual leaves while suspended from hanging walkways strung through the research site. The third member of the team is Neill Prohaska, a 36-year-old PhD candidate in Saleska’s lab in the University of Arizona’s Department of Ecology and Evolutionary Biology. Prohaska’s dissertation focuses on seasonal cycles of photosynthesis and respiration. His tree-climbing prowess allows him to install monitoring devices a hundred feet above the forest floor and take measurements directly from tree crowns, sometimes while eye-to-eye with the area’s howler and capuchin monkeys.
The 600,000-hectare (2,300 square mile [about the size of Delaware]) Tapajós forest is managed by the Chico Mendes Institute for Conservation of Biodiversity, named after the environmental activist who was assassinated in 1988. It sits toward the northern edge of the Amazon rainforest, which means it is drier than other rainforest regions. Because of its relatively long dry season—about equal in length to the wet season—it may be a harbinger for a warmer future that is projected to produce hot, dry conditions, here and elsewhere in the tropics.
In addition to the daily physical challenges of Amazon research, shifting political priorities and funding problems abound. Construction on the flux tower began in 1999, a collaboration between NASA and the Brazilian space agency INPE. The tower has enjoyed a series of benefactors since then, including the National Science Foundation. Today it is funded through a partnership between Saleska’s lab (funded by the U.S. Department of Energy and other sources) and the Brazilian government’s LBA, the Large-Scale Biosphere-Atmosphere Experiment in Amazonia. But that situation is in flux as a recent political crisis in Brazil has sent ripples throughout the country’s economy, and caused a reorganization of its national science funding agencies. Saleska’s own funding depends on maintaining a flow of research grants in an increasingly difficult domestic funding environment.
The field research station’s centerpiece is the soaring flux tower, surrounded by a locked yellow chain-link fence to deter unauthorized visitors. Taut guy-wires stretch out in four directions, holding the spire in place (with one calamitous exception in 2006, when a tree fell on several of the wires and the structure bent in two, like a performer taking a bow).
The flux tower is loaded with a multitude of sensors designed to track the flux—the movement of a gas in or out of a space—of CO2 and water vapor from the forest floor to above the forest canopy, which averages about 45 meters (148 feet) high. Sensors measure CO2 and H20 concentrations eight times per second at multiple heights along the tower, and also monitor daily and seasonal fluctuations.
The research station’s nerve center consists of two small lime-green concrete-walled shacks, continually air-conditioned to protect the data collection systems from the paralyzing tropical humidity and heat. The instruments are powered by a diesel generator located a kilometer away, with a high voltage line running through black plastic conduit alongside the dirt track like a long, undulating python.
Scattered inside one shack is an array of wires, cords, connectors, and of course computers to process and store data from the tower’s system. The second shack is filled with cables, spare parts, and what looks like a search-and-rescue squad’s arsenal of climbing harnesses, ropes, carabiners, and other mountaineering gear never intended to be used to scale 45-meter (150-feet) guaruburana trees.
Finally, the area surrounding the flux tower is booby-trapped with scientific measuring devices, from 10-meter (33-feet) deep pits used to measure soil moisture content to a variety of tree-mounted solar-powered devices that measure light quality in the forest dangling from trees like misshapen fruit.
Raising an Eyebrow
The 53-year-old Saleska is a hybrid between a rumpled, absent-minded professor and perhaps Indiana Jones’s slightly less swashbuckling older brother. Saleska was raised in Milwaukee, the son of a Unitarian minister. For him, growing up was more about science fairs than football games. He recalls his earliest interest in science may have found expression, quite literally, in learning to imitate Leonard Nimoy’s Vulcan character Dr. Spock in Star Trek. Spock, the science officer of the Starship Enterprise, could raise one eyebrow to express his puzzlement at human folly. It was something he did frequently as the smartest being on the spacecraft.
By age 12, Saleska was already “tinkering,” doing such things as taking apart his family’s washing machine and putting it back together. In high school, he successfully aimed his sights on attending M.I.T., where he would major in physics. He became interested in the intersection of science and policy during the chlorofluorocarbon (CFC) debates of the 1970s, a case in which scientific discovery about threats to humanity from a growing ozone hole led to swift global action.
Saleska worked for a stint at Ralph Nader’s Public Citizen as an energy analyst, and as a consultant for the Environmental Protection Agency’s nascent Global Change Division. He then attended the University of California, Berkeley for his doctoral work in the university’s Energy and Resources Group. There he met John Holdren, who would become President Obama’s science advisor, and whom Saleska says had a profound effect on him. “Holdren always talked about `science in the service of humanity,’” Saleska recalls.
While at Berkeley, Saleska attended a seminar by visiting Harvard professor Steven Wofsy, who had been doing early flux tower work in central Massachusetts’ Harvard Forest, about 110 kilometers (68 miles) west of Boston. Saleska burnished his already impressive academic pedigree by doing a post-doc with Wofsy, then got in, literally, on the ground floor of the new flux tower in the Tapajós. When Saleska first arrived at the site in 1999, he recalls, the tower was nothing but “a stake in the ground.”
Sink or source?
The day after the rain-soaked machete fest, we drove back to the tower and Saleska verbally roamed through some scientific history and philosophy that gives his work context. The scientific understanding that human activities could influence global climate began in the mid-1800s, he said, with discoveries by Joseph Fourier and John Tyndall. This was right around the same time that Charles Darwin and Alfred Wallace were formulating the theory of evolution—and Wallace was roaming the Amazon.
It would be another 100 years, however, before Charles David Keeling began constructing a long-term data set in the 1950s on Hawaii’s big island that definitively showed rising levels of carbon dioxide in the atmosphere. The data would change the course of scientific history.
Keeling’s measurements showed seasonal shifts in levels of atmospheric carbon dioxide, which fell during the Northern Hemispheric spring due to increased photosynthesis, and rose again during the winter months when forests stopped photosynthesizing. More importantly, over the decades, the “Keeling curve” also showed a marked upward trajectory of atmospheric carbon dioxide—from 315 parts per million in 1958 to more than 405 parts per million this winter.
This influx of CO2, largely from the burning of fossil fuels, coupled with the loss of forest cover and other land disturbances as well as huge human population growth, instigated a cascade of rapid changes in Earth’s systems, from record-setting global average temperatures and rapid glacial melt to dramatic shifts in the behavior, distribution, and longevity of countless organisms around the globe.
When Saleska began his career, a debate raged in scientific circles: Were rainforests sources or sinks, either releasing carbon into the atmosphere or sucking it up like giant, leafy sponges? “Nobody really knew what the forest was doing,” Saleska says. How did the movement of carbon dioxide in and out of individual leaves at different times of day, at different times of the year, and from year to year affect this critical equation, especially as conditions changed in a warming world?
The way to figure it out, Saleska thought, was to emulate the infamous bank robber Willy Sutton, who said he robbed banks because “that’s where the money is.” For Saleska, interested in what CO2 was doing in a changing world, he decided he needed to be where the carbon was: the Amazon rainforest.
Breaking new ground
After the flux tower became functional in 2000, Saleska didn’t have to wait long to collect enough data to make a scientific splash. In 2003, he was the lead author of a paper in the prestigious journal Science that challenged forest ecology orthodoxy. Conventional wisdom held that during the dry season (fall and winter in the northern hemisphere), leaves slow or even cease their photosynthetic processes, therefore absorbing less atmospheric carbon dioxide.
But measurements coming from the flux tower flew in the face of those expectations. In the Tapajós, early measurements indicated that many of forest’s trees were, in fact, actively “flushing,” or growing new leaves, in the dry season—and therefore, photosynthesizing at even higher rates at that time of year than in the wet season. Also, in this broad-leafed, evergreen, tropical forest, light seemed to be a more important factor than rainfall in the forest’s photosynthetic dance. The assumption had been that tropical forests would be very drought sensitive, since they had shallow roots and couldn’t access deeper soil water.
The measurements of dry-season flushing were so surprising, said Wofsy, Saleska’s post-doctoral advisor at Harvard, that Saleska and the team started to do what any good scientist should: “Try to break the data.” They looked for anomalies or artifacts that might have slipped into their instrumentation calibration, or in the analysis of those measurements. “When you get a contrary result, you look a little harder,” said Wofsy, in a phone interview from his office in Cambridge, Massachusetts.
It turns out that the conventional landscape ecology wisdom about seasonal leaf production had been derived from temperate (mid-latitude) forests and from satellite measurements, and those data had been baked into global climate models. Saleska’s work offered a new way of looking at the rainforest, which sent some global climate modelers back to their supercomputers to re-think their landscape-scale carbon-flux numbers. “Modelers and satellite people don’t like to be told their ideas are upside down,” said Wofsy. “It turns out the forest is growing like gangbusters in the dry season.”
The Science paper headline, with Saleska as lead author, was innocuous enough: Carbon in Amazon Forests: Unexpected Seasonal Fluxes and Disturbance-Induced Losses. But it helped chart a new direction in the field, and encouraged Saleska to keep returning to the Tapajós.
When we reached the field station on day two, Saleska climbed the tower to troubleshoot the equipment failures. Prohaska, the PhD student and master of aerial gambiarra, gave me a tour of the research site. He advised me to put on my newly purchased snake gaiters to protect my lower extremities from bushmasters (a species of pit viper) and other venomous forest denizens.
Prohaska is a Tucson native who started rock climbing at 15, then worked as a research technician and industrial climber for Biosphere 2 before plying his climbing skills as a rigger for circuses and concerts. He joined Saleska’s Tapajós crew as a technician while working on a Masters in Latin American studies, before starting his PhD in ecology and evolutionary biology. Fluent in Portuguese, Prohaska does everything from finding functioning ATMs in Santarém where he can withdraw cash to pay for generator fuel to coaxing expensive research equipment back from recalcitrant customs officials.
Prohaska’s high-flying skills have enabled him to set up a remarkable network of walkways 30 meters (100 feet) above the forest floor. He shoots slingshots and crossbows with fishing line attached to get his first access to high limbs capable of holding his weight. He then uses the line to pull ropes up into the canopy, testing the strength of the limbs as he does. When he’s ready to leave the forest floor, he climbs with uncanny speed using rock-climbing ascenders called jumars attached to nylon webbing that he uses to coordinate his feet and arms in a balletic vertical dance.
To finish our tour, Prohaska led me down a 200-meter (218-yard) boardwalk to the “walkup tower,” which looks like a precarious stack of freestanding scaffolding rising almost 140 feet above the forest floor, higher than almost all of the tallest trees. One anomaly breaks the endless sea of green: Rising above the canopy on the flux tower a few hundred meters away, we could see Saleska silhouetted against the coming dusk, still fiddling with the instruments.
“A rehearsal for hell”
Keeping all these sensors, computers, inverters, voltage regulators, air conditioners, and generators running is a constant battle against equatorial entropy. Mold, rust, power outages, rain, and insects eating through rubber wire insulation all create obstacles to gathering these tiny building blocks of scientific discovery.
Saleska, down from the tower, sat in the air-conditioned shack with Silva Campos, hunched over a computer on a wooden stool. The device that measures wind speed had been malfunctioning, as had another sensor designed to measure the relative amounts of water vapor and carbon dioxide. At last, they’d narrowed down one problem to a faulty wire or a faulty connector.
Saleska tinkered with a circuit board, replacing a broken component. I joked that he had to suddenly become an electrical engineer as well as an ecologist, and he off-handedly said, well, EE was his minor at M.I.T. He then coded the new information into the computer.
Outside in the dripping heat, Penha and Prohaska slipped into climbing harnesses draped with locking carabiners, ascending devices, descending devices, nylon webbing, a leaf porometer for Penha for her work measuring water vapor exchanges, and Prohaska’s hyperspectral camera, which measures leaf composition. The scientists don snake gaiters when on the ground, and mosquito head nets when they’re in the trees, more for the gnats that can carry leishmaniasis (a parasite-borne skin disease) than for the mosquitoes, which can carry malaria and Zika.
Penha completed her Master’s in environmental science and moved to the countryside with her husband and two children to teach high school biology, when the forest—and Saleska’s lab—beckoned. At first, Penha said, she had reservations about working with foreigners, whom she feared would exploit her as a local source of cheap labor.
Then she met Saleska and Prohaska, and her opinion quickly changed. She liked how they sought her opinion on how to organize their own work. Still, Penha recalled that the first day climbing up to Prohaska’s perches in the canopy felt like a “rehearsal for hell,” since she feared heights more than the tree snakes and spiders that are now her daily companions.
Penha says that as a kid growing up in the Amazon, the forest didn’t appear remarkable to her. But the more she studied it, the more the ecosystem’s intricacies charmed her. Now, she says, “Each little piece is surprising.”
Penha measures the hydraulic functions of leaves. Her field days begin at 3 a.m., when she gears up and climbs to the hanging walkways, which range from 18 to 36 meters (60 to 20 feet) off the forest floor, and are as much as 40 meters (130 feet) long. From 4 a.m. until 8 p.m., she takes her porometer and cruises two different walkways, taking four measurements on each leaf, five leaves per tree, 14 trees a day, every two hours. Which adds up to 280 separate measurements per day that collectively capture the movement of CO2 into and water out of specific leaves over time. It’s a laborious, intensive process, to say the least, but it’s also an essential building block of the Saleska lab’s research.
To help complete the connection between the leaf and the landscape, other researchers take measurements of leaf litter quantity on the forest floor, read dendrometer bands to chart trunk growth, take laser measurements of tree crown reflectivity, and conduct spectral surveys of leaf flushing. Having precise micro-data, such as how individual stomata in a leaf absorb carbon dioxide and release water over the course of a day, is the kind of ground-truthing that adds to the scientists’ understanding of the life cycle of leaves, trees, and the forest as a whole.
Ultimately, this information can be scaled up, cross-referenced with satellite data, and applied to global climate models. “It’s so important to have the data from these flux towers,” says Michela Figueira, a biology professor at UFOPA. The data that Saleska’s team gathers “paints a detailed picture of what’s really happening in the forest,”
That data, Figueira said, doesn’t stay in one scientific silo. One of Saleska’s gifts, she said, is that “he’s not stuck in one field. He has this view of every process to help him make sense of the tower measurements.” Other measurements, including satellite data, are important, she says, but “Scotch knows you can’t simply take a picture and say, `okay, the forest is like that.’”
The next day, Prohaska climbed to a high limb of a 40-meter (130-foot) tree, and suddenly broke into a free-hanging twerk. “Aaaaaaants,” he yelled, after he had disturbed a nest. The angry colony used his rope as a conduit to engulf him. After rappelling down, he shook the ants off in a series of emphatic, sweeping gestures.
Amazingly, Prohaska said after repelling the ant invasion, there have been few accidents and incidents among the fieldworkers over the years. True, one grad student did get stung by a black scorpion, but after 24 hours of basically being paralyzed, he emerged unscathed. There have been a number of open wounds, one case of leishmaniasis, some nasty insect bites and a car crash. For a station that’s been operating in the middle of the Amazon since 2000, that’s an impressive safety record.
Prohaska has become something of a leaf-whisperer. He can tell the age of a leaf the way a pediatrician can gauge a child’s age. Some leaves can live for years, he explained, while others die off and are replaced in months. Each of the 300 species of trees grows and drops leaves at different rates. “Trees here don’t just lose their leaves once a year,” he said. “It’s a constant turnover.”
How the forest takes up carbon fascinates Prohaska. Stomatal conductance, he explained, is essentially the relationship between evaporation and evapotranspiration. Leaf water potential describes, among other things, the optimal time for leaves to open their stomata and “eat” carbon dioxide. “A leaf is like a little factory powered by light,” he says. But leaves pay a price to eat their atmospheric fuel. “To gain CO2,” he said, “you have to lose water.”
Prohaska’s swiftness when climbing is especially helpful here because, he said, “the hardest part about tropical research is sample size.” It’s always difficult to measure the essential functions of leaves—photosynthesis and respiration—because leaves obviously live at the ends of tree branches. Inconveniently, some Amazonian trees don’t even bother with branches until they’re 20 or more meters (65 feet) off the forest floor, since the competition for light makes it a waste of energy to sprout low limbs.
Up in the walkways, Prohaska takes hyperspectral camera measurements that help determine the changing quantity of the leaves over time. Since leaves are reflective in the infrared, he can shoot laser pulses into the canopy, similar to radar. In this way he can document leaf density at different heights in the canopy, and how that changes over time. He can also calibrate a model that gives an average age of leaves and some sense of the leaf quality, vital components for any modeler trying to understand the landscape-scale implications of daily and seasonal changes.
Hot, Dirty Work
Research papers that Saleska has co-authored since that landmark Science paper in 2003 continue to build on his initial observations: Seasonal dry and rainy seasons per se are not the most important driver of leaf production, or the photosynthetic uptake of atmospheric CO2 in the Amazon. Perhaps more importantly, it has become clear to increasing numbers of scientists that satellite data and computer models alone cannot project what a warmer world will do to the forest’s role as a sink or a source. It’s going to take some hot, dirty work. “Changes to the carbon cycle in tropical forests could affect global climate, but predicting such changes has been previously limited by lack of field-based data,” wrote Christopher E. Doughty and colleagues in a special section of the American Geophysical Union’s publication Global Biogeochemical Cycles in May 2015.
The February 26, 2016 Science magazine with the cover line, “Amazon photosynthesis,” was devoted to the subject of carbon exchanges. In an article authored by Jin Wu, who works with Saleska at the University of Arizona (Saleska is a co-author), Wu wrote, “Ultimately, understanding the evolutionary and physiological basis for phenological mechanisms may be critical to predicting the long-term response and resiliency of tropical forests to changing climate.”
Which, in translation, means that in order to help scientists project future climate changes, people like Saleska and Penha and Prohaska will have to continue gathering their sweaty, buggy, gambiarra-infused data.
Like many scientists, Saleska doesn’t like to wade into contemporary political debates. But he won’t shy away from his conviction that human-caused climate change is breeding consequences that will only increase with inaction. “Not only will these forests be affected by climate change, they themselves will affect climate,” he said, as he prepared to leave the Tapajós to visit another flux tower farther up the Amazon, near Manaus.
“If we proceed with climate change the way we have been, the fate of this forest will likely be grim no matter what its degree of resilience,” Saleska said. “Resilience just isn't enough to counteract the massive hammer blow of the amount of climate change that would happen if humans do not slow their impact.”
Why, I asked, does he do this? “Because I'm a scientist and a human being, both. This is an incredible frontier of knowledge and it's an incredibly exciting experience to be part of the forefront of the advance of human knowledge about how the world works.”
Sitting in a folding chair and shedding his work boots for sandals, Saleska put on his human hat. “You can't help but come here and be completely awestruck by the beauty of the forest, and also sobered by the threat that the forest is under.” He pointed at the flux tower. “I may have climbed that tower 50 times, 60 times. Almost every single time I go up there, I stop and take a look around and look at the beauty of the forest and think, `This is my office. This is my lab.’”
He got up and finished the thought. “Once you've visited the Amazon, once you've been in the forest, it never leaves your imagination or your heart.”
A big enough lever?
My last day at the field station, I clipped in for my own vertical field trip up the flux tower at sunset. In every direction, I could see the slanting light illuminating a thousand shifting hues of green as the sun met the western horizon over the Tapajós River.
As the forest chatter at day’s end rose in volume and emanated into the canopy, I imagined the cornucopia of life forms below, settling in for their nighttime rituals: venomous bushmasters and benign blue butterflies, tiny leafcutter ants and giant anteaters, jacarandas and jaguars, all playing their intricate and essential roles in keeping the rainforest—and planet—vibrant.
The Amazon spread out below me like an undulating carpet. A soft, warm breeze moved like a melody and coaxed the trees into a swaying, rhythmic dance. I imagined the forest beginning to exhale, its daylong work of photosynthesis done, the nocturnal creatures beginning their shift.
I thought about what Saleska told me when I asked about his Archimedes quote: “Fossil fuel burning is Archimedes’ lever that will move the Earth unless we stop it,” he said.
Could science be a large enough lever to right it? I asked him.
Saleska paused. “That’s an interesting question,” he said, raising a single eyebrow.