Amazon plants' ability to absorb sunlight is being disrupted by climate change.

Researchers have been pursuing the surprisingly complicated topic of how tropical canopy trees avoid burning in the very sunshine they depend on for years in the hot crown of the Amazon rainforest, which is located almost 200 feet above the forest floor.

Amazonian trees are remarkably adept at balancing sunlight, according to a recent study performed by Michigan State University (MSU). However, as climate change makes the forest hotter, drier, and brighter, this equilibrium may break.

Scott Stark, a forestry ecophysiologist, guided the project, which was headed by doctoral candidate Leonardo Ziccardi. The team has conducted several field missions close to Manaus, Brazil, since 2019. They had ropes sturdy enough to carry scientists and equipment into the leafy heights, as well as portable photosynthesis instruments.

Ziccardi recalled hundreds of hours spent in the air, documenting minute-by-minute variations in leaf temperature, humidity, and photon capture, saying, "It's been a long journey."

Like solar panels, trees control light.

In order to power photosynthesis, leaves absorb photons, acting as organic solar panels. However, unless trees transform the excess light into harmless heat or a faint red glow called chlorophyll fluorescence, the photosynthetic apparatus may be scorched.

Ecologists have been using satellite-detected "solar-induced fluorescence" (SIF) as a proxy for photosynthesis across large areas for decades. The way that individual canopy leaves divide light between photosynthetic activity and defensive dissipation in the constantly changing conditions of an Amazon dry season, however, has not yet been thoroughly mapped.

Monitoring each photon

The researchers clipped onto thousands of leaves from dozens of tree species and all canopy levels using the MultispeQ, a portable device developed by co-author David Kramer at MSU's DOE-funded Plant Research Laboratory.

The device allows the scientists to track the fate of each photon by measuring absorbed light, photosynthetic electron transport, heat loss, and real-time fluorescence.

According to the data, canopy leaves function as enormous, sensitive antennas. In mild weather, leaves use the majority of light for carbon fixation, releasing very little in the form of heat and fluorescence.

The equilibrium changes when noon sun or dry-season humidity deficits increase. Safety valves that release excess energy as heat are left in place. When those protections are at their peak, fluorescence spikes as a last resort, even while photosynthetic rates fall. This is a sign that the cell machinery is about to burn out.

A stress signature in three phases

Across species and canopy positions, this complex reaction became a recurring three-phase pattern. In a balanced phase, photosynthesis and fluorescence increase simultaneously at low to moderate light levels. The effective use of energy by leaves keeps protective heat dissipation at a secondary level.

Higher light levels or drier air push leaves to convert more absorption into heat during the transition phase. At this point, fluorescence starts to separate from photosynthesis when it reaches a plateau.

Ultimately, heat dissipation reaches its maximum during the overload period. As photosynthesis declines, fluorescence increases even more, suggesting possible photodamage.

It was the first time that Amazonian forests had such high-resolution leaf physiology documented. Ziccardi attributed the breakthrough to his unrelenting canopy work.

"We've conducted several field campaigns throughout the seasons since 2019, scaling enormous trees in the centre of the Amazon to learn how these forests react to environmental changes," he stated. "We've done measurements up in the canopy for hundreds of hours; it's some of the most demanding and fulfilling work I've ever done."

Tree stress may be misinterpreted by satellites.

The finding raises a warning for worldwide remote sensing initiatives. To determine gross primary productivity, researchers frequently map SIF from space. This important indicator measures the annual amount of carbon dioxide that vegetation transforms into biomass.

According to the study, canopy leaves may glow brightly even as their photosynthetic capacity crashes during extreme droughts, when cloud cover thins and sunlight becomes harsher. Therefore, depending solely on SIF may overstate the forest's actual productivity during the periods when it is under the highest stress.

Clouds are thinner and insolation is increasing as the Amazon's dry season lengthens due to regional deforestation and warming temperatures. It is yet unclear if canopy leaves can continue their complex balancing act in the face of these combined demands.

According to the team's measurements, energy-dissipating pathways can adapt, but they have limitations, suggesting resilience but not invincibility.

Examining Amazon trees closely

Researchers can improve predictions of the forest's future by integrating satellite photos and leaf-level observations to improve algorithms that convert fluorescence into carbon uptake. However, such calibration necessitates ongoing boots—or, more accurately, ropes—in the canopy.

According to Stark, a wide variety of species with unique physiologies can be found in the Amazon's tall biomes. Whether drought-adapted trees have undiscovered benefits or if a tipping point where defensive measures fail collectively is imminent will be investigated by ongoing field campaigns.

A framework for tracking other tropical forests that are moving towards hotter, drier climates is also provided by the studies. A multiscale picture of photosynthetic health might be obtained by combining portable instruments like the MultispeQ with flux towers, drones, and satellites. Setting off alarms before permanent die-back occurs is essential.

A beautiful dance with light

The canopy trees of the Amazon have adapted to flourish in a radiant power Goldilocks zone. Though frequent clouds and humid air soften it, they receive enough sunlight to power one-fifth of the planet's photosynthesis. That equilibrium could be tipped by climate change.

Ziccardi's findings provide an up-close look at how the abundance and load of light affect each leaf on a massive tropical tree second by second. Whether the rainforest retains its ability to store carbon may depend on the intricate balance between photosynthesis, heat venting, and fluorescence. or starts releasing it back into a world that is already warming.