Climate change is disrupting how Amazon trees process sunlight

In the sweltering crown of the Amazon rainforest, nearly 200 feet above the forest floor, researchers have spent years chasing a deceptively complex question: How do tropical canopy trees keep from frying in the very sunlight they rely on?

A new study led by Michigan State University (MSU) has found that Amazonian trees juggle sunlight with remarkable finesse – yet that balance may falter as climate change turns the forest hotter, drier, and brighter.

Doctoral candidate Leonardo Ziccardi led the study under the guidance of forestry ecophysiologist Scott Stark.

Since 2019, the team has mounted multiple field campaigns near Manaus, Brazil. They carried handheld photosynthesis instruments and ropes strong enough to haul scientists and gear into the leafy heights.

“It’s been a long journey,” Ziccardi said, recalling hundreds of hours spent aloft, recording minute-by-minute fluctuations in leaf temperature, humidity, and photon capture.

Trees manage light like solar panels

Leaves are biological solar panels, absorbing photons to power photosynthesis. But excess light can scorch the photosynthetic machinery unless trees convert the overload to harmless heat or a faint red glow known as chlorophyll fluorescence.

For decades, ecologists have used satellite-detected “solar-induced fluorescence” (SIF) as a stand-in for photosynthesis across vast regions.

Until now, however, no one had mapped in detail how individual canopy leaves partition light between photosynthetic work and protective dissipation in the ever-shifting conditions of an Amazon dry season.

Tracking every photon

Using the MultispeQ – a portable device created at MSU’s DOE-funded Plant Research Laboratory by co-author David Kramer – the team clipped onto thousands of leaves spanning dozens of tree species and all levels of the canopy.

The instrument measures absorbed light, photosynthetic electron flow, heat loss, and real-time fluorescence, letting the scientists trace every photon’s fate.

The data reveal canopy leaves acting as huge, sensitive antennas. Under mild conditions, leaves direct most light to carbon fixation, releasing only small amounts as heat and fluorescence.

As midday sun or dry-season humidity deficits intensify, the balance shifts. Leaves ratchet up safety valves that vent surplus energy as heat.

When those safeguards max out, fluorescence surges as a last-ditch outlet, even while photosynthetic rates plunge – an early warning that cell machinery is approaching burnout.

A three-phase stress signature

This nuanced response emerged as a consistent three-phase pattern across species and canopy positions.

In a balanced phase, at low to moderate light, photosynthesis and fluorescence rise in tandem. Leaves are using energy efficiently, and protective heat dissipation remains secondary.

In the transition phase, higher light or drier air forces leaves to divert more absorption into heat. In this stage, photosynthesis plateaus and fluorescence begins to decouple from it.

Finally, in the overload phase, heat dissipation hits its ceiling. Fluorescence spikes even higher, signaling potential photodamage, while photosynthesis collapses.

Such high-resolution leaf physiology had never before been charted in Amazonian forests. For Ziccardi, the breakthrough sprang from relentless canopy work.

“Since 2019, we’ve run multiple field campaigns across seasons, climbing giant trees in the heart of the Amazon to understand how these forests respond to environmental changes,” he said. “We’ve spent hundreds of hours up in the canopy doing measurements – some of the most intense and rewarding work I’ve ever done.”

Satellites may misread tree stress

The discovery carries a caution for global remote-sensing efforts. Researchers often map SIF from space to estimate gross primary productivity. This key metric tracks how much carbon dioxide vegetation converts into biomass each year.

The study shows that during severe droughts – when cloud cover thins and sunlight becomes harsher – canopy leaves may glow brightly even as their photosynthetic capacity craters. Relying on SIF alone could therefore exaggerate the forest’s true productivity at precisely the times it is most stressed.

As the Amazon’s dry season lengthens under warming temperatures and regional deforestation, clouds are thinning, and insolation is rising. Whether canopy leaves can sustain their intricate balancing act under these compound pressures remains an open question.

The team’s measurements suggest resilience but not invincibility: energy-dissipating pathways can adjust, yet they have limits.

Studying Amazon trees up close

By combining leaf-level measurements with satellite imagery, researchers can refine algorithms that translate fluorescence into carbon uptake, improving forecasts of the forest’s future. But such calibration requires continued boots – or rather, ropes – in the canopy.

Stark notes that the Amazon’s towering biomes host myriad species, each with distinct physiology. Ongoing field campaigns will probe whether drought-adapted trees hold hidden advantages or if a tipping point looms where protective strategies fail en masse.

The work also provides a template for monitoring other tropical forests edging toward hotter, clearer climates.

Integrating handheld devices like the MultispeQ with flux towers, drones, and satellites could provide a multiscale view of photosynthetic health. This is crucial for sounding alarms before irreversible die-back sets in.

A delicate dance with light

The Amazon’s canopy trees evolved to thrive in a Goldilocks zone of radiant power. They receive enough sunlight to drive one-fifth of the planet’s photosynthesis, yet it’s softened by frequent clouds and moist air. Climate change threatens to tip that balance.

Ziccardi’s data offer a first-hand glimpse of how each leaf on a giant tropical tree responds second by second to light’s bounty and burden.

The delicate choreography between photosynthesis, heat venting, and fluorescence could determine whether the rainforest continues to trap carbon – or begins to release it back to an already warming world.

The study is published in the journal New Phytologist.

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