Amazon trees glow more under stress but photosynthesize less

In the dense green heart of the Amazon, a silent conversation takes place between trees and sunlight. From space, this conversation appears as a faint glow, a shimmer given off by leaves.

This glow, called solar-induced fluorescence (SIF), has helped scientists estimate how much photosynthesis is happening on Earth. The brighter the forest glows, the healthier it appears. But new research warns that this glow may not always tell the full truth.

As climate change makes dry seasons longer and harsher, researchers are looking closer at how the Amazon canopy responds. What they have found could change how we read satellite data and rethink what we know about forest health.

A tree may shine brightly but still struggle to breathe, grow, and store carbon. The Amazon, often described as the lungs of the planet, may be gasping in ways we fail to see from above.

How trees glow and handle sunlight

To understand how this vast forest manages sunlight, scientists at Michigan State University took their work into the forest. Led by doctoral candidate Leonardo Ziccardi, the team climbed trees nearly 60 meters tall in the central Amazon.

Armed with a handheld device called the MultispeQ, the researchers measured how leaves absorb, use, and release sunlight.

“It’s been a long journey,” said Ziccardi. “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.”

“We’ve spent hundreds of hours up in the canopy doing measurements, some of the most intense and rewarding work I’ve ever done.”

Over 3,000 leaf measurements showed how leaves manage energy in changing light, height, and moisture conditions. The team explored three energy paths: photosynthesis (ΦPSII), heat dissipation (ΦNPQ), and steady-state fluorescence (ΦNO).

Drought unmasks a complex dance

In dry seasons, sunlight floods the upper canopy. But instead of boosting photosynthesis, this intense light overwhelms the leaves.

As a defense, the leaves channel more energy into heat (ΦNPQ) and emit more fluorescence (ΦNO). Meanwhile, actual photosynthesis (ΦPSII) drops.

The researchers found a striking pattern. In dry conditions, photosynthesis and fluorescence first track together, then split, and finally oppose each other.

This “triphasic” behavior means fluorescence can increase even as photosynthesis crashes. The phenomenon creates a major red flag for interpreting satellite SIF readings.

Tree glow shifts across canopy layers

Each forest layer has its own light and stress profile. The lower canopy, shaded and cooler, showed higher ΦPSII and ΦNO but lower ΦNPQ.

Upper layers, exposed to harsh light, had the opposite pattern. They showed lower photosynthesis and more heat loss.

Interestingly, the strongest SIF signals came from these shaded, lower leaves. That means satellites may often be “listening” to the quieter, protected parts of the forest rather than the sun-drenched treetops.

Species matter more than expected

Different tree species do not respond to environmental stress in the same way. The study revealed that the fluorescence signal known as ΦNO varied strongly between species, particularly in the sun-exposed upper canopy during the dry season.

This means that some trees, under intense light and water stress, increased their fluorescence output more than others, depending on how they manage excess energy.

The species-level differences suggest that even if two forest regions appear to glow equally bright from a satellite’s perspective, their actual rates of photosynthesis and their ability to take in carbon could be very different.

In other words, the same level of fluorescence does not guarantee equal productivity. The forest’s composition, the types of trees present and how they deal with light, plays a crucial role in shaping what those glow signals really mean.

Remote sensing tools must account for these biological differences to avoid drawing misleading conclusions.

Tree glow can confuse satellite readings

SIF still offers a powerful tool, but it must be used carefully. The researchers recommend accounting for leaf age, species, and height.

Models that ignore these factors risk overestimating how much carbon forests absorb, especially during drought.

The study highlights how the Amazon’s structure, with its dense lower canopy and diverse species, complicates the reading of satellite data. Signals from above might miss crucial internal forest dynamics.

Where research goes from here

This work lays the foundation for smarter GPP models (gross primary productivity). By capturing the vertical, seasonal, and species-specific nuances of energy flow, it helps scientists avoid mistaking plant glow for plant growth.

Ziccardi’s team shows that to understand forest photosynthesis, we must look beyond surface signals. Forests whisper complex stories in light, and it’s time we listen closely.

The study is published in the journal New Phytologist.

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