At the top of Biosphere 2's rain forest canopy, leaves brown and wilt beneath the glass enclosure heated by the Arizona sun. Air temperatures inside can reach above 130 degrees Fahrenheit towards the tree tops, while temperatures in the real Amazon canopy never exceed about 108 degrees F. This makes Biosphere 2's sealed rain forest one of the hottest tropical canopies in the world – a critical preview of challenges the Amazon could one day face. 

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With some trees topping 90 feet, researchers needed an easier way up into the canopy to study rain forest resilience and better predict how real forests might fare under environmental stress.

During the spring 2026 semester, Biosphere 2 completed construction of three canopy walkways strung throughout the rain forest biome. The walkways give students and researchers safe access to the upper canopy, which is over 70 feet high.

The project is funded by a Provost Investment Fund grant and led by Joost van Haren, director of rain forest research.

“The walkways really give you nice accessibility, and it’s much easier than training students to actually climb the space frame – the biome’s extensive surrounding structural framework – that they previously had to scale by hand,” van Haren said.

The provost’s office and media are expected to attend an official opening of the canopy walkways on October 16.

Tracking resilience, leaf by leaf

Amid this year's exceptionally strong super El Niño, understanding rain forest resilience takes on a new sense of urgency. Biosphere 2’s new walkways are key to carrying out these experiments.

"Strong El Niños bring severe drought to much of the Amazon basin,” van Haren said. “Drought means fewer clouds and stronger sun, so with less shading, leaf temperatures rise. Drought and high heat always coincide with one another.”

The walkways sit at three distinct heights, roughly 20, 45 and 70 feet, deliberately chosen so researchers can compare how plants, such as Theobroma cacao and Coffea arabica, respond to different conditions at each level – cooler, shadier air down towards the forest floor and intense heat and light near the top.

Resilience comes down to whether a leaf can afford to protect and repair itself faster than the heat damages it. This recovery process requires carbon, which the leaf absorbs during photosynthesis. Heat slows photosynthesis, so the leaf earns less carbon just when it needs more. Leaves also cool themselves by releasing water through the same pores that take in carbon dioxide, but during periods of drought, those pores close, leaving the leaf even hotter and further in need of carbon. 

“At this stage, leaf mortality could slowly lead to tree mortality,” van Haren said. 

Easy access via the walkways enables researchers to measure the effects of this negative feedback loop high up in the canopy, helping to build on previous heat experiments the rain forest team ran in 2024.

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“We previously found that for the whole canopy there seems to be a trend where between 122 and 126 degrees F, leaf survivability for all species reaches a point of no return,” van Haren said.

This summer, students participating in a National Science Foundation-funded research program are among the first to use the walkways to conduct fieldwork. Students are tracking how water flows through the trees and out through the leaves. They are also measuring chlorophyll and protective compounds to test whether leaves higher in the canopy build chemical defenses against the pests that thrive in the heat.

Construction in the canopy

Rain forest researcher and terrestrial biome manager Jason DeLeeuw and crew members Cody Dykes, Michael Oldham and postdoctoral student Justin Beslity installed the walkway system. The team, which was certified to climb the biome’s space frame, first bolted heavy steel saddles together onto joints of the biome's framework at the three allocated heights, then ran cables between them. 

“I sat in a small sling suspended on a pulley system, and dropped down between the cables to measure their tensions,” DeLeeuw said. “A cable tension meter isn’t a small instrument – it was quite awkward to use, especially while hanging in mid-air.”

The bottom walkway also required one end of its cables to be anchored straight into the biome’s artificial mountain feature, which meant drilling through 12 inches of concrete, DeLeeuw said.

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After making final adjustments, the team laid aluminum walkway segments over the cables and implemented redundant systems into the walkway safety protocol.

Each walkway carries five cables: two along the bottom that the aluminum planks rest on, two along the sides that act as handrails and one more line overhead. Workers clip their lanyard to the top cable as an extra safety precaution, and fall-rated netting wraps beneath the entirety of the walkways to make the canopy walk less daunting. 

“It really facilitates a lot of movement in the canopy because we have one walkway that runs east to west and two that run north to south,” DeLeeuw said. “Now, if we want to work in the southern part of the biome, we just climb up and walk across the walkway instead of hanging onto the space frame.”

A potential Phase 2 might extend the walkways further around the rain forest biome to limit the need for climbing, van Haren said. Expansions may eventually help the team mount continuous, heat-tolerant sensors on harder-to-reach leaves.

For now, the walkways offer researchers their clearest path yet into the world's hottest canopy and, with it, a glimpse at how Earth’s rain forests might endure a warmer future.