On the death of giant trees

Litterfall

For Esquivel-Muelbert, discovering new aspects of nature is a lifelong love. Her childhood was spent climbing trees to see who could claim the biggest fruit, tracking animals, and collecting shells.

She remembers walking with her father in the forest, while he explained the importance of litterfall – when dead plant material falls to the ground and provides nutrients for the soil.

“Even though I was very little at the time, I remember thinking how beautiful litterfall was, and that more people should know about it,” Esquivel-Muelbert says. “That the forest was feeding itself by shedding its leaves.”

Esquivel-Muelbert was born in Panama, where her socialist father fought the Americans during their invasion of the country.

The family then moved to Southern Brazil, where Esquivel-Muelbert’s parents cultivated fingerling fish. They kept these juveniles in ponds and reproduced them in their lab, helping to preserve native Brazilian species. 

Esquivel-Muelbert remembers, “As children, we were always helping to run the fish farm. My parents instilled a strong sense of responsibility. That a worthwhile life is one that contributes positively to your society.

“We learned how to avoid polluting streams, and minimise the environmental impact of our farm. There was a constant stream of interns from universities in Brazil too, who came to help. I was always somehow connected to academia.”

While visiting her researcher uncle in Panama, Esquivel-Muelbert was enthralled by a thriving community of scientists who gathered to explore the forest.

“I thought: this is what I want to do.”

How do we track forest health?

Ecologists have many ways of assessing the health of a rainforest. Some are deceptively simple. 

Put a rope square on the ground, 100 metres by 100 metres. Then count and map all the trees within that square. Measure their diameter at chest height. Then collect branches from the trees – easier said than done, when some branches only start at 40 metres above the ground. 

“We train climbers to take branch samples, in these cases,” Esquivel-Muelbert says. “You put a rope around your foot, and lever yourself up the trunk. It requires a very strong core! Then it’s up to the people on the ground to catch the branches you collect, and make sure they’re the right ones.”

After taking these samples, botanists piece together the structure of the forest. It can take months to identify all the different species taken in by the sampling team. In one hectare of forest, ground teams might find up to 700 trees, and 300 different species. New species crop up frequently.

Ground teams have to come back to the same site every 2 years, to take the same measurements. In this way, researchers accumulate enough data to understand how forests are changing over time. 

This data can only be gathered through international co-operation. Esquivel-Muelbert works with colleagues across the globe to ensure these projects value and acknowledge every person in the network – especially given historical inequalities between the global north and south.

More carbon, bigger trees

Teams gathering data in this way have shown that forests are adapting to climate change – higher atmospheric CO2 levels and increased droughts – in surprising ways. 

First, drought-resistant trees now have an advantage. And second, bigger trees are thriving. 

“We found that so far, the Amazon has been able to pack more carbon in the same area,” Esquivel-Muelbert says. “This means bigger trees, on average.”

The rainforest has shown surprising resilience. So far, some of the negative impacts of climate change (more carbon in the air) seem to have been mitigated by the effects of increased plant resources. 

While this is good news for now, we don’t know the long-term consequences of trees packing in more carbon. They might be investing less in combating other threats as a result. 

Likewise with die-backs caused by pathogens, like Dutch Elm Disease. Tropical forests have so far been more resistant to these than UK forests, for example, perhaps because rainforests are so diverse that bugs taking out one species don’t take down the whole system. But this is speculation, and any system has its limits.

The increased frequency and intensity of droughts is still a major concern, causing spikes in tree mortality. Droughts can also turn off the forest’s carbon storing activities, decreasing their ability to limit the harmful effects of our emissions.

Regardless, the need to reduce carbon emissions is paramount – our natural systems can only cope with so much change. Esquivel-Muelbert’s work clarifies how the character of our forests is already shifting as a result of our actions.

At Cambridge’s Department of Plant Sciences, Esquivel-Muelbert has found the perfect place to continue her work. Here, she can collaborate with researchers at the forefront of plant molecular biology. She can also connect her projects to those in other disciplines, through initiatives like the Cambridge Conservation Institute. And together with her international network of tree detectives, she can help us understand the deeper workings of Earth’s forests, while there is still time to act.

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