Discovering life in the canopy

Interview by Katerina Markelova

UNESCO

You have spent four decades studying the forest canopy. What do you see and feel when you leave the dark forest floor and mount to the summits of the trees?

What has always amazed me about climbing into the canopy is that there are such dramatic differences, both in microclimate and biology just 30 meters above the forest floor. To reach my study trees in Monteverde, Costa Rica, I must first walk along the dark and humid forest floor for about an hour. The forest appears as a group of tall cylinders with the canopy that seems to be a big blob of green. The colors are muted. The sounds are muffled. There's no wind. 

But when you ascend, what you feel is this gradual change in the surroundings as you emerge into the treetops. There is a different microclimate up there: more sunlight, higher temperatures and more wind. Sounds are very present with a hummingbird whizzing by or the howler monkeys calling to each other. Orchids and bromeliads are abundant and incredibly lush. And there is a huge diversity of canopy invertebrates. All these species of plants and animals that we would never see on the forest floor have evolved over millennia to adapt themselves to this different microenvironment and architecture.

Until quite recently, forest ecologists studied complex canopy ecosystems without leaving the ground. What has changed since scientists penetrated the canopy forty years ago?  

In 1983, the entomologist Terry Erwin from the Smithsonian Institution (United States), used insecticidal fog to study the beetles and insects that live in the forest canopy. He would basically shoot this fog up into the canopy at dawn and the insects would fall to the forest floor. What he learned was that there was a tremendous amount of insect diversity up there that people never knew about. That led him to call the forest canopy the last biotic frontier.

Later on, a few pioneers started to use mountain climbing techniques to get up into the forest canopy. That was a real turning point in terms of our understanding of these ecosystems because it allowed studying the organisms where they live. Other methods rapidly developed, such as walkways between forest canopies to observe arboreal mammal and bird behavior, and the use of construction cranes to get above the forest canopy to study the interface of the atmosphere with the living biota. A French group headed by Francis Hallé invented the canopy raft, a hot air balloon with a raft that could be lowered on the top of the trees, and the canopy bubble that could navigate its way around the trees. And most recently, we've been using remote sensing, satellite imagery and drones. 

Epiphytes – plants that grow on other plants, such as orchids, mosses or ferns – are your main field of scientific research. What role do they play in the forest ecosystem?

Epiphytes are a very diverse group of plants that get support from the trees, but, unlike parasitic plants, don’t have root connections to vascular systems of their hosts. Over evolutionary time, epiphytes have evolved the physiological and anatomical capacity to intercept and retain the atmospheric nutrients present in droplets of rain and mist. 

They capture nutrients from outside the ecosystem and then make them available to plants and animals that are inside the ecosystem. One of our studies showed that one third of all foraging visits for nectar, sugar and mosses by birds and arboreal mammals aim at the epiphyte resources. And six bird species turned out to be epiphyte specialists, because they use them for over 90 per cent of their foraging visits. Our conclusion, corroborated by other studies, was that epiphytic plants fulfill a critical role in terms of nutrient cycling. 

You’ve conducted studies of forest canopies on four continents. What have been your most striking discoveries?

Epiphytes, by decomposing, create the canopy soil, which can reach a meter deep. They're rich in nutrients and are populated by invertebrates, microbes and even earthworms. Some trees can actually put out roots from their own branches and trunks that permeate these mats of canopy soils and take up nutrients and water from them. The fact that trees are capable of growing roots high above the forest floor literally blew my mind. 

I also learned that although epiphytes seem so vibrant and amazingly powerful, they are not resilient to physical disturbance. In 1987, I carried out a series of experiments in which I stripped branches of epiphyte mats for a meter in length away from the branch to see how the epiphyte would come back. I anticipated that they would grow back very rapidly and that they would encroach from the outside like grass. But I was wrong on both counts. It wasn't until 13 years after that I saw the first signs of recolonization and 22 years later, only 40 per cent of the original cover was restored. 

What new species have been discovered in the canopy? 

We're continually finding new species, especially orchids and invertebrates. But it's really difficult and probably impossible at this point to estimate how many more are yet to be discovered. That's really partly due to our ignorance of what's already up there. We can do that for other plant groups, for trees, for example. According to a 2022 study published in the American scientific journal Proceedings of the National Academy of Sciences, out of an estimated 73,000 tree species in the world probably 9,000 still remain unknown. The canopy researchers just don't have that kind of foundational database at this point.

Since the mid-1970s you have focused on the Costa Rican cloud forest in Monteverde. What makes it special? 

Although cloud forests make up a very small proportion of woodlands, they have a unique structure, composition and function. They grow on tropical mountains and their major climatic characteristic is the presence of wind-driven mist and cloud. When the warm water, which forms over the ocean, moves inland by the trade winds, it encounters mountains and cools down, becoming mist and clouds. This moist air provides nutrients to cloud forests. With climate change, it takes a longer time and a higher elevation for the condensation to happen. As a result, the forests get less exposed to the nourishing clouds that, in some cases, are moving all the way up and over the mountains without encountering the forests at all. 

Cloud forests’ contribution to world biodiversity is disproportionately high, because they are home to many endemic species. When I was a graduate student, one of those amazing cloud forest endemics was the golden toad. You could only see these brilliantly colored creatures during the very dry season when they were going out to breed. El Niño, which occurred in Costa Rica in 1988, caused their extinction. 

You dedicate a part of your time to raising public awareness of the crucial role played by forests. Is your message being heard?

Beyond just their ecological value, forests touch a wide spectrum of other crucial values, be it aesthetic, economic or spiritual ones. When we engage with the forest, we physically feel better, our stress and anxiety are reduced. I have spent many years trying to bring science lectures and conservation projects outside of academia, for example, to incarcerated populations who are denied access to nature. We've seen evidence of improvements in their mental health. 

Forests inspire art, poetry and music. When I bring artists to the canopy, they create pieces of visual arts, rap songs and poetry that they then perform in poetry readings or modern dance performances. If we can bring the values of nature to those places, we might have a better chance of expanding the audience and contributors to forest conservation. I think that scientists have a part to play in this.