Trees of Amazon Rainforest Are 'Time Capsule' of Human History, Absorbing Details of Their Surroundings Over Hundreds of Years

The tropical forests of the world act as important "time capsules" of human activity going back hundreds of years, according to a team of researchers.

For a review paper published in the journal Trends in Plant Science, the scientists describe how the biology of living tropical trees, combined with other archaeological and historical data, can shed light on the history of native peoples in the tropics, as well as the impacts of colonialism.

"Tropical forests have long been considered as 'pristine' wildernesses, untouched by humans until recent industrial forces have begun to invade them," Patrick Roberts, an author of the study from the Max Planck Institute for the Science of Human History, told Newsweek.

"However, archaeological data from the last two decades have shown that human societies have occupied and modified these environments over the course of millennia. How they changed these forests, and the implications that has for conservation and identifying sustainable coexistence strategies today remains a key question," he said. "We thought we would try to investigate this by analysing the largest 'witnesses' of changes in human activity in the tropics—trees."

Some tree species in tropical forests have been shown to grow for up to a thousand years. And in this time, they absorb details about their surroundings into their wood, the researchers said. For example, various factors such human settlements and industry can have an impact on how trees grow.

"We wanted to see and review how different existing methods for exploring tree populations, tree growth, and tree ages might be combined to explore how the growth of these forest giants has changed in response to humans over key periods in human history in the tropics," Roberts said.

In temperate regions of the world—such as those found in Europe and North America—scientists have long used trees to learn about climate change and environmental trends. Trees grow seasonally leaving a series of "rings" behind, which researchers can analyze to shed light on past climates.

However, these growth patterns can also be used to shed light on human activities—something that is often overlooked by ecologists, according to Roberts.

"For trees that like light, humans may clear their surroundings to promote their growth if they want to use them," he said. "This would lead to a change in their growth rings. Similarly, if humans cut down a lot of one species in one event then this would lead to all of the new growth of a species dating to a particular time period."

"In the same way archaeologists excavate layers to produce a 'stratigraphy' we can look at the 'living stratigraphy' of trees to look at how their growth changed, at what time, and how it might relate to climate and human activities," he said.

Scientists have also successfully used genetic analysis of trees in order to find out how different populations of trees relate to each other across space, and also how different characteristics may have been selected for.

"As trees of different ages can be sampled and dated, we can actually sample the 'ancient DNA' of trees in a single forest to look at how the population might have changed through time," Roberts said. "'Bottlenecks' in genes may occur following fierce deforestation, or certain genes may be selected for by humans looking for a particular trait."

Unlike in temperate regions, however, techniques such as genetic analysis and the study of tree rings, have not been well explored in the tropics for two main reasons, according to Roberts.

Brazil nut tree
A Brazil nut tree in Jaú National Park. Victor Caetano-Andrade

"Firstly, because it was assumed that tropical forests have no seasons and so no growth rings," he said. "Our review shows this to be incorrect and more than 200 tropical trees have been shown to form annual rings. Secondly, if there was an assumed absence of humans in these supposedly unattractive environments, then why would one expect to see any impact on the trees."

In the paper, the authors point to two case studies of recent research which reveal that this situation is changing in the tropics, and how scientists are beginning to glean more information about human history from the trees in this region.

The first case study looks at the work of Victor Caetano-Andrade—first author of the latest paper from the Max Planck Institute for the Science of Human History—who investigated massive Brazil nut trees in the Amazon. Caetano-Andrade's research has revealed how major periods in history have altered how these trees have grown near the city of Manaus, Brazil.

"Studying one stand of these trees, he showed that the arrival of colonists and development of Manaus led to a collapse of Indigenous population and the ending of Indigenous traditional management strategies of these trees," Roberts said. "As Indigenous people had favored these trees for their rich nuts, ensuring they had access to light, the loss of these people, and their management, impacted the growth of Brazil nut negatively."

The second case study discussed the work of several scientific groups which have shown that genetic variation of economically important trees—such as papaya, chocolate trees, peach palms, and Brazil nut trees—are closely associated with areas of intense pre-colonial human occupation in some parts of Central and South America.

"In the case of the chocolate tree, more detailed full genomic analysis even shows that humans apparently selected genes that reduced its bitterness and improved its resistance to disease," Roberts said. "In both cases, these types of study show that trees act as capsules of past human activity, and store these insights within their growing tissues. However, we argue that the combination of these approaches has even more to contribute."

In light of their paper, the authors argue that recognizing the worth of living trees as time capsules of human heritage highlights the need to protect tropical forests, which are some of the planet's most threatened environments.

"Ultimately, our paper argues that is essential that archaeologists and ecologists now work together to preserve not just the natural benefits of tropical trees but also the centennial and millennial scale records of human activities and knowledge that are imprinted within them," Roberts said.