What tropical trees can teach us about the environment
Rose Oelkers cores a tree in the Tucuman-Bolivian forest in southern Bolivia. Photo courtesy of Ross Oaks.
rose oaks is a tree-ring scientist with a difficult task: studying trees that don’t have rings — or at least trees that don’t have well-defined ones.
Currently a fifth-year PhD student at Columbia Climate Institute Lamont-Doherty Earth Observatoryshe analyzed tropical tree species in Bolivia and Peru to describe their growth patterns and responses to environmental changes.
But when Oakes attended William Paterson University in New Jersey as an undergraduate, she planned to study ecology. “I didn’t really know we could study tree rings,” she said.
That all changed when she met an enthusiastic tree-ring scientist in the William Paterson University Department of Environmental Science: Nicole Davey, who introduced her to the concept of trees as environmental storytellers. Oelkers knew she wanted to know more.
“I find it’s really nice that you can go to your local forest and look at tree rings from a specific period and link that to human memory – for example, if you were in the 1960s during the drought in the Northeast, you could look at Some old samples of hemlock trees in New York, look at the ringlets that happened in those years,” Oelkers said.
so when David Oelkers announced a research opportunity involving significant lab and field work, and took it – and hasn’t looked back since. “I fell in love with the idea of ​​being able to get your hands dirty and travel the world to try and learn more about different environments,” she says.
Via Davi, who is also a part-time senior research scientist at the university The Growth Ring Laboratory at the Lamont-Doherty Earth ObservatoryOelkers has since met other transformative tree-ring scientists and became the lab’s chief technician from 2015 to 2018, working on a variety of projects, including studying the density of tree rings in Alaska’s slow-growing white spruce.
Today, Oelkers is working with Leia Andrew-Hales and Roseanne Darrego Studying tropical trees in South America at Lamont-Doherty, she said it was very different from her experience of tree rings in the northern hemisphere. In the Q&A below, she tells us about the challenges of working with tropical species and how she discerns their hidden stories.
Why is your current study of tree rings in tropical climates important?
If we can study tree rings over time, we can understand different aspects such as forest age and climate change. South America doesn’t have a lot of tree ring data; it’s a huge gap in the network. Nonetheless, some of the oldest trees are known to be in the tropical Andes in Bolivia.
Trees grow very differently in these wet and dry environments without summer and winter. Before the 1980s, it was a common misconception that trees in the tropics just kept growing and didn’t even form annual rings. Or if they do, they are indistinguishable or inconsistent due to “lack” of seasonal climate change. In recent decades, the tree-ring community has made great efforts to explore tropical forests such as the Amazon and develop new tree-ring chronologies. They found that some trees, if they exist in places where the dry season lasts for more than two months, can actually form growth rings. By studying different parameters of these rings, we learned that they can respond to and record different changes that occur in the environment in which they grow.
Rose Oelkers coring a tree in the central Amazon of Bolivia. Photo courtesy of Ross Oaks.
What are the biggest challenges and opportunities of this job?
The sites I studied in Bolivia were very complex—there were high-altitude Andean environments with very slow-growing species, but also low-altitude, humid Amazon forests with millions of species that haven’t even been discovered yet. In some areas, we have little information about the history of forests and how tree species grow.
In the tropical Andes of Bolivia, we work with botanists at the National Herbarium of Bolivia; in general, our goal is to find and sample old trees that produce growth rings to learn more about the age and growth cycle of the forest. Studying new species for tree ring studies is exciting but challenging. Dendrochronology goes beyond determining the age of a tree by counting growth rings. For example, changes in these rings can help us understand how tree growth responds to climate change over time: Do trees grow faster or slower? Do they grow more in wet years? Are there signs of fire or changes in forest structure? Is it artificial? We really just get to know the history of the land from the perspective of the trees.
This is very different from the process of studying trees in a northern hemisphere environment, where trees are widely studied because a wide variety of species grow close to each other and trees have well-defined growth rings; constructing a well-replicated, robust dataset is more easy. Growing tropical tree species can be more complex because climates, ecosystems and overall growing conditions are quite different.
HHow do you study tropical species and determine their growth when the rings may not exhibit an annual pattern?
For non-tropical trees, the most traditional dendrochronological parameter is to measure the width of the rings at each core or cross-section to understand the growth pattern of each tree over time. Because we knew the year we took the sample in the field, we assigned preliminary dates to the rings and crossed over time in other nearby trees to determine the ring width pattern. If they were growing in a similar way each year, the number of rings and relative width would be positively correlated each year, which tells us that these trees are responding to similar environmental variables that aid growth.
In tropical species, you may find that one side of the tree has a different number of growth rings than the other, making the cross-dating process difficult.
To confirm whether annual rings are produced, independent dating methods, such as radiocarbon analysis, are often used, which is what we did for the Bolivian tree samples. If the dates we assign via dendrochronology exactly match the radiocarbon curves in the atmospheric data, then you can prove that you have annual growth.
Sometimes the ring cannot be identified even under a microscope. We looked at high-resolution images of tree samples to try to determine if we had a ring that had both earlywood — the wide, porous part of the ring that forms during the rainy season — and latewood: thin, in the Dense bands of cells formed under dry conditions. Zooming in on these anatomical cuts can help us identify growth layers. If we don’t have that kind of thin band of cells where the cell walls are growing tighter, that means growth has stopped, which could mean the trees aren’t officially stopping, maybe the trees are growing differently, not every year. Therefore, tree ring width, wood anatomy and radiocarbon analysis are key tools we use when studying trees in complex growing environments such as the tropics.
Samples of tropical tree species Noctilucentalso known as swamp cinnamon or cock thorn, from a moist forest in northern Bolivia. The tree samples varied in ring width patterns, highlighting the need to measure multiple sections of tropical wood for accurate growth.Photo: Ross Oakes
What stories do these trees have about their climate? What can they teach the local community?
In general, we can try to understand major past climate events or trends. So if there is a cyclical pattern, say, every 50 years, that these trees grow very low, and that time period happens to coincide with a known climate event, whether drought or otherwise, we can help predict whether or not the frequency of those droughts It may be to increase or try to communicate that if a 50-year drought is imminent, it may be wise to manage water resources.
In one of the species I studied at high altitudes, we observed a decline in tree growth since the 1980s. They are dying for some reason, which may be related to drought, or it may be related to an extended growing season and limited resource availability. We’ve been trying to talk to the locals to see how the growing season has changed? Are their crops growing later in the year? I think it’s important to connect what the community knows and values ​​to what the trees record in their community to enhance understanding of how the environment is changing.
What surprised you the most about studying trees in these tropical environments?
It’s really interesting to find out the age of these trees growing in wet conditions. At high altitudes, the fastest growing trees in the world can often reach 700 years old, but that’s because they are in very harsh environments and their species types are adapted to those conditions. But in lower latitudes, like some wetter environments, the trees are also much older than I expected. 200 years doesn’t seem like a lot by comparison, but in a really humid environment, large trees are likely to grow quickly and young, and I’ve learned that’s not always the case.
The human history and value of these trees to local communities is yet to be understood. When researching new tree species, we also learn about their uses (medicine, wood) and the stories they record. For example, you can see evidence of human settlement in the growth rate of tree rings. If there was deforestation in the forest, the live trees I sampled would show this huge release of rapid growth, or large growth rings over time. Talking to people and hearing them say, “Yeah, we actually settled in this neighborhood 20 years ago,” which coincides with that huge version of growth, is unbelievable. It’s amazing to see this connection between people and trees.
