Don’t eat that cookie! We need it for science.
How scientists use “tree cookies” and “root cookies” to study ecosystems
Mmmm, the smell of cookies is everywhere this time of year. Just imagine that lovely scent—so woodsy, so piney. We’d take a bite if not for the risk of splinters and breaking our teeth.
If that doesn’t sound like your typical holiday cookie, you’d be right—we're talking about tree cookies. “Tree cookie" is a term for a round slice cut out of a dead woody tree trunk (there are also “root cookies” from thick roots). It’s kind of like making slice and bake cookies, except with more power tools. Tree cookies are a showcase for tree rings, which can tell scientists all kinds of things about the tree and the environment around it.
You may be familiar with counting tree rings to see how old a tree is, but the thickness of tree rings and other patterns in the wood, like fire scars, can reveal how quickly a tree has grown and how its growth may have been affected by drought, fire, avalanches, erosion, or forest management. Tree and root cookies can be cut out of all kinds and sizes of trees—some are only a few inches in diameter, while others can be three feet in diameter. Every tree cookie tells a story.
Tree growth patterns are detectable in tree cookies because the wood looks different depending on how the tree was growing when it was formed. When the tree is growing quickly, the cells are big and thin-walled and the wood appears lighter. When it is growing slowly, the cells are small and densely packed, and the wood looks darker.
This is why you see annual rings—there’s a contrast between the wood laid down during the growing season and the dormant season. Rings appear wider when the tree is growing quickly and conditions are good and narrower when they are poor, as during a drought or during other times of stress.
Read on for three examples of how USGS scientists use tree and root cookies to learn about the environment across the country.
In Glacier National Park, scientists use tree cookies to understand how climate affects avalanches
While avalanches can be powerful enough to kill big trees, those that survive will show the scars of the avalanche in their rings. One pattern that scientists can observe is “reaction wood”, where the tree grows asymmetrically in response to a mechanical stress, such as the impact of an avalanche. If the avalanche bends the tree downhill, the tree will bend upwards as it grows to right itself, and that growth pattern will show up in its rings.
The stress of the avalanche can also cause slow growth, resulting in narrow rings. Trees wounded by avalanche debris (heavy snow, rocks, or wood) may also produce scars and traumatic resin ducts, structures that help the tree respond to injury that are visible in the wood long after the tree has recovered.
USGS scientists have been cutting hundreds of tree cookies from dead trees in avalanche paths in Glacier National Park, Montana. The goal is to understand how avalanches are affected by climate in this region. First, they use the tree cookies (as well as tree cores from living trees) to create a history of avalanches in the park.
Then they connect that history to climate records. Ultimately, they are able to understand how avalanche frequency and behavior changes as the climate changes, information that can better help people and communities at risk of avalanches prepare and protect themselves, their property, and infrastructure.
Background image: Stacks of tree cookies. (NPS)
Across North America, tree cookies build out a long history of fire, climate, and human activity
Other USGS scientists are also looking to tree cookies to understand past climate change with an eye towards the future, but rather than snow and ice, they focus on fire. Fire activity has long been shaped by both humans and climate, and that remains the case today, but figuring out how each has contributed and how they interact can help us understand how fire-prone ecosystems have developed and how they might best be managed in the present day.
To get at this puzzle, USGS scientists are using tree cookies and other tree ring records to build long-term histories of how humans and climate have shaped fire in the southwestern U.S., including the long history of fire use by Indigenous peoples.
When trees survive fires, the fire can damage the living tissue below the bark, leaving a fire scar. Tree-ring fire scars provide information about the year, season, severity, frequency, size, and fire-climate relationships of fires that occurred hundreds to thousands of years prior to modern records. Scientists can observe fire scar records in tree cookies or by cutting small samples from of living trees.
Scientists have collected tree-ring fire-scar records across North America, and in 2022 USGS scientists compiled all known tree-ring records from across the continent into the North American Tree-Ring Fire Scar Network, which has helped them identify continental fire-climate relationships and areas where there are gaps in the records.
Background image: Research Geologist Natalie Kehrwald cuts a “cookie” from a fire-scarred tree in the southern Sangre de Cristo Mountains, New Mexico. Cookies are cross-sections of trees that provide information on the interactions between past droughts and fires. (Becky Brice, USGS)
In the Mid-Atlantic, root cookies help scientists measure stream bank erosion
It’s not just tree trunks that hold secrets of the past – thick, woody roots also have annual growth rings, and like tree cookies, scientists can cut root cookies to learn about changes in the environment. In the Mid-Atlantic region, USGS scientists are using root cookies to measure stream bank erosion. Erosion exposes live woody roots that were previously buried.
When roots are first exposed to air or water following stream bank erosion, signs of stress are recorded in the new tissue as changes in wood porosity or color, changes in ring eccentricity (roundness), or presence of scars.
Scientists collect cross-sectional samples of these roots, or root cookies, and identify these signs of exposure under a microscope. They can then estimate how much time has elapsed since the root was exposed by counting growth rings inwards to the point where the signs of stress begin to appear.
They also measure how far the exposed root is from the current bank, which provides an estimate of how much soil has eroded since time of exposure. These two numbers are used to calculate an erosion rate for that spot. The scientists aggregate data from multiple root cookies along a stream to get a measure of erosion rate for a particular stretch of stream bank.
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