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When resource specialists with the National Park Service noticed some peculiar patterns in Ponderosa pine trees in their park, they sent out a call to Department of Interior partners. They were looking for a dendrochronologist (someone who studies tree rings) who could use their scientific expertise to help unscramble and translate the life story hidden within the rings of the trees. 

Recently, scientists working for the National Park Service in Dinosaur National Monument (Dinosaur), spotted something surprising and unexpected. They noticed Ponderosa pine (Pinus ponderosa) trees, tall and dramatic, growing alone or in small groups on the river meander benches of the Yampa River in Dinosaur. Although Ponderosa pine is common in the lower montane forests of western North America, isolated old-growth Ponderosa pine growing at low elevations and on river meander benches and islands is less common.

island in middle of river with single Ponderosa pine living on it
The majestic Ponderosa pine living a solitary existence on a Green River island in Dinosaur National Monument. Credit: Peter M. Brown, Rocky Mountain Tree-Ring Research
Scientists sampling Ponderosa Pine with incrememtn borer
Tree cores are collected from trees using a forestry tool first developed in Germany more than 100 years ago called an increment borer. The increment borer is a simple but powerful tool with a long, hollow drill bit that isolates a pencil-sized core spanning bark to middle of the tree. Because this is a manual tool, and because the coring cavity is so small, this manner of collecting tree-ring samples leaves minimal evidence of the extraction and no long-term damage to tree growth or tree health. Images credit: ReBecca Hunt-Foster, NPS Dinosaur National Monument

As well as being an ecologically important species, Ponderosa pine trees are very sensitive to variations in moisture, and their growth rings record that variation each year. This makes them a very useful, high-resolution environmental proxy of past climate conditions for scientists. In Dinosaur, Ponderosa pine tree rings hold the potential to reveal new insights about the environmental conditions that lead to their establishment and sustainability in the future. The tree rings can tell us about Ponderosa pine tree prevalence, age, establishment patterns, ecological relationships, and climate information. Understanding what environmental and climate conditions allowed for these trees to thrive in the past, may be crucial to their continued management and survival in the future with climate change.

Eager to learn more about the life history of these distinctive and rare groves of Ponderosa pines, Dinosaur scientists reached out to Department of the Interior researchers broadly in search of a dendrochronologist – someone who studies tree rings, to determine the dates and patterns of past events recorded in the rings.

Joining Forces to Collect Field Data

U.S. Geological Survey scientist and dendrochronologist, Becky Brice, excitedly answered their call to document this unique and possibly sensitive resource. So, the USGS began a partnership with Dinosaur to conduct fieldwork in which systematic measurements of individual river Ponderosa pine trees and their groups could be gathered.

Measurements to be taken in the field included trunk diameter at breast height (DBH), tree height, and ring growth. These dendrometric measurements can be used in combination with precise tree-ring dating to determine the statistical properties of each pine group population, estimate age structure, and model average demographics. Additionally, on-the-ground assessments allow the team to accurately map spatial extent and patterns of establishment and growth and can provide information critical for planning and management of park resources.

left image shows scientists measuring diameter of fallen tree and right image shows someone holding a pine cone
Scientists and volunteers collect tree measurements in Dinosaur National Monument. On the left, tree diameter is measured and on the right, cone dispersion is measured. Credit: Becky Brice, USGS
two scientists looking at hanging cliff stands of ponderosa pine from river raft
Emily Spencer (NPS) and Becky Brice (USGS) surveying the hanging cliff stands of Ponderosa pine. The deep knowledge held by the National Park Service staff about the landscape was integral to finding and accessing the trees. Credit: Jenny Biggs

Expedition One: Yampa River

The initial sampling trip traveled nearly 70 river miles over seven days on remote and difficult-to-access portions of the wild Yampa River of Colorado in May 2022. This expedition was comprised of seven boats and seventeen people including archaeologists, ecologists, paleontologists, and dendrochronologists from the U.S. Geological Survey, National Park Service, U.S. Forest Service, Bureau of Land Management, plus a group of volunteers unsurpassed in enthusiasm and dedication.

In order to gather all of the data they hoped for, the team was required to board river rafts, tackle class-four rapids, and stop at every Ponderosa pine tree accessible from the river. The team was on a mission to efficiently gather as much information as possible, aided immensely by the swift river waters of spring snowmelt.

scientists and volunteers in the field collecting samples together
Dr. ReBecca Hunt-Foster (NPS) with Dinosaur National Monument volunteers. Credit: Becky Brice, USGS
scientists rafting down river and surveying conditions
Expertise of volunteer boat operators navigating the river rapids and safely scouting complex rapids from the shore. Field sample collection by boat introduces a new level of organization, logistics, safety considerations, and gear management that requires expertise, cooperation, patience, resilience and trust. Images credit: Peter M. Brown, Rocky Mountain Tree-Ring Research

The first group of Ponderosa pines encountered on the Yampa were at the aptly named Ponderosa camp and guarded by a resident bald eagle. This old guard seem to rise defiantly from the river’s edge, their height was visually exaggerated by the shorter vegetation around them. This was one of only three such groups along the entire stretch of this river.

row of Ponderosa pine along Yampa River
A stop along the Yampa River to sample the "old guard" Ponderosa pine trees. Credit: Emily Spencer, NPS Dinosaur National Monument

The slopes rising from the Yampa River are not occupied by Ponderosa pine, so the seed source for the river trees has yet to be determined. After seeing them in person, many questions surfaced about their origin. How did the Ponderosa pine get to the Yampa? What allowed the old trees to establish and stay there? Is there potential for young trees to establish and grow as well?

green and brown water from two different rivers merge as the rivers meet
Confluence of the Green and Yampa Rivers (green color is the Green and yellow color is the Yampa). The color of the water is an indication of the very different streamflow patterns between the two rivers. Credit: Becky Brice, USGS

Expedition Two: Green River

The success of the Yampa River expedition led the team to ask another important question: Are there Ponderosa pine growing on the meander benches of the Green River in Dinosaur, the sister river of the Yampa, and do they show the same patterns as the Yampa trees?

The two rivers and their landscapes are drastically different. The Yampa River is one of the few remaining free-flowing rivers in the western United States. It originates in the high mountains of northcentral Colorado and traverses Dinosaur from east to west. By contrast, streamflow in the Green River is controlled by the Flaming Gorge Dam in neighboring Utah and enters Dinosaur from the north, meeting the Yampa along the iconic Steamboat Rock at Echo Park. Because of these differences, a second trip to the Green River was warranted to compare with the Yampa results.

weather extremes of snow and pouring rain in Dinosaur National Monument
Each river-based research trip elicits anticipation, nervousness, uncertainty and excitement about what will be found and what challenges will be faced. Early morning snowfall in May on the Yampa River (left), and a downpour in August on the Green River (right), can make daily camp tasks difficult. Credit: ReBecca Hunt-Foster, NPS Dinosaur National Monument

The team set off on their second expedition in 2023 from the Gates of Lodore along the Green River. This time carried out over 6-days with four National Park Service rafts loaded with twelve research staff and volunteers, journeying to remote regions of the Green River in Colorado and Utah.

scenic view of the Green River
A raft of scientists and volunteers floats down the Green River in Dinosaur National Monument. Credit: Peter M. Brown, Rocky Mountain Tree-Ring Research

Very early into the trip it became clear that the Green River and its Ponderosa pine were indeed distinctly different from the Yampa. The team found eight times as many Ponderosa pine trees between the Gates of Lodore and the confluence with the Yampa than along the entirety of the Yampa River, and the trees also grew in larger groups. While the estimated age of the Yampa River trees decreased with river mile, Green River tree ages were mixed. But, perhaps the most notable difference, was the Ponderosa pine sentinels keeping watch along the canyon rim and potentially providing a seed source to the trees below along the Green River.

Analyzing the Samples

Ponderosa pine tree rings core under microscope
One Green River Ponderosa Pine sample after mounting and sanding lab procedures. Credit: Becky Brice, USGS

While the field observations started to move the needle on our understanding of Ponderosa pines growing on the meander benches of Dinosaur, subsequent work in the lab offers the potential for new insights to be revealed. 

Back in the lab, tree core samples are carefully prepared for examination under the microscope and for ring-width measurement. Scientists start by gluing the cores into sample mounts. After progressive sanding and polishing, the plant cells within each tree ring become visible and the light-dark colored couplets representing one year of growth for each year in each tree can be clearly distinguished.

scientist looking at tree core through microscope
A scientist looks at a Ponderosa Pine tree core using a rotary measuring stage. Credit: Becky Brice, USGS

The width of each annual ring changes through the life of the tree, and the change in the width reflects the variable growing conditions for each year for each tree. These changes reveal multi-year patterns, and the patterns are compared across individual trees and across the landscape to verify year assignment. This process is called crossdating and is a foundational principle in the discipline of dendrochronology. Crossdating also allows scientists to approximate the birthdays of the trees.

Once the year of growth for every ring is known, a rotary measuring stage allows the width of each ring to be slowly and precisely measured to the micron, and for these measurements to be recorded over the hundreds of years that the Ponderosa pines lived.

As changes in the tree-ring widths of Ponderosa pine can be highly correlated with changes in water availability, narrow tree rings indicate periods of water stress and drought. Tree-ring reconstructions use the statistical relationship between instrumental records and tree-ring widths to extend our understanding of past environmental change to centuries before we used instruments to record it.


Finishing a challenging field expedition with promising samples in hand and having developed strong new bonds with people and the landscape is thrilling. The bittersweetness of parting with beautiful public lands such as Dinosaur National Monument is buffered by the anticipation of scientific discovery and advancement of knowledge.

sun reflecting off riverside cliffs along the Green River
A stunning farewell to the field expedition in Dinosaur National Monument with early morning sunshine gleaming off the riverside cliffs. Credit: Becky Brice, USGS

The partnership between the USGS and NPS, as well as collaborative contributions of other federal agencies on the ground, made these trips possible and ensured successful acquisition of data that would have otherwise required significant financial and personnel resources. Volunteers are also a key component of such work. Volunteers were Marshall Worsham (Denver, CO), Peter Brown (Fort Collins, CO), Jeremy Bailey (Steamboat Springs, CO), Marla Bailey (Steamboat Springs, CO), Dave Miller (Grand Junction, CO), Janet Kelleher (Grand Junction, CO), Rob Herring (Moscow, ID), Sarah Dentoni (Fort Collins, CO), John Saunders (Steamboat Springs, CO), Beth Honebein (Reno, NV), and Peter Honebein (Reno, NV).

While the team is still working to analyze and interpret all of their samples from the first two expeditions, the partnership between the USGS and NPS continues as plans are in the works to complete analysis of meander bench Ponderosa pine results this year, and to return to sample Ponderosa pine in 2024 with new questions about the connection between human history and drought in Dinosaur. The trees, as our stoic partners, still have many stories to tell!

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