Research

Which tree species are best at cooling down the city?

Gabe Allen — Tue, 12 Aug 2025 17:28:53 +0000

Which tree species are best at cooling down the city? Gabe Allen Tue, 08/12/2025 - 11:28

Gabe Allen

Summers in Boulder are hot and getting hotter. But the large trees that occupy the city’s parks, yards and green spaces can provide a welcome patch of shade for pedestrians, critters and understory plants.

One INSTAAR researcher is hyper-focused on this phenomenon. PhD student Advyth Ramachandran is investigating the cooling power of various species of urban trees. Each week, his field crew downloads data from carefully-placed temperature sensors and gathers samples of leaves and wood. The goal is to create guidance for urban planners and foresters looking to cool down cities with natural solutions.

Today, Ramachandran will present his methodology and preliminary findings at the Ecological Society of America’s Annual Meeting in Baltimore. He is one of several CU Boulder researchers who will attend, speak and present at the conference.

Earlier this week, INSTAAR sat down with Ramachandran to learn more about his project and how it came to be.

What is the origin story of your interest in urban trees?

Advyth Ramachandran clips a twig of an American linden tree in Boulder in August, 2025. Back at the lab, Ramachandran and his team will use the sample to assess wood density and leaf area for the tree. (Photo courtesy of Advyth Ramachandran)

I didn't have a very outdoorsy childhood, but my family would walk around a lot in urban parks, so I was always really interested in vegetation and biodiversity in urban areas. Then, when I was in college, three of my friends, who are computer engineers, recruited me as the science team member for a hackathon project using remote sensors to better manage urban forests. We were interested in soil moisture and how it impacts urban tree survival. Through that project, I got fascinated with urban forests, and I decided that it would be cool to study urban ecosystems more in graduate school.

When I started my PhD in Katharine Suding’s lab, I pitched the urban tree traits project to Katie and she gave me the green light. The idea is to study urban trees using some of the same methods that we use in natural ecosystems and also to think about urban vegetation dynamics through the frameworks we use in restoration ecology. One of my committee members, Laura Dee, connected me with the City of Boulder Climate Initiatives team, which led to a collaboration with them that is ongoing.

This is such a unique research question. How did it come about?

We know trees are good. We know they're good for a lot of reasons, but importantly they reduce urban temperatures. At this point, the evidence of that is overwhelming.

But, a lot of the evidence that we have for the role of trees in reducing temperatures actually comes from satellites, which are sensing land surface temperature — not actually sensors on the ground that are measuring the air temperature that people experience. The year before I came to CU, students in Laura Dee’s lab put out temperature sensors around Boulder and found that vegetation does actually cool down air temperatures in Boulder.

My research question ultimately arose from this finding, the existing literature and my interest in plant functional traits — basically characteristics that influence how plants interact with the environment. We converged on this idea of how tree traits might predict which species are the best at cooling.

Today in Boulder, we really don't know which tree species are the best to plant for cooling. When foresters decide which trees to plant they base that on a lot of things, such as disease resistance, drought tolerance, and fire risk. We’re trying to add an understanding of which species might work best for reducing temperatures to that selection process.

This is really an example of one of the main thrusts of my dissertation research, which broadly focuses on how plant functional traits can be used to design nature-based solutions. We wrote a paper about this last year, and one of the things that we learned is that this area is really understudied.

So, I did a pilot study in 2024 and then this year was the real deal. We got useful data last year, but we didn't have enough to really see clear differences among species. This year, I was finally able to see some clear differences arise after refining the methodology and expanding the size of the study.

What insights have you gleaned so far?

CU undergraduate researchers Jen Dugdale (left) and Rhiannon Danborn select and preserve leaf and wood samples In Boulder in August 2025. (Photo courtesy of Advyth Ramachandran)

The first insights came from last year's pilot study. We found was that the tree canopy has a significant effect on temperature, as you might expect. It can actually cool the air by around four degrees Fahrenheit. We also found that soil moisture and grass cover are really important determinants of temperature. One major takeaway was that temperature varies on really fine spatial scales. You could have sensors that are a stone’s throw away from each other and see a difference in average summer afternoon temperatures of five degrees Fahrenheit.

Coming into 2025, we decided to start zeroing in on the impacts of individual tree species by controlling for all of these factors that we identified last year. We only put sensors under isolated trees located in urban parks that are irrigated and green. Once we incorporated all these factors into our statistical model, we started to see differences between species. In the end, we were able to identify a gradient of tree species based on their cooling effects.

Can you get specific about which trees are the best at cooling?

I'm not ready to provide planting recommendations just yet, because we still need to refine our analyses and go through peer review. One thing we did find, though, is that dense foliage seems to be a useful trait for cooling. Interestingly, there are some trees, like blue spruce for example, that grow dense foliage and seem to be relatively drought-tolerant. More research is needed, but this is a promising indication that we might be able to find tree species that are both tolerant of a dry climate and good at cooling cities.

This week, INSTAAR PhD student Advyth Ramachandran is presenting preliminary findings at a conference in Baltimore. His work seeks to understand the cooling effects of various urban tree species in Boulder.

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CU undergraduate researchers Gavin Schoew (left) and Arin Oberley measure ground cover near a temperature sensor in Boulder in August, 2025. This data is used to disentangle the impact of trees from the effect of ground cover on urban temperatures. (Photo courtesy of Advyth Ramachandran)

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CU undergraduate researchers Gavin Schoew (left) and Arin Oberley measure ground cover near a temperature sensor in Boulder in August, 2025. (Photo courtesy of Advyth Ramachandran)

Spruce Gulch land gift enhances future ecological and academic studies at CU Boulder (CU Boulder Today)

Gabe Allen — Tue, 12 Aug 2025 13:00:00 +0000

Spruce Gulch land gift enhances future ecological and academic studies at CU Boulder (CU Boulder Today) Gabe Allen Tue, 08/12/2025 - 07:00

Linda Holubar Sanabria has donated a 476-acre wildlife and research reserve to CU Boulder. INSTAAR will manage research and education programs at the site, with Timothy Seastedt stepping up as director.

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Air pollution and warming are changing Colorado’s remote alpine lakes

Gabe Allen — Mon, 04 Aug 2025 20:49:08 +0000

Air pollution and warming are changing Colorado’s remote alpine lakes Gabe Allen Mon, 08/04/2025 - 14:49

Gabe Allen

On a hot summer day, Julia Pop and MJ Farruggia paddled a two-person pack raft to the edge of Sky Pond, an alpine lake at 11,000 feet in Rocky Mountain National Park.

“It’s blooming!” Pop told INSTAAR fellow and assistant professor of ecology and evolutionary biology Bella Oleksy, who waited on the shore.

To a passerby, Pop’s exclamation might have seemed odd. The lake was surrounded by granite boulders and snow. Visible vegetation was limited to a few patches of forest and sparse shrubs and wildflowers emerging from rocky crevices. But, a careful observer might have noticed a green tint to the water — which is what Pop was referring to. The lake was full of microscopic algae called diatoms.

In the past, diatoms mostly lived in the shallow waters near the shore of Sky Pond. But, in recent years, environmental changes have allowed diatoms and other algae to proliferate throughout the lake, according to research from Oleksy and collaborators. It’s just one insight that has come from decades of data collection at the site.

MJ Farruggia (left) and Bella Oleksy on the hike up to Sky Pond in Rocky Mountain National Park on June 19, 2025. (Gabe Allen)

In 2023, Oleksy’s lab took over a long-standing monitoring effort at the Loch Vale watershed, which includes Sky Pond, Lake of Glass and the Loch, all in Rocky Mountain National Park. For 42 years, the program has sought to unravel how human activity, both regionally and globally, affects alpine ecosystems.

Though the program has had a long tenure, Oleksy says there are many outstanding research questions. Alpine lakes are projected to heat up especially fast as a consequence of global warming. And, simultaneously, an influx of airborne chemicals from industry, agriculture and fuel combustion is changing their chemistry.

“So far, the big question is how nitrogen emissions from outside impact a remote watershed that experiences virtually no direct human impacts,” Oleksy said. “We’re also only just starting to really understand how this watershed works and all of the complicated ways it responds to both long and near-term climate patterns.”

This summer, Oleksy’s lab will return to Loch Vale once per month. The information they gather will fuel analyses to come, and contribute to one of the longest-running biogeochemical records in the Colorado alpine.

A changing landscape

In 1983, Oleksy’s future thesis advisor, Jill Baron, began a monitoring program in Loch Vale with funding from the National Acid Precipitation Assessment Program. The goal was to evaluate how acid rain, a major environmental concern at the time, was affecting alpine ecosystems.

The program's goals soon shifted when the researchers discovered that acid rain was having only a negligible effect. Instead, they observed high levels of “wet nitrogen” raining down on the watershed. At the time, the nitrogen mostly stemmed from vehicle and power plant emissions in front range cities. But, as the trend has persisted, these sources have gotten cleaner while large-scale livestock operations have become a more significant contributor.

“A lot of ammonia gets volatilized from these concentrated animal feeding operations around Greeley and elsewhere,” Oleksy said. “When we get these upslope storms in the summer, it arrives in precipitation. You can smell it on the wind all the way up here.”

Nitrogen, an essential nutrient for living organisms, is historically scarce in alpine environments. As such, its arrival en masse in Loch Vale spurred major ecological change. So far, Oleksy and collaborators have identified overall increases in several types of algae, including diatoms and filamentous green algae. They have also found that much of the nitrogen is washed downstream, where it might have further impacts.

In 2005, research at Loch Vale catalyzed a collaborative effort between state and federal agencies to reduce nitrogen emissions in Colorado. The effort is ongoing, and Oleksy’s monitoring efforts provide a way to assess progress.

Preventing toxic blooms

Alpine watersheds are in a period of rapid change, and the consequences of that aren’t always black and white. Using machine learning, Oleksy and collaborators published a report showing that, during anomalously hot and dry summers, mountain lakes are experiencing unseen levels of productivity. Basically, the total amount of organisms living in the lakes, largely algae, are much higher than normal.

The ecological consequences of this change are complex, but one knock-on consequence is that these lakes may be pulling more carbon dioxide out of the atmosphere through photosynthesis. When the organisms die, that carbon is stored in lake sediment.

“These historically super clear lakes are now producing a lot more carbon,” Oleksy said. “The question is: ‘are they now more of a carbon sink than we thought?’”

Bella Oleksy holds up a water filter tinted yellow from diatoms at the outflow of Sky Pond in Rocky Mountain National Park on June 19, 2025. (Gabe Allen)

When this process is pushed to the extreme, though, it can backfire. If a lake becomes too warm and nutrient-rich, it can trigger a proliferation of cyanobacteria that produce deadly toxins.

This issue is top of mind for Oleksy’s lab. In recent years, several alpine lakes in the San Juan mountains in Southern Colorado have experienced extreme algal blooms. Pop is currently working on a project that seeks to learn more.

“She’s looking at what species are there. Are there cyanobacteria that could produce toxins or just a lot of algae?” Oleksy explained.

The San Juans are an example of drastic and rapid environmental change. A combination of bark beetle outbreaks, wildfire and a dwindling snowpack have catalyzed a major ecological shift. For now, the Front Range has escaped environmental change at that level, but it’s not out of question as climate change progresses.

“Compared to Northern Colorado, the climate shifts and changes to snowpack in the San Juans have been profound,” Oleksy said.

While the San Juans might provide a window into the future, the Loch Vale monitoring program provides a window into the past. On top of 42 years of field data, sediment cores from the lake bed give the scientists an idea of biological activity in the lake going back hundreds of years. The goal is to glean an understanding of how these ecosystems evolve over a variety of climatic and atmospheric conditions.

“I think it’s really important to have these big datasets and combine different techniques,” Abby Ross, a PhD student in Oleksy’s lab, said. “With all of that background, we can actually tell what changes are unprecedented and need to be investigated further.”

Bella Oleksy filters water into a sample bottle held by PhD student Abby Ross at the inflow of Sky Pond in Rocky Mountain National Park on June 19, 2025. (Gabe Allen)

If you have questions about this story, or would like to reach out to INSTAAR for further comment, you can contact Senior Communications Specialist Gabe Allen at gabriel.allen@colorado.edu.

The Oleksy lab has taken over a 42-year-old monitoring project in Rocky Mountain National Park. Their investigations reveal how remote alpine watersheds are changing in the Anthropocene.

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Bella Oleksy directs postdoctoral scholar MJ Farruggia and undergraduate research assistant Julia Pop from toward a buoy from the shore of Sky Pond in Rocky Mountain National Park on June 19, 2025. (Gabe Allen)

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Report paints grim picture of how nuclear war could impact oceans (CU Boulder Today)

Gabe Allen — Wed, 30 Jul 2025 15:48:40 +0000

Report paints grim picture of how nuclear war could impact oceans (CU Boulder Today) Gabe Allen Wed, 07/30/2025 - 09:48

In a Q&A, INSTAAR director Nicole Lovenduski talks about her contributions to a report on the environmental impacts of a hypothetical nuclear war. Lovenduski was tasked with modeling impacts in the world's oceans, which could be global and long-lasting.

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Spotlight: Hunter Geist-Sanchez is pioneering restoration methods for Colorado grasslands and reconnecting with his ranching roots

Gabe Allen — Mon, 21 Jul 2025 20:58:52 +0000

Spotlight: Hunter Geist-Sanchez is pioneering restoration methods for Colorado grasslands and reconnecting with his ranching roots Gabe Allen Mon, 07/21/2025 - 14:58

Gabe Allen

Hunter Geist-Sanchez's summer field crew hikes to a field site near the National Renewable Energy Lab's Flatirons Campus. Left to right: Ava Boettiger, Zade Baldwin, Hunter Geist-Sanchez, Rose Young. (Gabe Allen)

All summer long, INSTAAR masters student Hunter-Geist Sanchez wakes up at dawn to meet a rotating cast of labmates and undergraduate research assistants at a grassy mesa bordering the National Renewable Energy Lab’s Flatirons Campus. The team spends long hours setting up experimental plots, measuring soil moisture and erosion, simulating grazing cattle with a weed whacker and, most of all, mapping plant species.

All the work is in service of the Suding Lab’s grassland diversity and grassland resilience projects. The overarching goal is to cultivate knowledge and management strategies that will help conserve Western grasslands as the climate becomes hotter and drier. This summer, funding from INSTAAR's Summer Scholars Program allowed Geist-Sanchez to expand his undergraduate research team to three members.

Hunter Geist-Sanchez shows undergraduate field technicians a map of a field site near the National Renewable Energy Lab's Flatirons Campus. (Gabe Allen)

Geist-Sanchez is relatively new to this work, but his passion for it is apparent.

"No one cares about grass, but I think grass is pretty great,” he explained during a recent day of field work. “It’s hard to really appreciate the beauty until you spend a lot of time in a certain place. Then you get it.”

Perhaps a piece of Geist-Sanchez’s love for grasslands is hereditary. His family has deep roots in the San Luis Valley, where his grandparents grew up working on local farms and ranches. His great uncle and cousins still own and operate a ranch in the area.

Though Geist-Sanchez has lived his whole life in front range cities, his extended family keeps him connected to Southern Colorado rangelands. Recently, his masters project has been a frequent topic of conversation with still-ranching family members.

“Hopefully this research helps them with restoration on the property,” Geist-Sanchez said. “My great uncle asks me about it all the time.”

For many, 10 or 12-hour stints of data collection in the blazing sun would seem a hefty price to pay for good science. But, Geist-Sanchez seems unfazed by long days in the field. For him, they are a chance to put distractions aside and revel in the details of an ecosystem. It’s a habit that he formed long before he entered academia.

A love of nature

Geist-Sanchez grew up splitting time between parents in Aurora and Fort Collins, with more family spread around Front Range cities. Growing up, his siblings and cousins were mostly captivated by sports and video games — things that Geist-Sanchez also had a passing interest in. But, his true passion was always nature.

“In all my free time, I would go to the library and read Nat Geo books and stuff like that,” he said. “I was really fascinated with wildlife growing up, and no one else was really like that. I knew from pretty early on that I was different.”

Geist Sanchez’s early education in the natural sciences didn’t all come from books. As a kid, he looked forward to family camping trips to Great Sand Dunes National Park, along the Poudre River and to other natural areas across the West. Most of all, he looked forward to spending time with his grandmother on a plot of family land in the San Luis Valley.

“My mom would drop me and my cousins off down there for weeks at a time,” he said. “Those are really fond memories.”

The property was stunning and rugged. To take a bath, Geist-Sanchez remembers filling up buckets from the well and boiling the water on the stove. Days were spent going on hikes and helping his grandmother tend the garden.

For Geist-Sanchez, it was idyllic.

“I just always loved being outdoors,” he said.

After high school, Geist-Sanchez went on to earn a degree in ecosystem science and sustainability from Colorado State University. As an undergrad, he took an unusually active role in several restoration ecology projects. Specifically, he worked with research groups testing new methods for restoring out-of-use agricultural fields and cheatgrass-invaded grasslands.

“After that, I knew I wanted to do more restoration. It’s really difficult work and I was fascinated by how we might be able to improve outcomes,” he said.

After college, one of Geist-Sanchez’s mentors connected him with Katharine Suding, who was looking for a grassland research assistant. After a year in that role, Suding asked Geist-Sanchez to join the lab as a graduate student and take the lead on the lab’s grassland resilience project. Geist-Sanchez jumped at the opportunity — it was a perfect way to sink deeper into his passion for restoring Colorado’s natural systems.

Restoring bare patches

Hunter Geist-Sanchez plants surveyor flags at the Hogan Ranch Property in Boulder. (Gabe Allen)

Restoration ecologists are looking for new methods to combat desertification as the West becomes hotter and drier. In his masters work, Geist-Sanchez has zeroed in on bare-patches, arid spots lacking vegetation that tend to grow over time.

The first step is to figure out how bare patches form and what leads to expansion. To investigate this question, Geist Sanchez is looking at thermal imagery of the landscape, measuring soil characteristics and cataloging the species that grow in and around the sites.

“One of the questions I’m interested in is, ‘What are the mechanisms behind this bare ground spread,” Geist-Sanchez said.

Next comes the question of restoration: how can land managers bring plants back to these sites so that they can once again become a functional part of the ecosystem? To address this, Geist-Sanchez is planting a mix of forb and grass seeds at the sites.

Importantly, he has balanced the seed mixes to represent plants with specific survival strategies. Plants that arise early in the season may be able to take advantage of spring moisture, and then offer shade later in the summer. Plants with large seeds might be able to survive periods of drought or heat by relying on energy stores.

These hypotheses are based on previous research, but only time will tell their efficacy.

“I guess we’ll have a better idea by the end of the season,” Geist-Sanchez said.

For now, bare patches are generally a small-scale problem in Boulder grasslands. But, if desertification becomes more extreme in the future, research like Geist-Sanchez’s will become even more important for both natural areas and rangelands.

“We don’t have extreme desertification here yet, which is great,” Geist Sanchez said. “If we ever start to see something like the dust bowl, which stemmed from extreme agriculture and land-use changes, I hope land managers will be able to apply this research. I want to be able to hand this off and expand on it.”

If you have questions about this story, or would like to reach out to INSTAAR for further comment, you can contact Senior Communications Specialist Gabe Allen at gabriel.allen@colorado.edu.

Geist-Sanchez is following his life-long love of nature to develop new methods for preserving Colorado natural areas and rangelands. As a sixth-generation Coloradoan, he hopes his research can help keep grasslands sustainable as the West heats up.

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What long-term science in Boulder’s alpine is teaching the world about mountain ecosystems (Boulder Reporting Lab)

Gabe Allen — Wed, 16 Jul 2025 15:50:32 +0000

What long-term science in Boulder’s alpine is teaching the world about mountain ecosystems (Boulder Reporting Lab) Gabe Allen Wed, 07/16/2025 - 09:50

In a Boulder Reporting Lab op-ed, Nancy Emery argues for the importance of the Niwot Ridge Long-Term Ecological Research Program. With over 45 years of uninterrupted data, scientists at Niwot Ridge provide unmatched data and insights into changing alpine ecosystems.

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Why flood prediction in the US falls short and how researchers are working to fix it (CU Boulder Today)

Gabe Allen — Mon, 14 Jul 2025 20:11:19 +0000

Why flood prediction in the US falls short and how researchers are working to fix it (CU Boulder Today) Gabe Allen Mon, 07/14/2025 - 14:11

New INSTAAR faculty Zhi Li chats with CU Boulder Today about flood modeling research. He explains why current flood warnings can leave communities unprepared — and how high-resolution forecasting and better risk communication could save lives.

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Meltpools in the Himalaya (Nepali Times)

Gabe Allen — Mon, 14 Jul 2025 16:50:46 +0000

Meltpools in the Himalaya (Nepali Times) Gabe Allen Mon, 07/14/2025 - 10:50

In an op-ed in the Nepali Times, Alton Byers outlines his work with a remote Himalayan community to identify a potentially dangerous glacial lake. Byers recommends management actions both locally and regionally.

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The power of pack mules: harnessing partnerships with land stewards for remote ecosystem research (ASLO)

Gabe Allen — Mon, 07 Jul 2025 21:52:09 +0000

The power of pack mules: harnessing partnerships with land stewards for remote ecosystem research (ASLO) Gabe Allen Mon, 07/07/2025 - 15:52

Bella Olesky and others in INSTAAR's mountain limnology lab are saddling up pack mules to investigate the causes of algal blooms in remote alpine lakes. Their results will help land managers understand and support these fragile and complex ecosystems.

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Western mountain snow melts fast and early (NASA)

Gabe Allen — Mon, 30 Jun 2025 20:57:49 +0000

Western mountain snow melts fast and early (NASA) Gabe Allen Mon, 06/30/2025 - 14:57

A report from INSTAAR's mountain hydrology group in collaboration with federal agencies reveals rapid spring snowmelt across the Mountain West in 2025. The analyses forecast potential drought conditions in the coming months, especially in the Pacific Northwest.

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Research

Which tree species are best at cooling down the city?

What is the origin story of your interest in urban trees?

This is such a unique research question. How did it come about?

What insights have you gleaned so far?

Can you get specific about which trees are the best at cooling?

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Air pollution and warming are changing Colorado’s remote alpine lakes

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